Applied
Geochemistry
Applied Geochemistry 19 (2004) 1255–1293
www.elsevier.com/locate/apgeochem
Natural organic matter in sedimentary basins and its
relation to arsenic in anoxic ground water: the example
of West Bengal and its worldwide implications
J.M. McArthura,*, D.M. Banerjeeb, K.A. Hudson-Edwardsc, R. Mishrab,
R. Purohitb, P. Ravenscroftd, A. Cronine, R.J. Howartha, A. Chatterjeef,
T. Talukderf, D. Lowryg, S. Houghtona, D.K. Chadhah
a
Department of Earth Sciences, UCL, Gower St, London WC1E 6BT, UK
Department of Geology, University of Delhi, Chattra Marg, Delhi 110007, India
c
School of Earth Science, Birkbeck, University of London, Malet St., London WC1E 7HX, UK
d
Arcadis Geraghty & Miller International Inc., 2 Craven Court, Newmarket CB8 7FA, UK
e
Robens Centre for Public and Environmental Health, University of Surrey, Guildford GU2 7XH, UK
f
Central Ground Water Authority, Bidhan Nagar, Salt Lake City, Kolkata 700091, India
g
Department of Geology, Royal Holloway University of London, Egham, Surrey TW20 0EX, UK
h
Central Ground Water Authority, 18/11 Jamnagar House, Mansingh Road, Delhi 110011, India
b
Received 16 September 2002; accepted 16 February 2004
Editorial handling is by Fuge
Abstract
In order to investigate the mechanism of As release to anoxic ground water in alluvial aquifers, the authors sampled
ground waters from 3 piezometer nests, 79 shallow (<45 m) wells, and 6 deep (>80 m) wells, in an area 750 m by 450 m,
just north of Barasat, near Kolkata (Calcutta), in southern West Bengal. High concentrations of As (200–1180 lg L1 )
are accompanied by high concentrations of Fe (3–13.7 mg L1 ) and PO4 (1–6.5 mg L1 ). Ground water that is rich in
Mn (1–5.3 mg L1 ) contains <50 lg L1 of As. The composition of shallow ground water varies at the 100-m scale
laterally and the metre-scale vertically, with vertical gradients in As concentration reaching 200 lg L1 m1 . The As is
supplied by reductive dissolution of FeOOH and release of the sorbed As to solution. The process is driven by natural
organic matter in peaty strata both within the aquifer sands and in the overlying confining unit. In well waters, thermotolerant coliforms, a proxy for faecal contamination, are not present in high numbers (<10 cfu/100 ml in 85% of wells)
showing that faecally-derived organic matter does not enter the aquifer, does not drive reduction of FeOOH, and so
does not release As to ground water.
Arsenic concentrations are high (50 lg L1 ) where reduction of FeOOH is complete and its entire load of sorbed
As is released to solution, at which point the aquifer sediments become grey in colour as FeOOH vanishes. Where
reduction is incomplete, the sediments are brown in colour and resorption of As to residual FeOOH keeps As concentrations below 10 lg L1 in the presence of dissolved Fe. Sorbed As released by reduction of Mn oxides does not
increase As in ground water because the As resorbs to FeOOH. High concentrations of As are common in alluvial
aquifers of the Bengal Basin arise because Himalayan erosion supplies immature sediments, with low surface-loadings
*
Corresponding author. Fax: +44-20-7679-4166.
E-mail address: j.mcarthur@ucl.ac.uk (J.M. McArthur).
0883-2927/$ - see front matter Ó 2004 Published by Elsevier Ltd.
doi:10.1016/j.apgeochem.2004.02.001
1256
J.M. McArthur et al. / Applied Geochemistry 19 (2004) 1255–1293
of FeOOH on mineral grains, to a depositional environment that is rich in organic mater so that complete reduction of
FeOOH is common.
Ó 2004 Published by Elsevier Ltd.
1. Introduction
In the Bengal Basin, some 25% of wells that tap
ground waters from alluvial aquifers contain high (>50
lg L1 ) concentrations of As, the consumption of which
threatens the health of millions of people (e.g. Chatterjee
et al., 1995; Chakraborti et al., 2002): by, for example,
inducing cancers that may prove fatal (Smith et al.,
2000; Yu et al., 2003). One model for the release of As to
ground water, viz. reduction of sedimentary Fe oxides
(FeOOH without mineralogical connotations hereinafter) and release of the sorbed As, was outlined by Gulens
et al. (1979), adopted by Matisoff et al. (1982), and
systematised by Korte and Fernando (1991). The process is driven by microbial reduction of organic matter
(Nealson, 1997; Lovley, 1997; Banfield et al., 1998;
Cummings et al., 1999; Chapelle, 2000; Lovely and
Anderson, 2000). This mechanism was used by Nickson
et al. (1998, 2000) to explain the occurrence of high
concentrations of As in aquifers in the Bengal Basin: it
was further developed by McArthur et al. (2001) and
Ravenscroft et al. (2001), who proposed that organic
carbon in buried peat was the driver of microbial reduction of FeOOH in the sediments, and that human
organic waste in latrines might also locally contribute to
As release by providing point-sources of organic matter
to enhance local reduction of FeOOH (thereby confusing regional trends of As distribution by adding local
signals). These authors also discussed, and dismissed,
other mechanisms proposed to explain the release of As
to ground water, such as oxidation of sedimentary pyrite, and competitive anion exchange of sorbed As with
inorganic PO4 from fertiliser; they thereby broke the
putative link between the development of irrigation with
ground water and the presence of As poison in ground
water.
Although the Fe-reduction model is now widely accepted as valid for the Bengal Basin (Bandyopadhyay,
2002; Dowling et al., 2002; Harvey et al., 2002; Anawar
et al., 2003; Tareq et al., 2003), the suggestion of
McArthur et al. (2001) and Ravenscroft et al. (2001) that
reduction is driven by buried peat has been disputed.
For example, Pal et al. (2002) found peat in an As-free
area, but did not find it in an As-affected area nearby.
Harvey et al. (2002) suggest that reduction of FeOOH is
driven by surface organic matter, from river beds,
ponds, and soils, that is drawn into the aquifer by irrigation drawdown: this model seeks to re-link irrigation
and high concentrations of As in ground water. Should
such a linking lead to renewed calls to curb or ban irrigation with ground water, the livelihoods of the rural
poor will be affected adversely.
To better define the source of the organic matter
driving reduction of FeOOH in the Bengal Basin and
to further examine the behaviour of As and Fe in
ground water, the authors installed 3 piezometer nests
in a shallow (<45 m depth) aquifer near Barasat, in
West Bengal (Fig. 1); measured vertical profiles of
water composition in them; and took cores for study
from the piezometer sites. One piezometer was sited
where As poisoning of wells did not occur (site DP),
one was where it was thought to be minor (site AP),
and one where it was severe (site FP): the sites are less
than 350 m apart (Fig. 1). Wells tapping the shallow
(<45 m) and deep (>70 m) aquifers in the area were
also sampled in order to ascertain the lateral extent,
and degree, of changes in water quality, including As
pollution. The content of thermotolerant coliform
(TTC) bacteria in wells and surface ponds was measured in order to assess their content of faecally derived
micro-organisms, the presence of which indicates the
influence of human and animal organic waste. In order
to investigate whether the wells changed their composition with time, they were analysed (TTC apart) on 3
occasions in 2 years. The results of these studies are
presented here.
2. Location of area
The study area is located at 22° 44.430 N, 88° 29.450 E
(Indian/Bangladesh datum) in the contiguous villages of
Joypur, Ardivok and Moyna (JAM hereinafter; Fig. 1)
in southern West Bengal. The area is just north of the
town of Barasat, Barasat-1 Block, 24 Parganas (North)
District, some 20 km NNE of Calcutta. The climate of
the area is tropical monsoonal with a dry season lasting
from December to May. The Hugli River flows from
north to south about 13 km to the west of the area, and
the Sunti River (dry in most months) runs north-south 2
km to the east, joining the east-west draining Sunti
Channel (also dry most months) at Mirathi, about 4 km
to the north of the area. The area looks flat but has a
regional slope to the north and east; i.e. towards the
rivers. Superimposed on this is a microtopography that
J.M. McArthur et al. / Applied Geochemistry 19 (2004) 1255–1293
1257
Fig. 1. Location of wells and piezometers in the study area on the northern outskirts of Barasat town, West Bengal. Waved areas are
ponds and rectangles represent some of the larger houses. Piezometer sites are double circles at AP, FP, DP. Arrowpoint in (b) shows
field area. Roads in C are double lines, prominent tracks are single lines. Unfilled circles are deep wells (>70 m depth), filled circles are
shallow wells (<45 m depth). N-34 is national highway 34.
influences local hydrology: relative relief of 2–3 m exists
where major roads are raised above ground level, and
where ponds, perched above the local water table by
several metres, have dry-season water levels up to 3 m
below ground level. These numerous ponds range in size
from <10 m diameter to around 100 m diameter, and are
excavated to a depth that preserves natural sealing by
the lower part of an aquitard that overlies the shallow
aquifer.
3. Methods
Piezometers consist of multiple, independent, wells
drilled to different depths and spaced no more than 2 m
apart. They were drilled using the reverse-circulation,
hand-percussion, method (Ali, 2003). Using this method, mud and clay samples are recovered with undisturbed cores. In sands, the samples are disturbed.
Separately at site AP (Fig. 1), percussive piston-sampling into plastic core-tubes was alternated with rotary
wash-boring. The coring gave good recovery from precisely known depths in all strata. Cores were stored at
room temperature until sampled for analysis, as the local
groundwater temperature is 25 °C. A LECO C/S 125
Analyser was used to analyse the <0.5 mm fraction of
sediments for total S (TS), total C (TC), and total organic C (TOC), the last after leaching samples with 10%
v/v HCl. The difference between TC and TOC is reported as a percentage of calcite. To obtain values of
d13 C for calcite, samples were analysed with a PrismÓ
mass spectrometer using a common acid bath and NBS
19, a calcium carbonate reference material supplied by
the National Bureau of Standards (now National Institute of Standards Technology).
Ground waters were sampled, after purging, in February, 2002, November, 2002, and March 2003, from
hand-pumped domestic and public wells. The depth of
each well was recorded at the time of sampling by asking
the owner how many pipes were used to install it:
commonly, individuals buy 20-foot (6.1 m) lengths of
pipe and employ a driller to install them. The depths of
some wells are uncertain because of a change of owner,
or some other circumstance. Water samples drawn from
wells were not filtered: an accurate measurement was
sought of the amount of As in water consumed by wellusers, who do not filter their water. All samples were
crystal clear, i.e. without visible turbidity. Piezometers
fitted with 1.8 m screens were installed in March 2003 in
eastern JAM where As poisoning was absent (DP); in
south-eastern JAM where it was believed to be minor
(AP); and in northwestern JAM where it was severe (FP,
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J.M. McArthur et al. / Applied Geochemistry 19 (2004) 1255–1293
Fig. 1). The level above ground of each well in each
piezometer nest, relative to an arbitrary level, was established by theodolite survey. Water levels were measured in order to establish hydraulic heads and the
direction of ground-water flow. After extensive purging,
piezometers were sampled via hand-pumps between 5
and 10 days after installation. Piezometer samples were
filtered through 0.45 lm membrane filters. All samples
for cation analysis were acidified to pH 2 with HNO3 .
Samples taken for anion analysis and for stable isotope
analysis were not acidified.
Ammonium and NO
3 were determined within 2
weeks of collection, and mostly much sooner. Measurements of conductivity, and temperature were made
at the well head. Measurements of dissolved O2 are not
given as they are devalued by pump aeration. For
piezometers and some wells, alkalinity was measured in
the field by pH-titration with H2 SO4 and is reported as
HCO3 : for wells where this was not done, HCO3 has
been estimated by difference of charge balance. No wellsample contained H2 S that was detectable by smell after
acidification of the sample with H2 SO4 and agitation.
Analysis of waters for cations, total-S (reported as SO4 )
and total-P (reported as PO4 ) was by ICP-AES and
analysis for anions and NH4 was by ion chromatography (including duplicate analysis for SO4 ). As a check
on ICP determinations of P, PO4 was also determined
spectophotometrically using the Mo-blue method.
Concentrations of As were measured on acidified samples using graphite-furnace AAS. In order to assess
particulate contributions to the dissolved load of well
water, the concentration of Al in waters was measured
and the turbidity of samples was recorded at collection.
All samples were crystal clear, i.e. without visible turbidity, and concentrations of Al were <0.1 mg L1 , so it
was assumed that the analysis is unaffected by particulate contributions. Measurement of d13 C of dissolved
inorganic C (DIC) was done with a Micromass MultiflowÓ preparation system on-line to an IsoPrimeÓ mass
spectrometer. After He flushing of samples, and addition
of H3 PO4 , the isotopic composition of CO2 was measured after equilibration for 9 h at 30 °C. Drift corrections were made using a reference-gas pulse that
bracketed each sample. Reference materials used were
an in-house carbonate and NBS-19. Every third sample
was accompanied by a standard, which was used to
make corrections for drift, which was assumed to be
linear; corrections were 6 0.4‰. Replicate analysis
showed that repeatability was 6 ±0.1‰ for solid reference materials and 6 ±0.2‰ for solutions made from
them. The average repeatability of the analysis was
0.06‰ for d13 C (n ¼ 134).
Microbiological analysis for thermotolerant coliform
bacteria was conducted in the field. The water was analysed, after purging, from 40 wells and 10 surface ponds in
JAM, and 14 wells in the As-free area of Mirathi, some
4 km to the north of JAM. The isolation and enumeration
of the TTC was carried out using membrane filtration
and growth on membranes with lauryl SO4 broth (Oxoid,
UK; Anon, 1994). The plates were incubated for 6 4 h at
ambient temperature to aid bacterial resuscitation, before
incubation at 44 0.5 °C for between 16 and 18 h. Yellow
colonies were classified and counted as thermotolerant
coliforms. Sanitary risks were quantified for each well in
order to identify faecal sources, contaminant pathways
and other factors that might contribute to contamination
of wells (e.g. Lloyd and Bartram, 1991). Risk factors
comprise observable faults in well construction and the
proximity of sources of contamination, e.g. faecal material, and the distance between wells and latrines.
4. Results
4.1. Sedimentology
A full description of the sediments and their composition will be presented elsewhere (Hudson-Edwards
et al., 2004), so only an outline is given here (Table 1).
Lithological logs of cored sediment were made on site
and are reproduced in Fig. 2 without change. An upper
aquitard of silts and silty clays (Fig. 2), of variable
thickness (9–24 m), overlies an unconsolidated sand
aquifer between 32 and 36 m in thickness (the shallow
aquifer): the aquifer is underlain by a clay aquitard
about 25 m thick, beneath which lies another sand
aquifer (the deep aquifer) that is >60 m thick.
The sediments were deposited between 22 and 1.2 ka
ago (14 C and OSL ages from Hudson-Edwards et al.,
2004) by the Ganges/Brahamaputra Rivers and their
distributary channels. Dating of quartz in cores from the
AP site by the OSL method showed that the aquifer
sands are around 17 ka old below 32 m depth and <8 ka
old above it (Hudson-Edwards et al., 2004), which explains why the lower part of the aquifer is twice as hard
to core as the shallower levels, when judged by the
number of hammer blows required to penetrate 0.5 m of
sediment (around 90 below 32 m, around 50 above 32 m;
McArthur et al., unpublished data).
Correlations between the piezometer sites are shown
on Fig. 2 and are based on the abundance of calcite and
organic matter (TOC) in the sediments. The aquifer at
site DP, and the lower 13 m of the aquifer at sites AP
and FP, contains little or no calcite and almost no organic matter (<0.1%) and are therefore regarded as
being of equivalent age (around 17 ka years). In the
upper aquitard, the calcite-free level around 6–8 m depth
in both AP and DP is assumed to correlate with the
calcite-free level between 5 and 5.5 m in DP. Finally, in
AP and FP, the upper few decimetres of the lower
aquitard are brown, oxidised, and correlate (drilling at
DP did not reach the lower aquitard).
Table 1
Composition of sediment from piezometer sites in Joypur, Ardivok and Moyna (JAM), West Bengal, India
Site DP
Site AP
Site FP
TS %
TOC %
m bgl
IC % as Cal
TS %
TOC %
m bgl
1.5
3.0
4.6
5.2
6.1
7.6
9.1
10.7
12.2
13.7
14.3
15.2
15.2
15.2
16.8
16.8
18.3
19.8
19.8
21.3
21.3
22.9
24.4
25.9
27.4
29.0
30.5
32.0
33.5
35.1
36.6
38.1
39.6
41.1
42.7
44.2
45.7
47.2
48.8
50.3
51.8
57.9
59.4
3.17
5.67
9.50
0.08
< 0.05
26.9
2.00
1.17
1.58
2.25
1.17
1.25
0.25
< 0.05
0.67
5.00
0.67
6.50
7.75
0.08
0.33
0.08
< 0.05
0.17
0.08
0.08
0.17
< 0.05
< 0.05
0.08
0.08
< 0.05
0.08
< 0.05
< 0.05
< 0.05
< 0.05
0.50
0.83
< 0.05
0.08
2.67
0.08
0.425
0.027
0.014
0.052
0.023
0.001
0.023
0.044
0.051
0.430
0.122
0.221
1.27
2.24
0.036
0.332
0.010
0.006
0.000
0.000
0.001
0.007
0.007
0.007
0.012
0.007
0.006
0.005
0.006
0.007
0.005
0.008
0.007
0.008
0.008
0.006
0.005
0.009
0.007
0.006
0.007
0.011
0.006
1.46
0.66
0.36
4.87
2.32
0.27
0.38
2.25
0.56
1.51
1.52
4.52
7.61
13.3
1.90
33.7
0.52
0.29
0.20
0.16
0.13
0.28
0.10
0.07
0.10
0.09
0.08
0.09
0.09
0.11
0.07
0.10
0.07
0.08
0.08
0.07
0.05
0.11
0.06
0.11
0.08
0.11
0.10
2.10
2.50
2.70
3.00
3.05
3.05
3.20
3.50
3.64
3.80
4.00
4.50
4.57
4.57
4.80
5.03
5.20
5.50
5.57
5.70
5.86
5.92
6.00
6.00
6.03
6.10
6.10
6.25
6.50
6.50
6.75
7.00
7.62
7.62
7.75
8.00
8.50
8.77
9.14
9.14
9.14
9.95
11.35
12.40
13.25
3.83
3.83
3.08
3.00
0.33
3.17
1.67
5.00
3.58
5.83
5.67
6.83
0.08
7.33
5.50
0.08
5.67
4.25
6.58
6.08
0.25
3.33
0.25
0.42
0.25
0.33
0.42
0.75
0.25
1.50
0.67
0.58
0.42
0.42
1.25
10.3
1.58
4.67
0.17
0.75
5.17
3.75
4.42
4.00
3.83
0.009
0.006
0.006
0.005
0.014
0.028
0.003
0.007
0.005
0.007
0.008
0.009
0.041
0.020
0.008
0.016
0.012
0.021
0.007
0.011
0.010
0.009
0.029
0.119
0.015
0.018
0.039
0.014
0.010
0.015
0.020
0.040
0.016
0.033
0.012
0.010
0.028
0.054
0.023
0.065
0.154
0.013
0.014
0.011
0.009
0.44
0.24
0.39
0.27
0.33
0.44
0.36
0.31
0.35
0.28
0.35
0.35
0.78
0.15
0.38
1.12
0.32
0.39
0.39
0.39
0.78
0.67
0.61
0.72
0.60
0.70
1.58
0.37
0.35
0.25
0.53
0.44
0.25
0.33
0.45
0.21
0.35
0.24
0.33
0.78
0.65
0.28
0.22
0.40
0.25
0.5
1.4
1.5
1.5
2.3
3.0
3.0
3.2
4.6
5.0
5.9
6.1
6.1
7.4
7.6
7.6
7.8
9.1
9.1
9.1
9.5
10.7
11.3
12.2
12.2
13.3
14.2
14.3
17.0
18.8
20.8
22.6
24.4
26.1
27.9
28.8
29.5
30.5
31.6
33.6
35.3
37.0
39.0
40.8
42.5
IC % as Cal
4.98
5.49
2.25
4.33
5.46
1.75
4.92
6.52
6.33
7.00
5.34
1.08
1.75
0.58
0.08
0.00
5.52
5.67
3.83
6.17
4.99
5.33
5.57
3.67
3.83
4.75
5.73
5.31
4.91
4.81
3.18
3.78
3.85
3.57
3.45
2.02
2.71
1.87
0.87
0.65
0.73
0.79
0.79
0.60
1.08
TS %
TOC %
d13 C OM
d13 C IC
0.04
0.00
0.00
0.00
0.02
0.00
0.01
0.04
0.02
0.06
0.01
0.00
0.01
0.00
0.04
0.08
0.01
0.06
0.01
0.03
0.00
0.05
0.00
0.01
0.01
0.00
0.01
0.01
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.01
0.62
0.04
0.00
0.00
0.00
0.01
0.02
0.01
0.01
0.53
0.24
0.49
0.40
0.45
0.42
0.40
0.53
0.56
0.44
0.66
0.86
0.79
0.89
2.92
2.63
0.20
0.62
0.58
0.42
0.72
0.58
0.35
0.60
0.78
0.17
0.17
0.17
0.18
0.33
0.14
0.13
0.14
0.17
0.23
0.10
7.20
0.17
0.18
0.12
0.10
0.09
0.08
0.09
0.08
)22.3
)21.2
)1.91
)1.61
)20.9
)0.69
)0.88
)22.0
)2.03
)21.4
)1.62
)20.2
)1.56
)23.6
)28.5
)25.0
)1.37
)9.77
)2.27
)1.80
)3.93
)2.55
)3.70
)2.99
)5.23
)4.57
)24.7
)24.3
1259
IC % as Cal
J.M. McArthur et al. / Applied Geochemistry 19 (2004) 1255–1293
m bgl
Site DP
m bgl
Site AP
IC % as Cal
TS %
TOC %
1260
Table 1 (continued)
Site FP
IC % as Cal
TS %
TOC %
m bgl
IC % as Cal
TS %
TOC %
d13 C OM
d13 C IC
13.50
13.50
14.25
14.50
15.50
15.51
16.20
16.50
16.50
17.30
17.50
18.50
18.50
19.51
20.30
20.50
21.15
21.45
21.49
22.25
22.35
22.45
22.50
23.30
23.50
23.50
24.51
25.50
26.00
26.50
27.00
27.49
28.00
28.50
28.96
29.50
29.50
30.00
30.50
31.00
32.50
39.50
43.50
44.50
45.11
45.42
4.25
4.42
2.92
4.67
3.42
3.42
6.25
6.83
6.08
6.25
6.08
5.00
5.17
5.17
4.17
4.83
5.25
4.75
5.50
5.42
5.75
3.25
5.33
5.58
4.67
5.25
4.92
4.92
4.25
4.00
3.08
2.42
2.25
1.92
1.25
0.50
1.92
0.75
0.50
0.42
0.08
0.75
2.17
10.3
2.67
7.58
0.005
0.016
0.010
0.009
0.011
0.007
0.015
0.018
0.018
0.012
0.014
0.008
0.006
0.005
0.009
0.006
0.013
0.009
0.009
0.018
0.014
0.020
0.009
0.008
0.015
0.006
0.007
0.009
0.006
0.002
0.005
0.005
0.004
0.004
0.002
0.006
0.002
0.004
0.003
0.002
0.005
0.005
0.005
0.005
0.002
1.26
0.16
0.15
0.35
0.16
0.22
0.17
0.49
0.30
0.48
0.20
0.21
0.24
0.13
0.13
0.32
0.16
0.42
0.52
0.28
0.52
0.31
0.76
0.41
0.17
0.49
0.30
0.18
0.34
0.17
0.18
0.14
0.14
0.14
0.12
0.10
0.24
0.08
0.11
0.09
0.10
0.09
0.11
0.11
0.21
0.42
1.43
44.4
46.4
47.1
48.0
48.3
48.4
48.5
48.6
48.6
48.7
49.0
50.0
51.9
53.7
55.5
57.2
59.0
60.8
62.6
64.3
66.1
67.8
69.6
2.09
2.31
2.16
9.84
6.58
8.58
6.50
9.08
9.50
10.0
8.87
9.79
6.24
8.49
9.02
8.16
8.84
9.53
10.7
10.5
10.4
10.0
9.70
0.01
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.01
0.03
0.32
0.28
0.23
0.02
1.21
1.33
1.09
0.50
0.31
0.22
0.25
0.45
0.28
0.06
0.18
0.16
0.09
0.24
0.30
0.28
0.34
0.41
0.68
0.83
0.72
0.49
0.42
1.57
1.48
1.30
0.69
0.68
0.53
0.50
0.66
0.83
)23.5
)3.70
)2.63
)3.08
)4.13
)24.9
)20.7
)0.61
)22.9
)2.47
)24.9
)0.83
)0.40
J.M. McArthur et al. / Applied Geochemistry 19 (2004) 1255–1293
m bgl
J.M. McArthur et al. / Applied Geochemistry 19 (2004) 1255–1293
At site DP, in the east of the JAM area (Fig. 1), the
upper confining layer is 24 m thick, with the lower half
comprising impermeable clay (17–24 m). A calcite-free
interval occurs around 5 m depth. Above 17 m depth,
the confining layer contains abundant organic matter,
especially at depths of around 5, 15 and 17 m: the
deepest occurrence comprises peat sensu stricto with a
concentration of total organic C (TOC) of 34% (Table
1); the peat contained well-preserved tree-leaves. Sulphur concentrations in the OM-rich units reach 2.2% at
15.2 m depth, but is only 0.33% at 17 m depth. Calcrete
occurs at 19.8 m. Around 21 m depth, the clay is brown
and oxidised and shows in Fig. 2 as a minimum concentration of total sulfur: the oxidised unit passes gradationally downward into the underlying unoxidised
grey clay. There is a sharp transition at 24 m into the
underlying aquifer sands, which are brown in colour
through the full thickness of the aquifer. The mean
concentration of TOC in the aquifer sands is 0.09%, with
no sample containing more than 0.11%, and the concentration of total S in the aquifer sand does not exceed
0.012%. At the greatest depth reached (59.4 m), cuttings
comprised brown sand mixed with grey silty clay, suggesting that the lower confining clay was nearby. However, the poor sample quality at this depth made it
impossible to discover whether the underlying aquitard
had an oxidised upper surface as is the case at the other
piezometer sites.
At AP, in southern JAM, the upper confining layer
consists of silts and clayey silts. An interval free of calcite occurs between 6 and 8 m depth. Around 9 m depth,
the silts grade downwards into fine sand in the upper
part of the aquifer (Fig. 2). The aquifer sands are grey in
colour above depths of 40 m, and brown in colour below
it. Organic-rich units exist in the upper confining silts
(1.6% TOC at 6 m depth; Table 1), and within the
aquifer sands at 17 m (0.49% TOC) and 23 m (0.76%
TOC). In aquifer sands, the mean concentration of TOC
is 0.27%. Concentrations of TOC appear to decrease
with depth below a depth of about 23 m, but an uncored
interval from 32.5 to 38.5 m depth prevents certainty in
this matter. Concentrations of total S reach 0.12% in the
upper confining silts but do not exceed 0.02% in the
aquifer sands. At a depth of 45 m, the aquifer sands give
way abruptly to the underlying clay of the basal aquitard. The upper few decimetres of this aquitard are
brown, oxidised, and contain <0.1% TOC and <0.001%
of total S. The oxidised horizon grades downwards into
grey clay in which the concentrations of these constituents are higher. A similar oxidised upper aquitard occurs
at FP (see below): at both sites, the total S content of
these intervals (Fig. 2) is low over several metres of the
clay, suggesting that the oxidation is more pervasive that
it appears.
At FP, in central JAM, the sediments consist of silts
and clayey silts in the upper confining layer (0–12 m)
1261
that grade into mostly medium sand in the main part of
the aquifer, which is 32 m thick. The upper aquitard is
free of calcite between depths of 6 and 8 m. The aquifer
sands are grey in colour, except at the base of the aquifer
(>40 m depth) where hints of brown occur in a general
grey colouration. Organic-rich units exist towards the
lower part of the upper confining silts (2.9% TOC at 8 m
depth; Table 1), and within the aquifer sands at 29.5 m
(7.2% TOC). A single sample at 22.9 m depth contains
0.71% TOC and may represent a lateral equivalent of the
organic-rich horizon at 23 m depth in AP. Elsewhere in
the sands, concentrations of TOC seldom exceed 0.3%
and are lower than this in the basal part of the aquifer.
Concentrations of total S reach 1.3% in the lower confining clay and 0.6% in the peaty unit at 29.5 m, but do
not exceed 0.04% in the aquifer sands, nor do they exceed 0.06% in the upper aquitard. The underlying
aquitard of grey clay has been proven by drilling to be
25 m thick at this site. The upper part of this clay is
brown and oxidised, as it is at AP, and contains <0.1%
TOC and <0.001% of total S; it grades downwards into
grey clay in which concentrations of these constituents
are higher.
4.2. Piezometer waters
The composition of water from the 3 piezometers
(Figs. 3–5), which tap only the shallow aquifer, is presented in the order of increasing concentration of As in
the ground waters drawn from the aquifer (DP none; AP
some; FP much), which is in the order of increasing
concentration of organic matter in the aquifer sands (but
not in the overlying aquitard; Fig. 2). The piezometer
profiles reveal that the ground water is chemically
stratified (Figs. 3–5), as expected (Parker et al., 1982;
Karim et al., 1997; DPHE, 1999; McArthur et al., 2001).
In the piezometer at DP, concentrations of aqueous
As do not exceed 5 lg L1 (Table 2; Fig. 3). Concentrations of Mn are high and peak in the middle of the
aquifer at 4.2 mg L1 , whilst concentrations of Fe show
an inverse trend to those of Mn, being lowest in midaquifer (0.44 mg L1 at 43.7 m) and highest (2.4 mg L1 )
in the lowest piezometer. Concentrations of PO4 are 0.35
mg L1 at 25.8 m and decrease downwards. Concentrations of SO4 are higher than at the other sites and increase downward to be around 10 mg L1 in the bottom
half of the aquifer. Values of d13 C(DIC) are around
)2‰ in the upper part of the aquifer and )6‰ in the
lower part and show no obvious relation to other
parameters.
In ground water at AP from depths <40 m (Fig. 4),
concentrations of Fe (3.9–6.2 mg L1 ), PO4 (0.8–3.2
mg L1 ), and As (240–420 lg L1 ) are high. The maximum concentrations occur at depths of around 38 m
(As), 26 m (Fe), and 21 m (P). The peak concentration
for Fe occurs just below the peak concentration of
1262
J.M. McArthur et al. / Applied Geochemistry 19 (2004) 1255–1293
Fig. 2. Lithological logs and profiles of total organic C (TOC, bold line), total S (TS, light line) and CaCO3 in sediment at the piezometer sites DP, AP, FP, in the JAM area. Lithological logs are those made in the field and are reproduced without alteration. Lines
of correlation are shown dotted. Note the logarithmic scale for TS and TOC.
organic matter (0.72% TOC) in the aquifer sands at 23 m
depth. At 26 m, the value of d13 C(DIC) also decreases by
around 2‰, but overall values of between )2‰ and
)10‰ are less negative than might be expected for a
system so reducing. Towards the base of the aquifer
(from the piezometer at 38.3 m depth downwards),
concentrations of dissolved Fe, As, and PO4 decrease.
Concentrations of SO4 decrease downwards from the
uppermost piezometer, which is in the upper confining
layers, and are <10 mg L1 below 32 m depth.
In the piezometer waters at FP, concentrations of Fe
are high (4–7 mg L1 , Fig. 5) over the entire aquifer
thickness, but are not related to those of dissolved As
nor to depth. Concentrations of As are around 430
lg L1 in the upper part of the aquifer, but are much
higher in most of the lower part (>32 m depth), where
they are >900 lg L1 in a considerable thickness of the
aquifer. Near the base of the aquifer (lowermost piezometer at 43.4m depth), the As concentration (239 lg
L1 ) is lower, but is still higher than it is in the base of
the aquifer at AP (7 lg L1 ). Values of d13 C(DIC) range
from +0.6‰ to )10.8‰ but show no strong relation to
lithology, including the presence of units rich in organic
matter. Concentrations of SO4 decrease downwards
from the uppermost piezometer (in the upper confining
unit) and are <1 mg L1 below 25 m depth. Concentrations of PO4 peak at 2.5 mg L1 around 20 m depth
and decline downwards.
4.3. Ground water flow
In the aquifers of the Bengal Basin, the natural flow
regime combines 3 scales of flow-system, as described
by Ravenscroft (2003). The first, at the kilometre scale
in the shallow aquifer, is driven by local topography.
On this are superimposed local systems, around microtopographic features such as levees, channels and
ponds, that extend laterally for a few tens to perhaps a
hundred metres, and vertically for 10–20 m. The third
scale comprises natural flow to the south in the deeper
aquifer.
Water levels in the piezometers at the 3 sites in the
shallow aquifer, recorded in March 2003, are shown in
Fig. 6: each has a perched water table in the upper
aquitard. The head differences between the aquifer and
overlying aquitard are 0.32 m at site AP, 0.82 m at site
FP, and 3.70 m at site DP, and reflect the degree of
hydraulic connectivity between aquitard and aquifer.
Within the aquifer sands, piezometric levels vary by less
than 0.2 m and decrease downward, with the gradient
being greatest (0.0013) at site AP and lowest (0.0007) at
site DP. Ground water elevations in the lower part of the
shallow aquifer indicate a hydraulic gradient of 0.0006
to the NNE at the time of measurement – in the dry
season in early March, 2003. During the monsoon season, the direction reverses as local rivers to the north
and east fill with water (McArthur et al., unpublished
J.M. McArthur et al. / Applied Geochemistry 19 (2004) 1255–1293
1263
Fig. 3. Lithologic log and piezometer profiles of groundwater composition in March, 2003, at site DP, Barasat, West Bengal. Horizontal dashed lines marks the boundary between the aquifer sands and the overlying confining unit: the base of the aquifer was not
definitely reached, so no lower boundary is drawn. Note the very much thicker upper confining unit, and the very much lower concentrations of As, compared to other sites (Figs. 4,5). Alkalinity is reported as HCO3 .
piezometric data). Time-series measurements of hydraulic heads, and the hydrogeology of the area, will be
presented elsewhere (Hudson-Edwards et al., 2004).
4.4. Well waters
In Table 3 are given the results of the chemical and
microbiological analysis of the ground waters sampled
from domestic and public wells. The compositions are
typical of those found elsewhere in the Holocene sediments of the Bengal Basin (DPHE, 1999; McArthur
et al., 2001; Harvey et al., 2002; Dowling et al., 2003).
Shallow wells in the central and NW of JAM (Fig. 7) are
rich in As (200–1180 lg L1 ), Fe (3–13.7 mg L1 ), and
PO4 (1–6.5 mg L1 ), and lack Mn and SO4 . A group of
shallow wells (14–18, 40–44) in the immediate vicinity of
piezometer DP contain <10 lg L1 of As and more than
2 mg L1 of Mn. With few exceptions, high concentrations of As are not spatially coincident with high concentrations of Mn (Fig. 7) nor with high concentrations
of SO4 (Fig. 7), and these parameters are not correlated
(Figs. 8(a) and (b)).
In cross-plots (Fig. 8) of species affected by redox
reactions (Fe, SO4 ) or Fe reduction (As, PO4 , Fe), the
data for shallow wells (excluding wells near the piezometer site DP, owing to their distinct sedimentological setting; Fig. 2) appear to fall into 3 groupings,
on the basis of their As/Fe values, and their concentration of As: those with <17 lg L1 of As; those
with >32 lg L1 of As and high As/Fe; and those
with >32 lg L1 of As and low As/Fe. These 3 groups
of wells appear to bear a relation to depth, a relation
explained in a later section (Discussion). Wells with
high As/Fe also have high As/PO4 (Fig. 8(d)) and
distinctly less Fe and PO4 (Fig. 8(e)) than do wells
with low As/PO4 (Figs. 8(c)–(e)).
The ground waters are anoxic because they contain
dissolved Fe. Concentrations of nitrate are generally < 1
mg L1 and the range in pH is small (6.76 to 7.32; Tables
2,3). Strong correlations exist between dissolved concentrations of Ca, Mg, and HCO3 (Figs. 9(a) and (b)).
1264
J.M. McArthur et al. / Applied Geochemistry 19 (2004) 1255–1293
Fig. 4. Lithologic log and piezometer profiles of groundwater composition in March, 2003, at Site AP, Barasat, West Bengal. Horizontal dashed lines mark the boundaries of the aquifer sands with confining units. The uppermost piezometer is screened within the
silts of the upper confining unit. Alkalinity is reported as HCO3 .
Shallow wells have Na/Cl ratios that scatter widely
about the value for marine-salt (Fig. 9(c)) showing that
some wells have gained Na and some lost it, relative to
Cl. Many wells plot along a trend of increasing Ca and
Na (Fig. 9(d)), as sodic feldspar and detrital calcite are
weathered, but some plot to the right of this line, in a
direction compatible with ion-exchange of Ca in solution for Na on mineral ion-exchange sites (trend of line
B, Fig. 9(d)).
The deep ground waters are fresher (mean EC 753 lS
cm1 ) than are shallow ground waters (mean EC 957
lS cm1 ): they contain less Ca, Mg, Cl, and HCO3 , but
more Na than most shallow wells (Fig. 9). Deep wells
(>70 m) have higher Na/Cl and Na/Ca values than do
shallow wells (Figs. 9(c) and (d)). Water from deep wells
contains <17 lg L1 of As.
Saturation indices, calculated for waters from shallow wells with Geochemist’s Workbench (v. 3.0; Bethke,
1998), show that most of the Joypur waters are oversaturated with calcite (SI +0.5 to +2.0), dolomite (SI
+0.1 to +2.0) and aragonite (SI +0.1 to +0.5). Water
samples with low concentrations of Fe are at equilibrium
or undersaturated with siderite (SI +0.2 to )0.9) and
vivianite (SI +0.3 to )3), and at equilibrium or oversaturated with rhodochrosite (SI +0.02 to +0.7). By
contrast, samples high in Fe are saturated with vivianite
(SI +0.8 to +3.7) and siderite (SI +0.3 to +1.5) and
undersaturated with rhodochrosite (SI )0.3 to )1.3).
Because Ba solubility is limited by the solubility of
barite, concentrations of Ba in a few well waters (Tables
2, 3) approach the guideline value of 0.7 mg L1 (World
Health Organization, 1993) where concentrations of SO4
are low.
4.5. Ground water abstraction
Inhabitants draw water for domestic supply mostly
from private wells in the shallow aquifer, typically with
12-foot (3.6 m) screens that have their screen-centres at
depths around 25–45 m, but 6 wells in the study area tap
the deep aquifer (>70 m depth). The deep aquifer is also
exploited by 3 irrigation wells near the JAM area: one is
600 m to its east, a second is 1000 m to the north, and
the third is 200 m to its SW. The 3 deep wells irrigate 34
ha of agricultural land that borders the villages: the
shallow aquifer is not used much for irrigation. During
J.M. McArthur et al. / Applied Geochemistry 19 (2004) 1255–1293
1265
Fig. 5. Lithologic log and piezometer profiles of groundwater composition in March, 2003, at site FP, Barasat, West Bengal. Horizontal dashed lines mark the boundaries of the aquifer sands with confining units. The uppermost piezometer is screened in the silts of
the upper confining unit. Alkalinity is reported as HCO3 .
the development of both aquifers, abstractions must
have modified the natural ground water flow regime.
Nevertheless, natural flow at the kilometre scale appears
still to dominate the shallow aquifer, as irrigation water
is drawn from the deep aquifer only and shallow abstraction is but a small fraction of recharge. Deep abstraction for irrigation will induce regional down-flow
through the clay aquitard between 45 and 70 m, but its
thickness ensures that the effect is small. Human modifications have applied, at most, for 40 a, whereas the
natural flow regime has existed in its present form for up
to 1.2 ka, since this is the age of OM in the upper
aquitards (Hudson-Edwards et al., 2004).
The deep irrigation wells to the north and east of the
JAM area are screened from 100 to 130 m and pump for
8 h a day in February and for 12 h a day in March and
April to irrigate about 27 ha of rice and 7 ha of vegetables. The third, sited 200 m to the SW of the area, is 95
m deep and has a comparatively small abstraction. No
abstraction volumes are available for these wells but,
assuming typical crop water requirements (Ravenscroft,
2003) and that deep percolation comprises 33% of abstraction, the authors estimate that the annual abstraction from these wells is of the order of 400,000 m3 , with
around 130,000 m3 being returned to the shallow aquifer
as deep percolation losses. Because of the clay aquitard
at 45–70 m, the cone of depression created by these wells
must have spread widely in the deep aquifer as downward leakage balances withdrawal of water from confined storage. In JAM, there are about 100 hand
tubewells taking water from the shallow aquifer. By
assuming that each well is used by 10 people with an
average water use of 20 L/d, the annual abstraction from
the shallow aquifer is estimated to be of the order of
7300 m3 , which is 2% of the abstraction from the irrigation wells.
5. Discussion
5.1. Piezometer sediments
The data and the correlations between the piezometer
sites (Fig. 2) suggest that the sediments below a depth of
32 m in FP and AP are equivalent in age, and of the
same age as the aquifer at DP; they also suggest that the
sediment above 32 m in FP and AP is distinctly younger
(<8 ka) than underlying sediment, which is at least 17 ka
1266
Table 2
Composition and hydraulic heads of ground waters from piezometers in Joypur, Ardivok and Moyna (JAM), West Bengal, India
Depth
mbgl
Water
level
Temp.
°C
EC lS
cm1 @
25 °C
pH
As
lg l1
Units are mg l1 unless specified otherwise
Na
K
Mg
Sr
Ba
Fe
Mn
NH4
HCO3
37.5 10.3
29.7 10.3
21.1 2.2
18.0 2.8
20.0 3.6
19.1 3.1
25.2 3.4
26.3 3.1
39.6 3.1
164
135
107
111
138
137
130
131
138
35.1
34.2
25.4
30.7
37.4
40.1
45.3
46.6
47.7
0.49
0.50
0.43
0.50
0.63
0.64
0.74
0.84
0.84
0.37
0.33
0.23
0.26
0.22
0.13
0.11
0.17
0.28
4.0
4.7
7.4
4.8
6.0
7.5
5.2
4.7
4.8
1.0
0.8
0.1
0.2
0.2
0.2
0.2
0.2
0.1
3.50
2.30
2.30
2.60
5.00
1.25
1.30
579
562
487
526
601
601
584
598
611
89.7
58.0
20.9
26.5
52.4
78.4
72.4
63.3
94.2
Cl
SO4
Bal
%
NO3
PO4
H4 SiO4
65.5
33.2
0.7
1.5
0.4
0.6
0.4
0.4
0.1
0.23
0.53
0.33
0.40
0.18
0.49
0.56
0.17
0.47
0.54
0.72
2.50
1.57
1.84
1.21
1.05
1.08
0.88
52
47
70
66
61
59
51
57
68
)8.4
)10.8
0.6
)0.6
)3.5
)4.3
)5.0
)6.2
)4.7
)0.7
)1.0
1.7
)0.9
)0.2
)2.5
0.9
1.0
0.8
Site FP: location 22 deg, 44.500
6.9
94.45
26.7
14.5
93.62
19.4
93.58
26.7
22.4
93.57
26.7
27.9
26.7
31.2
93.58
26.8
37.0
93.56
26.9
39.8
93.57
26.8
43.4
93.57
26.7
min N, 88 deg, 29.430 min E
1210
6.54
35
1018
7.21
94
764
6.84
443
795
6.96
411
975
6.96
447
1022
6.96
448
1028
7.10
960
1024
7.08
958
1139
7.04
239
Site DP: location 22 deg, 44.511
14.3
97.22
25.8
93.52
26.1
31.9
93.54
26.3
37.6
93.52
43.7
93.52
26.3
50.1
93.53
26.3
55.6
93.51
26.4
min N, 88 deg, 29.561 min E
76.8
50.2
53.6
55.6
47.6
48.2
0.9
1.0
0.6
1.3
1.6
2.4
146
154
139
129
118
151
27.5
38.6
37.3
33.8
38.8
42.9
0.44
0.55
0.53
0.51
0.48
0.66
0.19
0.07
0.08
0.09
0.09
0.14
1.8
1.5
0.6
0.4
0.5
2.4
1.6
4.2
2.7
2.6
2.1
0.5
0.08
0.02
0.13
0.12
0.45
0.17
744
724
670
640
606
728
51.2
62.6
58.8
40.2
47.6
80.4
0.4
1.5
5.4
11.5
6.3
9.7
0.58
0.31
1.32
0.41
0.19
4.56
0.35
0.17
0.09
0.02
0.04
0.05
58
70
77
75
65
71
)2.1
)1.9
)6.8
)6.4
)6.4
)5.5
)2.2
)1.3
)1.2
)0.2
)0.5
)4.0
Site AP: location 22 deg, 44.347
6.9
94.13
26.7
14.3
93.81
26.5
20.6
93.72
26.3
25.9
93.71
26.1
31.9
93.70
26.3
38.3
26.5
41.0
26.5
41.8
26.4
42.8
93.69
26.5
min N, 88 deg, 29.505 min E
3152
6.70
135
250
1406
6.78
327
87.6
1307
7.04
258
58.8
1075
7.04
226
42.1
925
6.96
279
28.4
757
7.07
419
23.3
859
6.91
187
33.4
919
6.88
110
34.4
939
7.07
7
44.7
3.2
1.4
2.1
2.3
1.5
1.6
1.4
1.5
2.4
364
168
186
143
122
109
112
116
115
87.2
39.0
51.4
39.6
37.3
33.3
35.3
36.3
38.0
0.97
0.49
0.83
0.67
0.62
0.55
0.56
0.57
0.55
0.31
0.40
0.39
0.37
0.28
0.15
0.09
0.09
0.14
2.0
5.2
5.3
6.2
4.4
3.9
2.0
1.5
1.0
4.1
0.8
0.3
0.2
0.2
0.2
0.3
0.5
0.6
1.30
1.30
1.95
1.55
1.10
1.30
1.50
1.30
701
690
670
562
551
526
579
593
622
558
100
74.1
66.8
65.2
8.1
13.4
30.0
29.3
490.3
98.5
80.1
53.0
11.9
2.8
2.4
8.4
3.9
0.12
0.13
0.18
0.36
0.30
0.12
0.35
0.16
0.22
0.24
0.82
3.16
3.12
3.00
2.25
1.15
0.49
0.13
54
62
64
67
69
66
65
68
64
)6.8
)7.3
)7.4
)9.8
)6.6
)2.5
)1.2
)1.4
5.4
1.8
)1.9
2.8
1.3
)1.1
)0.5
8.8 11.7
16.1 11.2
4.3 1.0
18
29
12
6.1
9.4
2.2
0.07
0.11
0.08
0.04
0.04
0.04
0.3
0.0
0.24
0.16
10.1
16.9
4.0
6.0
4.6
6.0
0.35
0.35
1.5
0.5
0.7
30
25
5
Pond A 1
Pond A 2
Rainwater
1142
1175
1048
1031
999
1131
6.88
6.84
7.08
7.00
6.96
6.95
4
1
1
<1
1
4
20
17
12
Notes: depth mbgl is depth in metres to mid-screen of each piezometer, Water levels are m above an arbitrary datum.
)3.6
J.M. McArthur et al. / Applied Geochemistry 19 (2004) 1255–1293
Ca
d13 C
DIC
‰
J.M. McArthur et al. / Applied Geochemistry 19 (2004) 1255–1293
1267
0
10
Depth m
20
97.22
30
40
AP
FP
50
DP
60
93.4
93.6
93.8
94.0
94.2
94.4
94.6
Water level (m above datum)
Fig. 6. Water levels in March, 2003, piezometers at sites DP, AP, and FP, in metres relative to an arbitrary datum. At each site, the
uppermost piezometer was completed close to the base of the upper confining unit. The higher head in the upper confining unit at Site
DP arises from the impermeable clay unit that locally caps the aquifer at this site. At FP and AP, the aquifer sands are capped by more
permeable silts.
old. The lack of calcite in sediment at DP, and below 32
m depth at AP and FP, might be due to its removal
during oxidative weathering for a period of around 9 ka
prior to the deposition of overlying sediment, or to the
deposition of the calcite-poor units by the Brahamaputra River: today, sediment of the Ganges River contains
detrital calcite but sediment of the Brahamaputra River
does not. This contrast is best seen by examining its
reflection in the Sr content of ground water in Bangladesh (P. Ravenscroft; Maps of Ground-Water Composition for Bangladesh, http://www/es.ucl.ac.uk/research/
lag/as): where detrital calcite occurs, concentrations of
Sr in ground water are high (>0.3 mg L1 ); where calcite
is absent, concentrations of Sr are low (<0.3 mg L1 ).
At DP, the aquifer sediments are brown over the
entire depth of the aquifer. At AP, the aquifer sands are
grey in colour (Fig. 10) above 40 m depth, and brown at
greater depth. At FP, the sediments are grey throughout
the full aquifer thickness (Fig. 10) but, at the very base
of the aquifer (>40m depth), the grey colour is accompanied by hints of brown at some levels (44.5 m). The
upper few decimetres of the underlying aquiclude at AP
and FP is also brown in colour and oxidised. As others
have done (van Geen et al., 2003a,b), the authors assume
that the brown colour arises from the presence of FeOOH and that the grey colour occurs when these oxides
have been destroyed (or given a sequestering surface
coating) by reduction. Attempts to confirm this assumption, and to quantify the cause of the colour difference, using sequential chemical leaches were
unsuccessful owing to uncertainty about the homogeneity, solubility, grain-size, and crystallinity, of likely
reduced phases and of FeOOH. Further work on this
issue will be reported elsewhere (Hudson-Edwards et al.,
2004). Nevertheless, the colour difference does suggest
an explanation for why concentrations of dissolved As
and Fe seldom correlate well in ground water from alluvial aquifers where FeOOH-reduction is the dominant
mechanism of As release: it is that most of the Fe(II)
that is reduced is sequestered in solid phases, such as;
siderite (Pal et al., 2002), Fe sulphide (Korte and Fernando, 1991; Rittle et al., 1995; Chakraborti et al., 2001)
vivianite, magnetite (Harvey et al., 2002), and green rust
(Frederickson et al., 1998). That not all reduced Fe stays
in solution is clear from the fact that the highest concentration of Fe in the waters (13.7 mg L1 ; Table 3)
requires the dissolution of only about 5 mg kg1 of solid
FeOOH. Although the amount of FeOOH in the sediment is low (see later), it is not that low: the reduction of
FeOOH must, therefore, result largely in the production
of solid phases containing Fe(II), with only a small
fraction of the Fe remaining in solution. The notable
difficulty, then, in explaining the composition of ground
water in alluvial basins is to account for those instances
(as found in JAM; see later) where dissolved Fe and As
do correlate: the expectation is that they should not.
The brown colour occurs distally from the sources of
organic matter, which are in the upper aquifer and
overlying aquitard. At AP, only the base of the aquifer is
brown (i.e. contains FeOOH), a level well below that
where most of the organic matter is found. At site DP,
the entire aquifer is brown, yet has abundant organic
matter in the unit above it. An examination of that unit
explains why: the upper confining layer is 24 m thick,
but the lower half comprises impermeable clay (15–24
m) which, despite its high content of organic matter
between depths of 15.2 and 16.8 m (Table 1, Fig. 2),
prevents hydraulic connectivity being attained between
1268
Table 3
Chemical composition of, and content of thermotolerant coliform bacteria in, ground waters from water wells in Joypur, Ardivok and Moyna (JAM), West Bengal, India. Some TTC determinations were
replicated, mostly on different days
Well
Lat.
Lat.
(deg.) (min.)
Long. Long.
San.
(deg.) (min)
Risk
TTC
a
2
b
Depth Temp
(cfu/100ml) (m)
(˚C)
EC
pH
(µS cm-1)
Units are mg L–1 unless specified otherwise
As
Na
K
Ca
Mg
Sr
Ba
Fe
Mn
NH4
µg L-1
88
88
88
88
88
88
88
88
88
88
88
88
88
88
88
88
88
88
88
88
88
88
88
88
88
88
88
88
88
88
88
88
88
88
88
88
88
88
88
88
88
88
88
88
88
88
88
88
88
29.4413
29.4145
29.4295
29.4515
29.4515
29.3215
29.2765
29.2965
29.3205
29.4979
29.4745
29.4766
29.5605
29.5505
29.5538
29.5661
29.5615
29.5751
29.2745
29.3115
29.3085
29.3105
29.3095
29.4515
29.4465
29.4347
29.4245
29.4331
29.3953
29.3725
29.3875
29.3705
29.3805
29.3675
29.3460
29.3425
29.3405
29.5155
29.5776
29.5455
29.5538
29.5677
29.5924
29.4996
29.4594
29.4437
29.2764
29.3124
29.3436
8
11
12
0, 0, 1, 10
0
0
11
7
10
11
3
8
7
1
0
0
0
1
7
11
11
8
4
7
0
5
0
61, 29
0
1
6
7
0
0, 0
7
4
9
3
10
0
0
2
0
6
12
12
8
11
7
11
8
10
9
3
6
9
5
7
1, 5
0
6, 7, 6, 20
0
1
1
63, 0
4
8
1
1
0
33.5
39.6
18.3
45.7
30.5
39.6
45.7
39.6
39.6
42.7
26.3
26.5
26.4
26.6
26.2
26.5
26.6
26.9
27.7
26.2
26.0
26.1
26.0
26.1
26.2
26.0
1087
949
1045
909
1160
685
911
937
672
1009
1197
1107
1090
1011
951
1265
26.2
26.8
26.9
27.0
26.8
27.0
26.6
26.5
26.6
26.6
998
1051
1024
1003
1133
1110
791
1038
957
971
33.5
39.6
42.7
29.3
33.5
51.8
29.0
21.3
21.3
41.1
49.4
45.7
32.3
26.4
26.4
26.2
26.1
26.4
26.2
26.3
26.0
26.3
26.2
26.0
26.0
848
1212
991
932
1217
743
939
688
802
1090
1001
1226
7.18
7.01
7.12
6.98
7.03
7.12
7.02
7.04
7.10
6.99
6.88
7.00
6.91
6.95
6.93
6.84
7.32
7.02
6.96
7.03
7.04
7.08
6.99
7.07
7.18
7.01
7.15
7.07
7.16
6.99
7.04
7.04
6.99
7.08
7.09
7.08
7.13
7.22
7.02
7.02
43.3
26.2
867
7.10
909
908
988
1181
1195
800
6.94
7.08
6.78
6.88
7.01
6.97
33.5
44.5
45.7
48.8
33.5
43.9
44.2
30.5
33.5
33.5
33.5
39.6
39.6
39.6
41.1
45.7
33.5
33.5
27.4
16.8
44.5
361
1180
1090
131
626
96
317
310
163
<1
391
459
32
<1
<1
<1
<1
<1
233
245
330
415
370
4
<1
42
98
56
<1
167
135
174
380
270
590
390
673
6
5
<1
<1
<1
4
<1
145
152
138
310
142
30.3
25.3
26.4
21.6
36.5
13.4
47.6
47.5
17.2
25.4
45.7
46.8
38.2
38.1
30.2
49.5
43.4
35.4
23.8
50.3
43.7
43.9
41.3
21.8
25.6
22.3
33.1
35.5
21.3
39.8
22.3
39.2
30.6
19.2
25.2
18.3
21.6
31.8
36.6
58.7
41.9
35.0
44.1
22.8
29.9
26.1
45.4
56.7
23.2
3.3
3.3
3.5
2.1
2.9
1.4
3.5
4.5
2.4
2.3
2.5
1.7
1.3
1.6
1.3
1.1
1.0
1.1
3.2
3.6
3.6
3.8
3.8
1.8
2.7
2.5
2.3
2.0
1.9
3.1
1.9
1.9
2.6
1.7
2.2
7.4
2.3
1.9
1.1
0.9
1.1
0.9
1.0
1.7
1.8
2.9
4.0
3.6
3.3
133
120
128
114
144
91
100
105
86
127
160
132
143
131
127
181
145
134
129
108
120
136
134
105
128
124
117
109
111
143
118
101
141
91
113
81
100
141
127
163
136
114
130
125
118
130
147
134
100
45.1
36.1
40.1
31.5
42.5
28.6
34.5
33.8
24.1
37.9
35.0
37.5
37.5
34.5
34.5
35.1
40.4
33.6
43.4
36.9
37.2
43.2
42.7
30.1
43.5
39.2
36.4
32.5
34.0
48.1
41.2
36.8
45.9
31.8
38.3
31.6
30.7
40.4
32.8
32.5
36.1
31.3
33.3
33.3
29.8
36.4
41.2
42.9
30.4
0.72
0.66
0.72
0.58
0.71
0.44
0.55
0.54
0.41
0.57
0.60
0.59
0.69
0.56
0.63
0.35
0.55
0.52
0.72
0.58
0.60
0.68
0.67
0.46
0.66
0.60
0.58
0.54
0.54
0.78
0.60
0.57
0.70
0.49
0.63
0.49
0.50
0.59
0.50
0.50
0.53
0.44
0.46
0.48
0.56
0.64
0.64
0.63
0.51
0.04
0.12
0.16
0.42
0.31
0.32
0.35
0.32
0.10
0.34
0.10
0.05
0.05
0.05
0.06
0.04
0.03
0.58
0.33
0.41
0.58
0.60
0.03
0.14
0.05
0.08
0.06
0.08
0.14
0.52
0.42
0.18
0.25
0.09
0.38
0.10
0.05
0.04
0.24
0.03
0.03
0.05
0.10
0.06
0.14
0.56
0.30
0.39
8.8
6.3
7.0
3.4
11.3
4.7
3.4
4.6
7.1
0.2
8.1
13.7
0.1
0.6
1.1
0.6
0.2
0.1
10.9
3.8
7.2
8.3
7.7
0.1
0.2
0.2
2.2
1.6
0.1
3.1
7.9
7.9
5.1
5.7
5.7
4.8
6.1
0.1
0.2
0.1
0.4
0.1
2.1
0.2
3.3
7.2
10.8
7.4
7.7
0.15
0.06
0.05
0.52
0.06
0.05
0.02
0.14
0.07
2.25
0.05
0.14
1.76
2.14
2.94
5.29
2.80
2.36
0.07
0.04
0.04
0.06
0.06
0.94
0.56
0.74
0.64
0.76
1.17
0.41
0.07
0.08
0.14
0.05
0.07
0.04
0.08
1.40
2.35
2.97
2.36
2.07
3.01
2.1
0.8
0.2
0.1
0.1
0.1
2.9
2.8
0.5
1.0
0.3
0.2
1.5
0.9
3.5
0.1
0.0
0.1
0.0
0.1
0.4
0.5
1.4
0.6
1.4
1.0
1.0
0.7
1.8
1.8
0.1
0.7
1.8
0.6
1.2
1.5
0.1
0.0
SO 4
594
533
547
475
622
430
572
582
414
485
680
625
627
552
571
768
647
600
570
608
638
703
678
489
527
521
525
514
498
574
512
526
502
478
531
453
478
539
573
700
83.0
64.7
83.5
58.4
91.3
15.8
27.8
30.2
23.5
101
80.0
71.4
60.6
69.6
39.6
60.2
69.0
45.1
73.8
31.9
30.8
43.4
46.4
29.9
87.6
70.9
63.6
46.2
43.3
116
78.5
48.7
144
16.1
56.0
14.6
34.7
88.0
47.7
72.4
0.0
0.0
0.0
0.6
0.0
11.2
0.1
0.3
0.1
0.0
0.3
0.1
0.3
7.8
9.0
0.1
2.2
0.2
6.0
1.4
0.0
0.1
0.1
0.1
6.3
1.1
2.8
3.7
0.1
22.1
3.8
5.8
7.4
0.1
0.0
0.1
0.0
29.2
0.2
0.3
538
37.4
0.2
615
539
565
750
703
510
34.6
60.8
33.0
53.5
19.0
0.4
0.2
0.5
0.6
0.2
557
513
565
635
423
581
583
407
487
682
658
612
526
591
819
612
583
661
646
736
666
NO3
PO 4 H4 SiO 4
δ13C
DIC
Bal
(‰)
0.3
0.2
0.3
0.1
0.1
0.5
0.4
0.2
0.1
0.2
0.4
0.3
0.3
0.3
0.3
0.2
0.1
0.1
0.1
0.3
0.1
0.2
0.1
0.3
0.1
0.5
0.1
0.6
0.4
0.4
0.2
0.3
0.7
0.3
0.3
0.1
0.3
0.5
0.3
0.0
0.3
0.0
1.7
1.5
1.3
0.7
2.3
2.3
2.1
3.1
0.3
1.8
0.9
0.2
0.0
0.1
0.3
0.0
3.5
2.1
1.1
3.3
3.7
0.1
0.1
0.1
0.3
0.0
0.2
0.2
3.0
3.4
1.1
2.9
1.1
3.0
1.7
0.1
0.1
0.2
0.1
0.0
0.2
0.2
1.2
1.5
5.8
4.9
6.5
48
59
58
71
65
67
59
60
98
72
67
53
77
68
81
67
68
76
68
59
67
67
67
75
68
71
71
67
66
68
66
68
66
63
59
67
59
72
78
63
-6.9
-10.1
-7.9
-3.6
-9.0
-6.6
0.9
-7.9
-8.2
-5.4
-7.4
-1.3
-6.4
-5.6
-6.6
-1.2
-0.3
0.3
-6.2
-7.7
-6.5
-4.0
-6.6
-7.8
-10.1
-5.1
-8.8
-9.0
-6.5
-10.3
-7.5
-7.8
-6.2
-7.5
-7.1
-0.5
71
-2.6
73
63
72
63
81
-7.6 2.5
-4.8 1.4
-5.3 -1.3
-0.9
0.6
-0.9
-0.2
0.6
-0.2
-0.2
-2.2
1.0
1.9
-1.6
-2.8
-0.9
-1.1
-3.9
-0.6
-2.2
0.6
J.M. McArthur et al. / Applied Geochemistry 19 (2004) 1255–1293
JAM, Shallow Aquifer
Ba 1b 22
44.5097
Ba 2 22
44.5430
Ba 3 22
44.5439
New Ba 4 22
44.4748
Old Ba 4 22
44.4748
Ba 6 22
44.4748
Ba 7 22
44.4830
Ba 8 22
44.5041
Ba 9 22
44.5067
Ba 10 22
44.4818
Ba 11 22
44.5007
Ba 12 22
44.5300
Ba 13 22
44.4673
Ba 14 22
44.4697
Ba 15 22
44.4777
Ba 16 22
44.4756
Ba 17 22
44.5284
Ba 18 22
44.5200
Ba 19 22
44.5178
Ba 20 22
44.5218
Ba 21 22
44.5262
Ba 22 22
44.5352
Ba 24 22
44.5469
Ba 25 22
44.4540
Ba 26 22
44.4334
Ba 27 22
44.4246
Ba 28 22
44.4316
Ba 29 22
44.4490
Ba 30 22
44.4248
Ba 31 22
44.4165
Ba 32 22
44.4158
Ba 33 22
44.4287
Ba 34 22
44.4321
Ba 35 22
44.4391
Ba 36 22
44.4297
Ba 37 22
44.4411
Ba 38 22
44.4604
Ba 39 22
44.4459
Ba 40 22
44.5003
Ba 41 22
44.5333
Ba 42 22
44.5080
Ba 43 22
44.5051
Ba 44 22
44.5100
Ba 45 22
44.4486
Ba 46 22
44.4918
Ba 47 22
44.5190
Ba 49 22
44.6118
Ba 50 22
44.6058
Ba 51 22
44.6238
HCO3c HCO3 Cl
Field
(diff)
29.4405
29.4645
29.3148
29.3082
29.2362
29.827
29.2260
29.2296
29.2662
29.5209
29.4807
29.4864
29.4516
29.4570
29.4210
29.3874
29.3646
29.3268
29.2986
29.2788
29.2458
29.4870
29.5314
29.5000
29.5030
29.5595
29.5572
29.5632
29.5948
29.4972
29.5940
29.6172
29.6736
29.7396
29.7510
29.3070
29.2296
9
7
7
0
0
6
137.2
115.8
125.0
88.4
115.8
121.9
32.0
35.1
44.2
43.6
34.4
44.5
45.1
41.1
36.6
25.9
33.5
29.0
39.6
44.5
44.5
39.6
40.2
44.5
46.0
39.6
28.0
38.4
38.4
26.2
38.4
44.5
40.8
42.1
33.5
32.0
26.7
26.8
27.7
26.5
26.5
26.5
26.4
26.4
26.3
26.4
26.3
26.2
26.4
25.9
26.0
25.9
26.1
Notes:
a
San. Risk is Sanitary Risk; see text for details.
b
Thermotolerant coliforms; colony-forming units per 100 ml.
c
Bicarbonate calculated from imbalance of ionic charge.
Bal = ionic balance as 100*(sum cations - sum anions)/(sum cations + anions) as meq l
88
88
88
88
88
88
JAM, Deep Aquifer
Ba 1a
22
44.5053
Ba 5
22
44.4735
Ba 23
22
44.5600
Ba 48
22
44.4324
Ba 65
22
44.5212
176-T
22
44.419
88
88
88
88
88
88
88
88
88
88
88
88
88
88
88
88
88
88
88
88
88
88
88
88
88
88
88
88
88
88
88
22
22
22
22
22
22
22
22
22
22
22
22
22
22
22
22
22
22
22
22
22
22
22
22
22
22
22
22
22
Ba 56
Ba 62
Ba 64
Ba 81
Ba 82
Ba 83
Ba 84
Ba 85
Ba 86
Ba 87
Ba 88
Ba 89
Ba 90
Ba 91
Ba 92
Ba 222
Ba 223
Ba 224
Ba 225
Ba 226
Ba 227
Ba 228
Ba 229
Ba 230
Ba 231
Ba 232
Ba 233
Ba 234
Ba 235
44.6124
44.6208
44.6118
44.5554
44.5536
44.3814
44.3664
44.3766
44.3796
44.3364
44.3250
44.3900
44.3988
44.3538
44.4174
44.4552
44.4408
44.2965
44.2907
44.3470
44.3508
44.3910
44.4054
44.4018
44.4450
44.4216
44.3604
44.3682
44.3604
44.3724
44.3700
22
22
Ba 52
Ba 53
-1
766
780
749
687
760
775
1126
872
1085
1120
1066
999
1069
1154
1039
1067
963
615
964
893
600
879
837
859
919
332
904
768
872
916
694
691
747
734
1070
1097
1205
6.78
7.15
7.28
7.20
6.76
7.30
7.01
6.98
7.00
6.91
6.98
6.93
7.06
7.03
7.03
6.89
7.10
7.05
6.99
6.80
7.16
7.18
7.22
6.91
6.88
7.00
7.03
7.03
7.00
6.99
7.13
7.04
6.98
7.02
7.01
7.03
7.08
6
17
3
7
7
3
192
405
444
4
34
1
49
1
<1
<1
<1
135
530
178
87
<1
189
167
105
8
16
<1
<1
<1
186
1130
5
<1
<1
237
151
47.9
44.3
51.8
44.9
54.0
56.5
56.1
41.5
47.5
36.7
33.6
34.9
31.9
34.1
25.2
32.7
27.3
13.0
28.5
25.1
12.8
35.5
27.6
33.9
35.6
27.1
22.6
18.2
23.5
34.3
18.8
16.3
30.6
19.9
29.1
30.8
59.1
4.4
3.6
3.8
3.7
3.3
4.6
4.1
3.5
3.9
2.1
2.1
2.1
2.3
2.6
2.3
2.5
2.2
4.3
2.5
2.0
4.0
1.6
2.1
1.4
1.5
1.9
1.8
1.7
1.2
2.6
1.9
1.5
1.5
1.2
1.4
3.8
4.0
86
90
82
72
82
87
128
102
124
139
136
128
140
147
142
132
123
83
121
118
80
112
113
112
118
122
123
108
120
126
103
105
106
103
146
137
135
24.3
26.7
22.6
31.4
22.4
24.2
41.1
31.5
38.9
42.8
40.7
38.8
40.1
44.5
39.0
39.7
37.3
22.1
37.7
33.1
20.8
35.3
33.3
34.9
36.5
35.9
37.1
30.5
33.3
35.7
29.7
29.4
29.4
31.7
42.9
42.1
44.3
0.51
0.52
0.51
0.61
0.50
0.54
0.60
0.49
0.62
0.69
0.66
0.63
0.66
0.69
0.68
0.64
0.61
0.26
0.59
0.54
0.31
0.58
0.54
0.56
0.58
0.58
0.58
0.60
0.51
0.54
0.50
0.45
0.51
0.41
0.57
0.62
0.67
0.16
0.16
0.15
0.18
0.15
0.16
0.44
0.34
0.53
0.15
0.13
0.12
0.09
0.14
0.07
0.08
0.04
0.19
0.14
0.38
0.25
0.06
0.12
0.09
0.09
0.04
0.04
0.04
0.04
0.08
0.36
0.03
0.05
0.02
0.05
0.8
1.4
1.0
0.6
0.8
0.8
6.2
4.8
5.8
0.3
0.3
0.2
0.8
0.3
0.2
1.9
0.4
5.7
6.0
8.2
3.7
0.3
6.7
2.0
1.6
0.1
1.1
0.1
0.1
0.0
5.7
0.6
0.1
0.0
2.7
0.42 10.7
0.46
5.8
0.15
0.19
0.15
0.1
0.1
0.13
0.1
0.0
0.1
1.5
0.6
0.8
0.7
0.6
0.8
0.7
0.5
0.1
0.1
0.1
0.2
0.77
0.22
0.35
0.50
2.10
1.04
1.84
2.16
0.88
0.11
0.74
1.46
2.69
4.33
0.1
0.1
0.1
1.3
0.0
0.0
0.0
1.0
2.4
2.9
2.5
2.5
2.6
2.6
2.5
1.9
2.4
2.0
2.3
1.7
1.7
2.0
0.0
0.8
0.9
0.2
0.0
0.2
0.0
492
516
477
477
482
526
766
595
653
537
573
685
568
547
580
699
523
394
570
615
345
606
606
598
591
570
586
530
544
588
505
513
507
519
582
652
699
523
498
534
474
546
583
485
526
495
513
615
604
579
593
559
579
0.7
0.2
5.8
1.5
7.8
2.6
4.3
0.9
0.1
1.9
8.4
7.3
4.9
0.2
4.8
3.2
0.6
0.1
9.7
0.1
9.4
27.9
11.0
3.6
13.4
30.4
36.6
27.0
8.3
33.7
45.2
16.5
6.8
26.0
11.1
98.3
16.1
13.5
14.6
13.9
13.2
12.2
2.5
0.4
0.8
24.2
44.6
0.4
70.9
95.3
69.8
65.5
13.2
46.0
0.3
41.9
30.6
0.9
0.3
37.6
91.4
61.2
51.1
64.8
0.1
0.3
0.4
0.4
0.4
0.2
0.4
0.2
0.2
0.2
0.1
0.4
0.2
0.3
0.2
0.5
0.3
0.4
0.0
0.0
0.0
0.0
0.0
0.1
3.0
2.9
3.3
0.2
0.2
0.1
0.4
0.0
0.2
0.1
0.4
2.9
1.6
2.2
2.2
0.2
1.7
1.2
0.6
0.2
0.3
0.1
0.2
0.1
2.3
1.0
0.3
0.0
0.1
6.1
2.2
81
75
76
64
71
72
52
69
62
65
68
69
74
73
72
69
63
85
71
73
74
66
67
58
69
67
69
62
61
74
66
58
0.3
-5.1 -0.5
-7.0 -1.6
-4.5 0.2
-0.1
0.3
1.8
-1.3
1.1
0.6
-0.7
0.1
1.5
-0.2
0.8
0.5
J.M. McArthur et al. / Applied Geochemistry 19 (2004) 1255–1293
1269
1270
J.M. McArthur et al. / Applied Geochemistry 19 (2004) 1255–1293
Fig. 7. Map of the distribution of As, Fe, Mn and SO4 in JAM wells. Symbols as on each figure. Wells near piezometer DP are rich in
Mn and lack As: wells in northwestern JAM are rich in As and Fe. For a discussion of why, see text.
the peat and the underlying aquifer sands. The best
proof of the absence of downward transport is the
presence of the brown clay at 21 m depth, which resulted
from oxidative weathering; it has not been re-reduced
during burial, and the resumption of reducing conditions, as would have happened had organic-rich water
from the overlying peaty layers been percolating
through it. The high water-level in the upper confining
unit at DP (>3 m higher than that at AP or FP; Fig. 6)
provides further proof of impermeability.
5.2. Piezometer waters
The development of anoxia must be driven by consumption of O2 and NO3 in the units rich in organic
matter, and by the reductive effect of small organic
molecules (carboxylic acids, amines, sugars), derived by
fermentative degradation of the organic matter, which
are dispersed through the aquifer (Bergman et al., 1999).
Where the sediments contain FeOOH (DP, base of AP),
concentrations of dissolved As are low (<10 lg L1 ),
despite the presence of dissolved Fe which attests to a
degree of reduction of FeOOH. The authors attribute
these low As concentrations to resorption, by residual
FeOOH, of As released by partial reduction of FeOOH.
For example, towards the base of the aquifer at AP
(>38.3 m), concentrations of dissolved Fe, As, and PO4
decrease markedly (Fig. 4). The Fe concentration (0.97
mg L1 ) in the lowermost piezometer shows that reduction of FeOOH has occurred, but the brown colour of
the sediment shows it has not gone to completion. The
low concentration of As in the lowermost piezometer at
AP (7 lg L1 ) is therefore attributed to resorption of As
to residual FeOOH. Given sufficient time, organic
matter migrating downwards will reduce all the Fe oxide
in the lower part of the aquifer at AP and release all
of its sorbed As to ground water, before continued
recharge flushes the system clear of As and other
constituents.
A contrast between brown and grey coloration of
sediment (particularly, between grey Holocene sediment
and orange/brown Pleistocene sediment) and an association of brown coloration with low-As water has been
noted in the area of Araihazar, Bangladesh (van Geen
1250
1250
1000
1000
As µg L-1
As µg L-1
J.M. McArthur et al. / Applied Geochemistry 19 (2004) 1255–1293
750
500
0
1
2
(a)
3
Mn mg L
4
0
5
-1
10
20
1250
1250
1000
1000
750
500
250
30
SO4 mg L
(b)
As µg L-1
As µg L-1
500
0
0
40
50
-1
750
500
250
0
0
0
5
10
15
Fe mg L-1
(c)
0
2
4
6
8
PO4 mg L-1
(d)
50
7
SO4 mg L-1
6
PO4 mg L-1
750
250
250
5
4
3
40
30
20
2
10
1
0
0
0
(e)
1271
5
10
0
15
Fe mg L-1
(f)
5
10
15
Fe mg L-1
Fig. 8. Relation in March, 2003, in JAM wells between (a) As and Mn (b) As and SO4 , (c) As and Fe (d) As and PO4 (e) Fe and PO4 ,
(f) Fe and SO4 . Wells are categorized using As/Fe and As concentration as proxies for depth. Open squares are wells with >32 lg L1
of As and low As/Fe values; filled circles are wells with >32 lg L1 of As and high As/Fe values; small open circles are wells containing
<16 lg L1 of As. See text for an explanation of the relation of these groupings to well depth.
et al., 2003a), but in Araihazar, unlike in JAM, grey
sediment does not always host As-rich water. That a
grey Holocene aquifer overlies a Pleistocene aquifer of a
‘‘deep burnt orange’’ colour was also noted in Munshiganj, Bangladesh (Harvey et al., 2002) but, again, low
concentrations of As prevailed over much of the thickness of the grey Holocene aquifer. The differences between these sites and JAM awaits explanation.
The concentrations of As in most of the lower part
(37.0–39.8 m) of the aquifer at FP are higher (>900
lg L1 ) than they are in the upper part of FP (94–448
lg L1 ) and are higher in both than they are at AP
(maximum 420 lg L1 ). The authors attribute the differences to the fact that FP contains more organic
matter than does AP, and higher peak concentrations of
organic matter (e.g. 7.2% at 29.5 m in FP, 0.76% at 23 m
in AP). This additional organic matter must be driving
reduction of FeOOH in FP further than it has gone in
AP, and results in ground water with higher concentrations of both Fe and As. This organic distribution
1272
J.M. McArthur et al. / Applied Geochemistry 19 (2004) 1255–1293
800
HCO3 mg L-1
Ca mg L-1
200
150
100
700
600
500
400
300
50
15
25
35
45
Mg mg L-1
(a)
50
55
100
(b)
200
200
70
A
Ca mg L-1
60
Na mg L -1
150
Ca mg L-1
50
40
30
150
100
B
20
50
10
0
0
0
(c)
50
100
Cl mg L
150
200
-1
0
(d)
20
40
Na mg L
60
-1
Fig. 9. Relation in March, 2003, in JAM wells between (a) Ca and Mg (b) Ca and HCO3 , (c) Cl and Na (d) Na and Ca. Line in (c)
shows Na/Cl in marine salt. Arrow A in (d) represents a weathering trend: arrow B represents stoichiometric ion-exchange of Ca in
solution with Na on mineral exchange-sites. Symbols as in Fig. 8, with the addition of large, open, circles for deep wells (>70 m), and
crosses for wells in the vicinity of piezometer site DP, which, owing to their proximity to piezometer site DP, are inferred to be tapping
an aquifer different sedimentologically from that tapped by other wells.
does not, however, appear to explain the fact that the
maximum As concentrations in AP and FP occur well
below the maximum concentrations of PO4 and Fe: the
control on dissolved P and Fe appears to be partially
decoupled from the control on dissolved As. The different depths of the maxima in As, Fe, and PO4 is
probably due simply to differences of the As/Fe/PO4
ratios in the aquifer sands above and below 32 m, given
their different hardness and age (see Section 4): variable
lithology is another reason why As and Fe seldom correlate well in ground water from alluvial basins.
In DP, the brown colour of the aquifers sands at all
depths (Fig. 10) attest to the presence of FeOOH
throughout the aquifer: that the composition of ground
water is quite different from those at sites AP and FP is
therefore not unexpected. Concentrations of As are low
(<5 lg L1 ), whilst concentrations of dissolved Mn and
SO4 are high. The profiles and compositions at DP are
best explained as resulting from the preservation of
originally oxic connate water, which has subsequently
been subject to creeping reduction as organic matter
moves in laterally from surrounding sediments (e.g. site
FP, which is 250 m to the west) to cause (presently)
incomplete reduction of FeOOH and Mn(IV) oxides.
Given the U-shaped profiles of As, Mn and Fe with
depth (Fig. 3), organic matter must be diffusing upwards
from the underlying organic-rich clay at this site (so its
upper part cannot be oxidised?) as well as entering the
aquifer laterally at shallow depths, so that the reduction
potential decreases towards the mid-point of the aquifer:
there, a peak in dissolved Mn (2–4 mg L1 , Fig. 3) coincides with a minimum in dissolved As (<2 lg L1 ) and
Fe. Reduction of FeOOH dominates over reduction of
Mn oxides towards the top and base of the aquifer.
Concentrations of As are kept low (<5 lg L1 ) by resorption of As to residual-FeOOH. Oxides of Mn sorb
As (Manning et al., 2002; Foster et al., 2003), so the As
released by reduction of MnO2 must be re-sorbed to
residual oxides of Mn and Fe, rather than remain in
solution (as noted by St€
uben et al., 2003).
The values of d13 C in dissolved inorganic C (DIC) are
high for a redox-dominated system. They are kept from
becoming more negative, and so from reflecting organic
inputs more strongly, by the low pH of the ground water
(6.76–7.32; Tables 2 and 3), which promotes rapid
equilibration between DIC and detrital calcite, with a
J.M. McArthur et al. / Applied Geochemistry 19 (2004) 1255–1293
1273
Fig. 10. Colour of wet sediment from West Bengal, showing the grey colour of sediments in which reduction of FeOOH has gone to
completion, and the brown colour of sediments that retain FeOOH. Groundwater from the former is enriched in As, that from the
latter is not. (For interpretation of the reference to colour in this figure legend, the reader is referred to the web version of this article.)
d13 C around )1‰ (Table 1; Hudson-Edwards et al.,
2004). At AP, values of d13 C(DIC) decrease by around
2‰ at 26 m, a change that may reflect an organic contribution to DIC from the degradation of organic matter
at 23m depth. At FP, the abundance of calcite approaches zero below 30 m depth; below this level, the
more negative values of d13 C(DIC) reflect this diminished buffering capacity (Table 1). Surprisingly, the organic matter at 29.5 m in FP appears not to influence
values of d13 C(DIC). In the upper part of the aquifer at
DP, values of d13 C(DIC) are around )2‰: they are
around )6‰ at greater depth. These high values must
reflect equilibration with traces of calcite in the aquifer,
although the analytical data suggests calcite is absent
from DP.
At sites AP and FP, concentrations of SO4 decrease
downwards from the uppermost piezometers, which are
in the upper confining layers (Figs. 4, 5), but it cannot be
said with certainty whether these profiles represent
downward passage (dilution and dispersion) of recent
anthropogenic SO4 from the surface, infiltration of
natural amounts of SO4 concentrated by evaporo-tran-
spiration, or remobilisation of S in Fe sulphide from the
organic-rich layers as oxic water passes through the
uppermost part of the upper confining layers. It seems
unlikely that they represent steady-state consumption of
natural SO4 , at depth, by SO4 reduction, because the
sites are <350 m apart and would have similar nearsurface concentrations were the profiles natural. In fact,
concentrations at 7 m depth are 65 mg L1 at FP and 490
mg L1 at AP. The authors speculate that the high
concentrations of SO4 at 7 m at AP result from surface
loadings of gypsum as a result of house building, as it is
used in plaster and the locality has several substantial
houses. If this is the case, the profiles imply that SO4
from a surface source can penetrate to 30 m depth within
a few tens of years.
The chemical stratification revealed by the piezometers explains why wells may differ in composition despite
being close to each other (<5 m) and of similar depth.
Arsenic concentrations in the bottom 4 piezometers at
the AP site decrease linearly with depth from 419 lg L1
at 38.3 m to 7 lg L1 at 42.8 m, a rate of change of 92
lg L1 m1 . The rate is 200 lg L1 m1 in the basal part
1274
J.M. McArthur et al. / Applied Geochemistry 19 (2004) 1255–1293
of the aquifer at FP. Such high gradients will cause wells
to differ substantially in As concentrations if they differ
by no more than a metre in depth, a distance within the
uncertainty with which a driller would record the depth
of a well.
Chemical stratification also compromises interpretions drawn from well-surveys of As concentrations, in
the Ganges Plain and elsewhere: low concentrations of
As in wells (e.g. as found near AP; Fig. 7) do not indicate that the aquifer they tap is free of As: using local
knowledge of aquifer colour, a driller may emplace wells
in the small thickness of good water in the brown sand
at the base of polluted aquifers, if it exists (as it does at
AP). Consequently, As in ground water would go undetected until breakthrough to wells, and its detection
by analysis or the development of arsenocosis in consumers. Breakthrough and detection might be separated
substantially in time and have serious consequences for
health. Furthermore, the finding in an area of dangerous
concentrations of As in only a small percentage of randomly distributed wells does not signal that the As is
localized and the aquifer generally safe for development:
it may show only that most wells tap water from above
or beneath a poisoned part of the aquifer.
5.3. Well waters
The chemical stratification of the ground water (Figs.
3–5), the differing depths of wells and the uncertainty
about some well-depths, and the differing amounts of
organic matter in the aquifer sands from place-to-place,
conflate to obscure areal patterns in water quality
(Fig. 7) and cross-plots of some dissolved constituents
(Fig. 8): nevertheless, some patterns emerge. Wells near
piezometer DP (14–18, 40–44) contain <10 lg L1 of As
and more than 2 mg L1 of Mn, so the aquifer they tap
must be sedimentologically and chemically similar to
that seen in the DP piezometer, and be in a setting that is
sedimentological distinct from those of other wells in the
JAM area that lack Mn. The wells near DP must tap a
part of the aquifer that is both free of organic matter
and protected from direct infiltration of organic matter
from above by a thick upper-aquiclude of clay. For wells
near DP, the low concentrations of As and Fe, and the
high concentration of Mn must be due to the fact that
reduction of MnO2 dominates over reduction of FeOOH, and that where the latter occurs, it does not reduce all the FeOOH, so, in both cases, re-sorption keeps
concentrations of As low. That concentrations of As are
low when Mn is present is an observation not confined
to the DP area: as Fig. 8(a) shows, As and Mn appear to
be largely mutually exclusive in solution, a situation that
extends to most shallow wells within 2 km of the JAM
area (McArthur and Banerjee, unpublished data) and
more widely in Bangladesh (data in DPHE, 1999). High
concentrations of Mn might therefore indicate the
presence of As-free connate water in an aquifer where
reduction of FeOOH is incomplete. The exploitation of
water at such sites will constitute mining of a finite resource and may draw in As-rich water. Such transport
will be moderated by sorption to unreacted FeOOH.
Regular monitoring of water quality is essential in such
a complex situation.
The composition of wells away from site DP can be
interpreted if it is assumed that the vertical pattern of
chemical stratification in the aquifer across the rest of
the JAM area resembles that seen at AP and FP. The
assumptions are: that As concentrations are low (<50
lg L1 ) where FeOOH is present to sorb As; that this
occurs only at the base of the aquifer; that As concentrations reach a maximum at 35–40 m and decrease
sharply below this depth; that ground water above this
depth contains less As, although concentrations are still
high; and that As concentrations reflect the amount of
reduction that has occurred, and so the amount of organic matter in the sediments. A well-by-well analysis of
As concentrations, however, does not reveal a clear relation between concentrations of As and well depth,
probably because some well-depths are wrong. There is,
however, some relation between well-depth and both As/
Fe and As concentrations: if the lowest concentrations
of As (<20 lg L1 ) are assumed to represent wells emplaced in the base of the aquifer where FeOOH persists,
as at AP, and if the remaining wells are divided into two
groups on the basis of their As/Fe values, 3 divisions
emerge (Fig. 8), viz.
Group A; 19 wells with <1–16 lg L1 of As.
Group B; 24 wells with >32 lg L1 of As and As/Fe
between 13 and 56.
Group C; 24 wells with >32 lg L1 of As and As/
Fe between 36 and 2025.
The mean well-depths for each group are: 135 14
feet (41.1 ± 4.27 m) (A); 124 24 feet (37.8 ± 7.32 m) (B);
and 108 20 feet (32.9 ± 6.1 m) (C), where all uncertainties are 1 s.d. Although these differences are not statistically significant, they are interesting because wells are
constructed from 20-foot (6.1 m) lengths of plastic pipe,
topped by 3–6 (0.91–1.82 m) feet of Fe pipe. The mean
depths of these 3 groups suggest that they reflect groups
of wells that are mostly made with 7, 6, or 5 pipes, with 7pipe wells (group A) being screened in brown sand at the
base of the aquifer, 6-pipe wells (group C) being screened
in the As maximum seen in the piezometer profiles to
occur in the lower part of the aquifer, and 5-pipe wells
(group B) being screened above the As maximum. The
high standard deviation probably arises because some
well depths are incorrect: nevertheless, enough are correct
to allow the overall depth-control to be seen. The use of
As/Fe, and As concentrations, as a proxy for depth seems
reasonable because the As/Fe values in AP and FP
(Fig. 11) change with depth, reflecting the fact that the
maximum dissolved concentrations of As occur at a
J.M. McArthur et al. / Applied Geochemistry 19 (2004) 1255–1293
depth greater than do those of Fe or PO4 , which
are themselves not much separated in terms of depth
(Figs. 4,5).
The different As/Fe/PO4 values present at different
depths (Fig. 11) probably arises because the younger
sediment (<32 m depth) releases more Fe and PO4 on
reduction, and less As, than the older sediment (>32 m
depth): such lithological differences, reflected in ground
water composition, must be one reason why Fe and As
often correlate poorly in the well waters of the Ganges
Plain. Dividing the wells onto groups A–C, however,
yields correlations in the As-rich groups between As and
Fe that are better than for the combined population,
probably because the effect of mixing populations from
different depths is reduced. The coefficient of correlation
between As and Fe in the 3 populations (Fig. 8(c)) are:
0.01 for A (p P 0:1); 0.58 for B (p ¼ 0:01); and 0.86 for
C (p ¼ 0:001, if one outlier, with 0.56 mg L1 of Fe and
1131 lg L1 of As, is excluded); the pooled population
has a coefficient of 0.58 (p ¼ 0:001 excluding the outlier).
In a pooled population of 213 wells within 2 km of JAM,
the correlation coefficient between Fe and As is 0.39
(p ¼ 0:001; Fig. 12; Banerjee and McArthur, unpublished data). If the correlations have meaning, it can be
only because the lower and intermediate concentrations
of As, and the accompanying Fe, reflect a dynamic
process that is proceeding extremely slowly, with the
dissolved As and Fe concentrations representing a
snapshot of slow, microbially mediated release, prior to
sequestration of Fe into solid diagenetic phases, and
resorption of As: the As and Fe are ‘kinetic species’ in
the sense used in Fig. 13 (see next section). Higher
concentrations of As may represent a state where all
reactive FeOOH has been consumed, leaving the As in
solution with no sorber to adhere to: this idea is also
developed in a later section. For groups A–C, the correlation coefficients between PO4 and Fe (Fig. 8(e)) are
0.00 for A (p > 0:1); 0.50 for B (p ¼ 0:01); and 0.62 for
C (p ¼ 0:001): the coefficient for the pooled population
is 0.72 (p ¼ 0:001) and is enhanced by mixing populations B and C, one high in PO4 and Fe, the other lower
in both. In a pooled population including data for 213
other wells within 2 km of JAM (Fig. 12; Banerjee and
McArthur, unpublished data), the correlation coefficient
between Fe and PO4 is 0.76 (n ¼ 213, p ¼ 0:001).
Given the discussion above, the high concentrations
of As and Fe in the northwestern part of JAM (Fig. 7) is
probably due to two factors viz. that most wells there are
emplaced at a depth near to the As maximum (are 6-pipe
wells) and that organic matter around 30 m depth (as
seen at FP) is more abundant in central and NW JAM
than it is elsewhere. In much of southern and eastern
parts of JAM, concentrations of As must be low (<50
lg L1 ) because wells there are screened at the base of
the aquifer in the small thickness of brown sand revealed
at AP.
1275
The considerations above show that real element
associations can be disguised or enhanced by pooling
data on too regional a scale, and that piezometer profiles
are essential to fully understand the source, transport,
and sinks, of As in the Bengal Basin. This discussion
also serves to emphasise that the results of well-surveys
must be evaluated in combination with a knowledge of
the local geology, particularly aquifer colour, and well
depth.
5.4. Diagenetic Iron sulphide
Diagenetic Fe sulphide is a sink for dissolved As
(Korte and Fernando, 1991; Rittle et al., 1995), and is
present in the sediments in West Bengal (Fig. 2; Chakraborti et al., 2001). As much of the ground water in the
JAM area lacks SO4 , as do most ground waters in surrounding areas (McArthur and Banerjee, unpublished
data) and in the Ganges Plain generally (DPHE, 1999,
2001; Nickson et al., 2000; McArthur et al., 2001;
Dowling et al., 2002), it must have been removed by SO4
reduction from most recharging water. As SO4 reduction
is driven by microbial metabolism of organic matter, it is
localised where organic matter is present. At all sites,
total S is almost absent from aquifer sands but is relatively abundant in the organic-rich units (Fig. 2; Table
1), which are (or have been) the site of SO4 reduction
and formation of diagenetic Fe sulphides. The traces of
S in the sands probably represents Fe sulphide formed
very early in the history of anoxia from reduction of SO4
in connate water. Reduction of SO4 is probably not
occurring in the aquifer sands now, either because they
lack SO4 (FP) or because SO4 -reducing bacteria are
outcompeted by Fe-reducing bacteria (Chapelle and
Lovley, 1992). The presence of SO4 in the upper aquifer
at AP and FP (Figs. 4 and 5), where it has penetrated to
levels below some organic-rich units, might show that
the organic matter has lost much of its original reducing
power through biodegradation. From these arguments,
and the fact that most of the organic-rich units are
stratigraphically above As-rich aquifers, the authors
infer that little As is being removed now from ground
water in the aquifer sands by sequestration in diagenetic
sulphide.
5.5. Resorption as a control on aqueous As
5.5.1. The shallow aquifer
Knapp et al. (2002) showed that aquifer sands from
Virginia, USA, sorbed SO4 least where FeOOH in the
sediments had been removed from grain surfaces by the
establishment of reducing conditions. In JAM, As concentrations are low where the sediments are coloured
brown by FeOOH, and are high where sediments are
grey because all FeOOH has been reductively consumed
(Figs. 10). Reduction is incomplete in those parts of the
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J.M. McArthur et al. / Applied Geochemistry 19 (2004) 1255–1293
0
Depth, metres bgl
10
AP
FP
20
30
40
50
60
0.00
DP
0.05
0.10
0.15
0.20
As / Fe
0
Depth, metres bgl
10
20
30
40
50
60
0.0
0.2
0.4
0.6
0.8
1.0
As / PO4
Fig. 11. Relation of As/Fe to depth at piezometer sites AP and FP in March, 2003. Values of As/Fe, and As concentration, are low
where wells tap brown sediment at the base of the aquifer (more than about 40 m depth); are high where wells tap the aquifer between
35 and 40 m depth, and lower, but still high, where wells tap the aquifer at shallower depths.
aquifer that are distal from sources of organic matter
(e.g. the base of the aquifer at AP), or where no hydraulic continuity exists between organic-rich sediments
and aquifer sands (at DP). Where reduction is incomplete, resorption to residual FeOOH will keep As concentrations low (as mooted by Ravenscroft et al., 2001),
until all sorption sites become saturated by re-sorbed As,
or all FeOOH has been consumed: this idea is implicit in
the model of Welch et al. (2000) that explains the generation of high concentrations of As by progressive reduction of FeOOH. In Fig. 13, this model is shown more
explicitly: As released to solution by reduction will exist
in solution temporarily (this is termed ‘kinetic As’) until
it is resorbed to residual FeOOH. Laboratory studies of
microbial FeOOH-reduction show that As resorbs over
a timescale of a few days to weeks (Figs. 3B and C in
Dowdle et al., 1996; Figs. 6 and 7 in Jones et al., 2000).
Resorption will decouple concentrations of As from
those of Fe and leads to poor correlations between these
constituents: this is one reason why these constituents
seldom correlate well in ground waters in alluvial aquifers. Resorption of As will occur as ground water moves
away from the influence of peaty sediment, where reduction is complete, to areas where it is not complete
and FeOOH is available to sorb it (DPHE, 1999).
5.5.2. The deep aquifer
Resorption may be one reason why the deep aquifers
in JAM, and widely across the Bangladesh, are free of
As (a very limited exceedance of 50 lg L1 of As in deep
wells may be explained as contamination or mis-identification of wells). Sands of the deep aquifer in Bangladesh are typically described as ‘‘burnt-orange’’ (Harvey
et al., 2002) or as ‘‘yellowish-brown to pale grey sands
J.M. McArthur et al. / Applied Geochemistry 19 (2004) 1255–1293
1277
1250
As µg L-1
1000
750
500
250
0
0
2
4
6
8
10
12
14
10
12
14
Fe mg L-1
7
PO4 mg L-1
6
5
4
3
2
1
0
0
2
4
6
8
Fe mg L-1
Fig. 12. Relation of As, PO4 , and Fe, in ground waters sampled from 213 wells in JAM in November, 2002, within 2 km of the study
area (McArthur and Banerjee, unpublished data). By analogy with piezometer profiles, the poor correlation between As and Fe
(r ¼ 0:39, n ¼ 213) arises because As/Fe values in ground water reflect As/Fe values in aquifer sediments, and are lithology-dependent.
The better correlation between Fe and PO4 (r ¼ 0:76, n ¼ 213) reflects a more uniform distribution (less lithology-dependent concentration) of PO4 than of As in aquifer sands.
Fig. 13. Schematic of dissolved concentrations of As as a function of the percentage of FeOOH reduced. Kinetic-As is that released by
partial reduction of FeOOH but not immediately re-sorbed: its concentration represents a balance between the rates of FeOOH reduction and As sorption. Filled-arrow 1 shows position in the sequence of reduction of groundwater at site DP and the lowermost
piezometer (42.8 m) at site AP; filled-arrow 2 shows the likely state of sediments and ground waters at site FP and the middle and upper
aquifer at site AP.
1278
J.M. McArthur et al. / Applied Geochemistry 19 (2004) 1255–1293
. . . cemented by secondary clay and Fe oxide’’ by Ravenscroft in DPHE (1999; Vol. S1-Main Report), because
they were subjected to a long period of oxidative
weathering during the lowering of sea-level that occurred between 125 and 18 ka. This weathering must
have generated FeOOH by oxidation of biotite and
other Fe-bearing minerals in the sediment. As a consequence, the higher content of FeOOH in Pleistocene
sands of the deep aquifer distinguishes them (Stanley
et al., 2000) from the Holocene sands of the shallow
aquifer. The authors suggest that the establishment of
anoxic conditions in the deep aquifer, as a consequence
of the deposition of overlying, organic-rich, Holocene
aquifers, has been sufficient to mobilise some Fe into
ground water (Table 3; DPHE, 1999), but has been insufficient to reduce all its FeOOH, the remains of which
may now act as a sorptive buffer to prevent As from
accumulating in solution. An alternative explanation for
the lack of As in waters from the deep aquifer, previously suggested by Ravenscroft (DPHE, 1999), McArthur et al. (2001), and Ravenscroft et al. (2001), and
repeated in Smedley and Kinniburgh (2002), is that
sorbed As was flushed from the deep aquifer during an
extended period of oxidative weathering and flushing
that occurred during the lowering of sea level between
125 and 18 ka. With available data, the authors cannot
be certain which explanation is correct.
In JAM, however, (where CH4 is not recorded) the
DP profile shows that peat sensu stricto is encapsulated
in impermeable clay: the deposition of the peat must
have occurred in response to local waterlogging caused
by the clay. The low permeability of this clay now prevents recharge from carrying soluble organic matter
downwards into the underlying aquifer sands. The sediment profiles at FP and AP show that sites marginal to
the peat depocentre (DP) developed lesser amounts of
organic matter, in peaty layers that are lateral extensions
of the main peat (i.e. are of a similar age; Hudson-Edwards et al., 2004). In these marginal environments, the
thinner, perhaps intermittent, clay will allow greater
hydraulic connectivity with underlying aquifer sands
and so allow ingress of organic matter. During sedimentation, however, such sites would have been less
waterlogged and so would have accumulated less organic matter than basin centres, and so would host less
organic matter. Beyond the basin margin, permeability
will be higher, but the amount of organic matter will be
negligible. These ideas are illustrated in Fig. 14, in which
a schematic cross-section through the JAM area shows
the relation of permeability, organic matter, and sedimentary architecture. This model suggests that chemical
reduction in the JAM aquifer (and by implication,
elsewhere in the Bengal Basin) is driven by organic
matter in peaty units that develop on the margins of peat
basins where there is enough organic matter to drive
redox, and sufficient permeability to allow organic
matter to invade the aquifer in soluble form.
6. The driver for FeOOH reduction
6.1. Natural organic matter in sediments
6.1.1. The JAM Area
The data for the JAM area suggest that reduction of
FeOOH is driven by natural, subsurface, organic matter,
in organic-rich units, formed at the periphery of local
peat basins, that are in hydraulic continuity with the
aquifer. The concentrations of organic matter in JAM
sediments (low% levels) is more than adequate to account for the amount of reduction present: at 25 °C (the
mean annual air temperature in JAM) infiltrating water
contains around 8.3 mg L1 of dissolved O2 , an amount
consumed by <0.001% of reactive organic matter in the
sediment. That peaty sediment is connected with the
presence of unwholesome water is confirmed by local
experience in JAM: Ashraf Ali (Ardivok driller, pers.
comm., 2003) has noted an association between bad
water and ‘khalu mati’ (black soil, which the authors
interpret to be peat, or peaty units) and between brown
sediment and good water. The association suggests that
connectivity (permeability) exists between aquifers and
peat deposits in some parts of the Ganges Plain, perhaps
where they are cut through by post-depositional channelling (H. Brammer, pers comm., 2001).
6.1.2. Other areas in the Bengal Basin
The coincidence of high concentrations of As in
groundwater and wetland environments, where plant
productivity is high, was noted by Ravenscroft (DPHE,
1999) and has been reproduced in Smedley and Kinniburgh (2002), whilst both McArthur et al. (2001) and
Ravenscroft et al. (2001) argued that peat, which forms
in such wetlands, was the driver for reduction of FeOOH in the Ganges Plain. One reason for an emphasis
on peat is that CH4 in combustible amounts is found at
depth in some ground waters (Ahmed et al., 1998;
Harvey et al., 2002) and the fact that CH4 in such
amounts can be generated only by anoxic degradation of
high concentrations of organic matter. If organic matter,
accumulating marginally to peat depocentres, drives
reduction of FeOOH (and so As release to ground water) it is crucial to establish the extent of peat deposition
in the Bengal Basin.
Peat is common beneath the Old Meghna Estuarine
Floodplain in Comilla (Ahmed et al., 1998), Sylhet, and
the Gopalganj-Khulna Peat Basins (Reimann, 1993;
Brammer, 1996). Many wells in the area around Faridpur may be screened in waterlogged peat (Safiullah,
1998) and the shallow aquifer system in Lakshmipur
contains peat (DPHE, 1999). Peat is often found in
J.M. McArthur et al. / Applied Geochemistry 19 (2004) 1255–1293
geotechnical borings (piston samples), although it is
rarely recorded during rotary drilling for water wells
because such drilling masks its presence, unless the peat
is very thick. Peat occurs extensively beneath the Asaffected areas of southern Samta village and Deulhi
village in southwestern Bangladesh at a depth of about
10 m (AAN, 2000; Ishiga et al., 2000). Peat has been
found in Holocene sediments around Mymensingh
(Ishiga et al., 2000) and is well documented at Panigati,
in SW Bangladesh (Islam and Tooley, 1999). Organicrich sediment has been reported to occur at 6 m and 25
m depth in Faridpur and at 28m depth in Sonargaon
(Fig. 3 of Tareq et al., 2003). Sediment from a depth of
2.1 m at Gopalganj (100 km SW of Dhaka) contained
6% TOC (Nickson et al., 1998); sediment from a depth
of 23m at Tepakhola (Faridpur municipality) contained
7.8% TOC (Safiullah, 1998). Peat is repeatedly mentioned by Umitsu (1987, 1993) and by Goodbred and
Kuehl (2000) as being present in Bangladesh sediments;
it has been reported to occur in southern West Bengal by
Banerjee and Sen (1987), and also by Pal et al. (2002)
who note that peat was present in the subsurface in an
As-free area, but not in a nearby As-affected area. These
authors do not state whether their peat was underlain by
impermeable strata, as at the DP site, but it seems likely.
It could be that the apparent lack of peat at their Asaffected site means that it was missed during coring.
Harvey et al. (2002) also reported that no peat was
found at an As-affected site in Bangladesh, but they
report the presence of CH4 (marsh gas) in combustible
amounts; this proves the presence of much organic
matter in the aquifer close to their site.
Thus, in the Bengal Basin, peat deposits are widespread, and so the margins of such deposits must be
1279
common. Despite the importance of peaty sediment as
the redox driver in the Ganges Plain, there are two other
potential drivers of redox: these are organic waste in
latrines (McArthur et al., 2001; Ravenscroft et al., 2001),
and organic matter leached from surface sources, such
as soils, river beds, and ponds (Harvey et al., 2002).
Each is considered in turn.
6.2. Other sources of organic matter
6.2.1. Organic matter in latrines
Human organic waste in latrines does locally promote reduction of FeOOH in aquifers and contributes
As to groundwater to locally enhance As concentrations, thereby confusing the regional signal of As derived from reduction driven by natural organic matter.
The impact of point-sources can be seen in Singair, an
As hot-spot in Holocene sediments some 10 km west of
Dhaka, Bangladesh: of 15 shallow wells within 300 m of
Singair mosque (Safiullah and McArthur, unpublished
data), the 3 wells sited within 3 m of latrines contained
900 lg L1 of As, whilst the remaining 12 wells, all >15
m from latrines, contained between 100 and 300 lg L1
of As. Such an influence is not uncommon; new wells
drilled through new concrete pit-latrines have been observed (McArthur, unpublished data). Point sources will
be particularly strong where the polluting organic matter is in hydraulic continuity with the aquifer e.g. where
the depth of a latrine is greater than the thickness of the
upper confining layer and/or where wells and latrines are
adjacent. But organic matter in latrines cannot be the
main driver of As release to ground water because it is
free of As in many areas of Bangladesh, despite the fact
that the country is densely, and fairly evenly, populated.
Fig. 14. Schematic of the relationship of peat basins to the occurrence of As enrichment in groundwater. Soluble organic matter to
drive FeOOH-reduction can invade the aquifer only at the margins of peat basins, where the sediments are sufficiently permeable to
allow its passage downwards in recharging ground water and the sedimentary source of OM is sufficiently abundant to supply it.
Towards the basin centre, permeability decreases; away from the basin, the abundance of organic matter decreases: in both cases,
enrichment of As decreases.
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J.M. McArthur et al. / Applied Geochemistry 19 (2004) 1255–1293
Where the depth of dug (pit) latrines is much less
than the thickness of the upper confining layer, where
wells are far from latrines, where sewage collection is
engineered, or where the unsaturated zone is very thick
(as it is on parts of the Madhupur and Barind Tracts),
the impact of human sources of organic matter will be
small, or negligible. Such is the case in JAM, where most
dwellings have concrete-lined holding tanks with tops
above ground level and where the depth of the upper
confining layer is between 9 m (at FP and DP) and 24 m
(at DP). Several considerations suggest that there is little
connectivity between latrines and aquifer water in JAM.
Firstly, thermotolerant (faecal) coliforms (presumptive
Escherichia coli) are bacterial indicators of faecal pollution. Despite the very high risks of faecal pollution
from poor well-completion, open access of many wells to
animals, and the fact that there is generally a broad
relationship between log(TTC) and sanitary risk scores
(Cronin et al., 2003), the study wells mostly have a low
TTC count (Table 3; summary statistics in Table 4):
about 85% of the wells contained <10 cfu/100 ml of
water. Furthermore, there is no correlation between
TTC count and As concentrations in JAM wells. Given
the high number of observable risks to JAM wells, their
low TTC counts probably stem from the fact that most
are >30 m deep, are sealed around the annulus by the
clayey silt of the thick ( P 9 m thick) upper confining
layer, and many have concrete completions of good
quality, although the completions are mostly small in
diameter. These factors reduce access of microbially
polluted water into the aquifer via the annulus of the
well. The implications of these results is that microbial
contamination of the aquifer in the JAM area is on a
small scale and so organic pollution of the aquifer by
human waste is limited in volume and distribution.
Hence, it is unlikely that faecal organics do much to
drive reduction of FeOOH in JAM aquifers.
sources of organic matter (rivers, ponds, and soils),
drawn into the aquifer by irrigation drawdown, as has
been suggested to occur in Bangladesh (Harvey et al.,
2002). The authors note, however, that the occurrence of
CH4 in combustible amounts around 30 m depth in
Munshiganj, but in lessening amounts as depth diminishes (Harvey et al., 2002), and the presence of CH4 in
ground water elsewhere in the Bengal Basin (Ahmed
et al., 1998), can be due only to the presence of subsurface organic matter in high concentrations. Furthermore, if surface sources of organic matter in the Bengal
Basin were of much importance in driving the release to
ground water of large amounts of As, high As concentrations would not be confined to about 25% of wells
(DPHE, 1999), or to Holocene sediments (it is largely
absent from the Barind and Madhupur Tracts; DPHE,
1999; McArthur et al., 2001; Ravenscroft et al., 2001),
nor would they be highest around 20 to 40 m depth
across most of the Bengal Basin (Karim et al., 1997;
McArthur et al., 2001; Ravenscroft et al., 2001; Harvey
et al., 2002; van Geen et al., 2003a). Finally, although
the 14 C ages of Harvey et al. (2002) for DIC show that
very young water is penetrating to depths of around
20 m, the piezometer profiles of those authors show that
concentrations of dissolved organic carbon (DOC) increases with depth to around 30 m depth, below which
they scatter but remain high to 80 m depth, thereby
proving the DOC has a deep source. The As maximum
(and a maximum concentration of CH4 ) at 35–40 m
depth cannot be caused by organic matter migrating
downwards against a concentration gradient and then
reacting at depth. The discrepancy noted by Harvey
et al. (2002) between young 14 C ages for DIC and older
14
C ages for DOC is explicable in terms of a reservoir
age for DOC and its derivation from older buried peat.
6.2.2. Surface sources of organic matter
For reasons given above, it seems unlikely that reduction of FeOOH in the JAM area is driven by surface
The results given here provide strong evidence that
reduction of FeOOH across the Ganges Plain is driven
overwhelmingly by buried, natural, organic matter.
6.3. The capacity to reduce FeOOH
Table 4
Summary table of content of thermotolerant coliform bacteria in waters from wells, ponds, and piezometers, from Joypur, Ardivok
and Moyna (JAM), and Mirathi, West Bengal, India, (March 2003). Colony forming units (cfu) per 100 ml. TNTC ¼ too numerous to
count
Summary statistics
JAM wells
Mirathi wells
JAM ponds
JAM piezos
Numbers are cfu/100 ml
Number, incl. repeats
Maximum TTC
Minimum TTC
Non-detects (TTC ¼ 0)
Mean TTC counts in wells with positive detects
49
63
0
23
10
15
65
0
1
14
11
TNTC
4400
0
–
21
TNTC
0
1
–
Notes. cfu is colony-forming units.
TTC is too numerous to count.
J.M. McArthur et al. / Applied Geochemistry 19 (2004) 1255–1293
According to the equation for reduction of FeOOH,
where FeOOH is used to represent Fe oxides
8FeOOH þ CH3 COOH þ 14H2 CO3
$ 8Fe2þ þ 16HCO
3 þ 12H2 O
ð1Þ
Given: the stoichiometry of this reaction; a density of 1
for buried peaty sediment; a C content of 7% in the
sediment; a thickness of 50 cm for a peaty layer (as at
29.5 m depth in FP) with half the C bioavailable; and a
porosity of 40%, then there will be about 1.1 g of bioreactive C under each cm2 of aquifer. The organic
matter will consume 25 g of FeOOH which, given a
porosity of 40%, a density of FeOOH of 2.4, and an
aquifer containing 0.2% FeOOH, will be contained in an
8 m thickness of aquifer sand, which is half that of the
aquifer thickness at FP below the peaty layer at 29.5 m
depth, and is similar to the thickness in which concentrations of As exceed 900 lg L1 . Towards the base of
the aquifer at FP, concentrations of As decrease whilst
remaining high. Our calculation is very basic, but suggests that few aquifer volumes of water have passed
vertically through the sands: had they done so, the organic matter, or the FeOOH, would be exhausted by
now and the aquifer would be clear of both dissolved Fe
and dissolved As.
Although approximate, the calculations above suggest (and observation confirms) that there is enough
organic matter in the aquifers in JAM for the entire
aquifer thickness to be at risk: confusing the calculation
are 3 factors. Firstly, the organic matter in the sediments
at JAM is P 1.2 ka old (Hudson-Edwards et al., 2004),
so it has been degrading for a substantial period of time.
On deposition of the sediment, the amount of organic
matter would have been greater and it would have been
fresher, and more biodegradable, than it is now. Secondly, there is some uncertainty regarding the estimate
of 0.2% for the amount of FeOOH in the aquifer. This
figure represents the Fe, recalculated to FeOOH, that is
leachable by ammonium oxalate at pH 3 (Table 5) from
sands in mid-aquifer at site DP, which is the least reduced of the 3 piezometer sites (Figs. 3–5 and 10). The
figure of 0.2% is a maximum because the leaches contained P, S, Al, Fe, Mg, (but not Ca, because Ca oxalate
is insoluble) and it is not possible to apportion the
leachable Fe between FeOOH and possible diagenetic
phases formed by its partial reduction viz. magnetite,
green rust, vivianite, siderite, Fe sulphide (Lovley et al.,
1987; Frederickson et al., 1998; Chakraborti et al., 2001;
Glasauer et al., 2003), or to possible detrital phases such
as Fe-rich chlorite. Thirdly, the formation of magnetite
(Fe2þ Fe3þ
2 O4 ) by reduction of sedimentary FeOOH reduces only one-third of the available pool of Fe(III)
whilst sequestering the remaining two-thirds in the
mineral structure (Frederickson et al., 1998) where it
1281
may be unavailable for reduction. Thus, the availability
of FeOOH to sorb As in Bangladesh sediments will be
some 70% less than supposed if magnetite is the main
product of FeOOH reduction across the Ganges Plain.
It can be inferred from the results of Harvey et al. (2002)
that magnetite comprised most of the Fe that was extractable with oxalic acid solution from aquifer sediments from, Bangladesh, and so magnetite constituted
up to 6% of total-Fe (although this inference is compromised by uncertainty about whether the mineral
formed by oxidation of green rust during storage, or by
reduction in the aquifer).
7. Other mechanisms of As release to ground water
This study confirms that reduction of FeOOH is a
good model for the release of high concentrations of As
to anoxic ground water in the Ganges Plain and, by
implication, worldwide, since reduction of FeOOH is
not a site-specific process. Other mechanisms, such as
reductive dissolution of Mn oxides (Smedley and
Kinniburgh, 2002), competitive exchange with HCO3
(Appelo et al., 2002), and pH-dependent desorption
from, or recrystallization of, FeOOH (Welch et al., 2000;
Smedley and Kinniburgh, 2002), are not needed. The
reasons why are discussed below.
7.1. Reduction of Mn oxides
In JAM wells, concentrations of dissolved As are low
(<33 lg L1 ) where concentrations of Mn exceed 1
mg L1 (Table 3; Fig. 8(a)). The reduction of Mn oxides
is thermodynamically more favourable than is the reduction of FeOOH (Edmunds, 1986; St€
uben et al., 2003)
and occurs before it. It follows that As, originally sorbed
to Mn oxides but released by reduction, must be
resorbed to FeOOH (or residual Mn oxides) rather than
be released to ground water (St€
uben et al., 2003). This
finding is contrary to that of Smedley and Kinniburgh
(2002), who state that ‘‘Manganese oxides also undergo
reductive desorption and dissolution and so could contribute to the As load of ground waters in the same way
as Fe’’.
7.2. Exchange of sorbed As with HCO3 in ground water
Potentially toxic concentrations of As in ground
water are not common, and are particularly uncommon
in potable water from oxic aquifers, in which HCO3 is
the commonest and most abundant anion worldwide: it
follows that the suggestion that As is released to solution
by competitive exchange with HCO3 (Appelo et al.,
2002) is unlikely to be correct. Ground waters from a
variety of aquifer types in the UK contain HCO3 in
concentrations of hundreds of m L1 , yet As concen-
1282
J.M. McArthur et al. / Applied Geochemistry 19 (2004) 1255–1293
Table 5
Provenance, and content of oxalate-extractable Fe, of sediment from aquifers in alluvial and deltaic settings worldwide
Setting
Source of sediment
Nature of source
% of oxalate-extractable Fe
Min
Max
Data source
Bangladesh
West Bengal
Himalayas
Himalayas
Ig, Met
Ig, Met
0.1
0.05
0.5
0.16
West Bengal
Himalayas
Ig, Met
0.12
0.12
Campania, Italy
Sarno & Latari
Mountains
Allegheny Mountains
Ig, Sed (Lmst)
0.83
1.6
DPHE (1999, 2001)
This work, site DP,
oxidised
This work, site AP,
oxidised
Adam et al. (2003)
?
0.4
41
Knapp et al. (2002)
Coastal Virginia,
USA
Huhhot Basin,
Inner Mongolia
North Island,
New Zealand
NW Mississippi
Lower Kissimmee
Basin, Florida
Da Qing & Man Han
Mountains
North Island uplands
Met
0.67
3.4
Smedley et al. (2003)
Ig, Sed.
3.7
8.1
Mississippi uplands
Florida uplands
Sed
Sed Lmst?
1.2
0.07
1.9
0.25
Nguten and Sukias
(2002)
Rhoton et al. (2002)
Reddy et al. (1995)
trations are typically <10 lg L1 and natural concentrations above 50 lg L1 are rare (BGS, 1989). Moreover, concentrations of HCO3 in JAM are highest (744
mg L1 at site DP, Table 2; Fig. 3) where the ground
water contains less than 5 lg L1 of As. Furthermore,
the large range of As concentrations at AP and FP bear
no relation to concentrations of HCO3 (Figs. 4, 5): this is
best seen in the lower part of the aquifer at AP (Fig. 15),
where concentrations of As decrease from 419 lg L1
(38.3 m) to 7 lg L1 (42.8 m) whilst pH varies by less
than 0.2 units, HCO3 increases from 526 to 622 mg L1 ,
dissolved silica (as H4 SiO4 ) varies by only 4 mg L1 ,
dissolved PO4 decreases from 2.3 to 0.13 mg L1 , and dissolved Fe decreases from 3.9 to 0.97 mg L1 . These data
emphasise the minimal role played by pH, HCO3 , and
silica, in affecting concentrations of As in ground water
in JAM and, by implication, in sediments elsewhere.
Competitive exchange with natural organic matter
(Redman et al., 2002), or other ligands (e.g. silica), may
become important when considering mechanisms of As
release that lead to concentrations of As of <50 lg L1
but, even at low concentrations, partial reduction of
FeOOH and release of kinetic As (As released by reduction but soon resorbed to residual FeOOH, Fig. 13)
may be adequate to explain low concentrations of As in
anoxic environments.
7.3. Desorption of As induced by increasing pH
Sorption of As to solids, notably sedimentary FeOOH, is generally held to be pH dependent, leading to
the suggestion that desorption of As from mineral surfaces may contribute As to ground water as pH rises
(Welch et al., 1988, 2000; Robertson, 1989; Smedley and
Kinniburgh, 2002). A general observation that waters of
higher pH tend to have higher concentrations of As
(Welch et al., 1988; Robertson, 1989) is not in itself
adequate to prove a causal connection between the two:
other factors, such as residence time, evaporation, and
weathering, may cause an increase in both. Whilst pHdependent desorption may release As to ground water in
plumes of landfill leachate, or hydrocarbons where pH
can exceed 10, there is little direct evidence that this
process releases As to potable ground water, in which
pH rarely exceeds 8.5. In waters from JAM, As concentrations range from <1 to 1180 lg L1 , despite the
narrow range of pH (6.8–7.3; Table 3), a fact best
illustrated by data from the base of the AP site (Fig. 15).
It follows that pH is not a control on any sorption or
desorption of As that may occur in JAM aquifers.
Arsenic concentrations were linked to pH by Welch
et al. (1988): the mechanism of pH-dependent desorption was used by Robertson (1989) to explain the presence of As in oxic potable ground water from 24 alluvial
basins in Arizona, and has since been widely invoked.
Robertson (1989) noted that concentrations of As increased as pH increased along flowlines in aquifers in
several alluvial basins, and assumed the relation was
causal. Many indicators in his data suggest otherwise: in
Verde Valley ground waters, As concentration decreased
with increasing pH (as he noted); the overall correlationcoefficient between pH and As concentration for his 24
sites was low (0.18, n ¼ 456; his Table 2); salinity increased along flowlines by a factor of between 4 and 8
(judged from his Fig. 3) suggesting either extensive
evaporo-concentration, mixing with deeper more saline
waters, or weathering, occurred as the water passed
down the aquifer. It seems likely that As accumulated in
J.M. McArthur et al. / Applied Geochemistry 19 (2004) 1255–1293
1283
Fig. 15. Detailed profiles of water composition in the basal aquifer at Site AP, showing the invariance in pH and in the concentrations
of HCO3 , and H4 SiO4 , in contrast with the change in concentration of dissolved As, PO4 and Fe.
solution along some flowlines in response to in-situ
weathering of volcanic rocks, and that the accompanying increase in pH was a consequence of such weathering, not a cause of As accumulation. Robertson (1989)
noted that ‘‘the basins containing the largest concentrations (of As) are bounded primarily by basalt and
andesite’’ and so the basin-fill must be weathered materials from those rocks, which may have moderated the
concentrations by resorption to weathered sediments.
Robertson’s (1989) observation that concentrations of
As correlated well with concentrations of Mo, F and V,
at sites examined in detail, parallels the observation of
Nicolli et al. (1989) that As, fluorine, and V, are associated in As-enriched ground water in central Argentina;
both the association and the As enrichment were ascribed by Nicolli et al. (1989) to the weathering of dacitic
volcanic ashfall layers in the loess aquifers.
The relation of pH to As in solution can be further
illustrated by reference to ground waters in aquifers of
metamorphic bedrock in New England, USA (Ayotte
et al., 2003). Higher concentrations of As (>10 lg L1 )
occur in waters with higher pH (7.5 to 9.3), but many
samples, with pH ranging from 5 to 8.8, contain <2
lg L1 of As (Fig. 4 of Ayotte et al., 2003): this would
not be the case if As concentrations were linked to pHrelated desorption. These authors note that the primary
control on As concentrations is bedrock type. Furthermore, concentrations of As in UK aquifers are not related to pH (BGS, 1989): concentrations of <4 lgl
occur in aquifers with a pH range from 7.0 to 9.5 (e.g.
the Lincolnshire Limestone, Table 5.3 of BGS (1989))
and concentrations rarely exceeded 50 lg L1 , whatever
the pH. Thus, the observation made by Welch et al.
(1988) of a weak positive correlation between pH and
concentrations of As in a large dataset of compositions
of US ground waters may have explanations other than
pH-induced desorption.
Finally, Dixit and Hering (2003) show that sorption
of both As(III) and As(V) to Fe oxides becomes less
sensitive to pH as As concentrations, and Asaq /Fesediment
ratios, decrease. At their lowest As concentration (750
lg L1 ), which approaches those found in the Bengal
Basin, these authors showed that sorption is almost independent of pH over the range of pH common in most
ground waters. Their data confirm that pH-dependent
sorption does not contribute significantly to the release
of As to anoxic potable ground waters of the composition, pH-range, and salinity typical of potable waters
from alluvial aquifers.
8. The Himalayan connection
8.1. The Bengal Basin
The authors postulate that the widespread occurrence of As concentrations above 100 lg L1 occurs only
when sorption sites for As in an aquifer are saturated i.e.
when reduction of FeOOH has gone largely to completion. Because high concentrations of As are common in
Holocene sediments, they evidently have a particular
susceptibility to undergo complete reduction of FeOOH.
the authors explain this as follows. The Holocene sediment in the Bengal Basin derives from Himalayan rocks
1284
J.M. McArthur et al. / Applied Geochemistry 19 (2004) 1255–1293
that are (mostly) igneous and metamorphic. Other than
adjacent to the Indo-Burman ranges, surface coatings of
FeOOH, with sorbed As, are not inherited from previous erosional cycles but must be created from crystalline
rocks by weathering. The aquifer sands largely derive
from erosion at a time of extended glaciation in the
Himalayas when physical weathering dominated and the
intensity of chemical weathering was limited by the low
(glacial) temperatures during erosion. The immature
sediment was transported rapidly to the lower reaches of
the basin and buried in a waterlogged, anoxic environment and so had little opportunity to acquire much
FeOOH through oxidative weathering. Further, much of
the fine fraction was winnowed during transport and
deposited offshore, thereby reducing the surface area of
sediment available to host coatings of FeOOH. The
abundance of fresh-looking muscovite and biotite in the
Holocene sediments attests to the limited degree of
chemical weathering they have undergone. Older sediments, such as the deep aquifer in Bangladesh, and
sediment at DP, and below 32 m depth in JAM, has
suffered more weathering.
In Table 5, the content of diagenetically available Fe
(as acid-oxalate leachable Fe) in oxidised brown sediments from JAM (from the base of the aquifer at AP,
and from DP) is compared to that in sediments from
elsewhere and shown to be low (<0.2%) in comparison.
As the brown sediment at depth in JAM is of pre-Holocene age and has probably suffered a period of oxidative weathering (that would have increased its content
of FeOOH), the original Holocene sediment may have
contained even less. Admittedly, such comparisons are
fraught with uncertainty: the leachable Fe may be in
vivianite, green rust, magnetite, Fe sulphides, chlorite,
and carbonate, as well as in oxides. Nevertheless, this
data lends some support to the idea that ground water in
the Bengal Basin is polluted by arsenic because its sediments contain little FeOOH (to sorb As) but have a
superabundance of organic matter with which to reduce
it, so total reduction is common. In this context, the
comment of Harvey et al. (2002, p. 1604), that greycoloured sediment in an As-enriched profile in the
Ganges Plain ‘‘contains little, if any, purely ferric
[Fe(III)] oxyhydroxides’’, confirms that, where high
concentrations of As are common, reduction of FeOOH
has gone effectively to completion.
8.2. Implications for other areas
In aquifers that contain much FeOOH and/or little
organic matter, reduction of FeOOH will be incomplete
and the release of As by reduction will be moderated by
resorption to residual FeOOH. Such a process may be
moderating As concentrations in groundwater in, for
example, the alluvial aquifers of the Tualatin Basin,
Oregon (Hinkle, 1997), where ground waters contain
modest concentrations of As (<1–73 lg L1 ), and where
those that exceed 50 lg L1 also contain some Fe (0.16–
1.9 mg L1 ) and PO4 (0.36–2.0 mg L1 ). Hinckle and
Polette (1999) speculated that FeOOH reduction was
one plausible mechanism of As release to ground water
in reduced ground waters of the basin. The authors
agree, but add that resorption of As to residual FeOOH
has probably kept its concentration low. A more extreme example of resorption control may occur in alluvial aquifers of eastern Arkansas, where ground water
contains up to 42 mg L1 of dissolved Fe, but less than
52 lg L1 of As (Kresse and Fazio, 2003). Although the
aquifer sediments contain lignite and peat, the amount
of organic matter available (i.e. biodegradable) appears
to have been insufficient to reduce all of the sedimentary
FeOOH, so concentrations of As are probably being
moderated by resorption to unreduced oxides. Conversely, the Song Hong (Red River) Basin, in the vicinity
of Hanoi, Vietnam, appears to be another basin where a
superabundance of peat (up to 5 m in thickness; BGS,
1996) occurs in the area most poisoned by As (Fig. 16)
and gives rise to ground water compositions similar
from those in the Bengal Basin (Fig. 17): ground water
in both areas is rich in As, Fe, NH4 and PO4 , and
probably for the same reason – the organic matter in the
Song Hong basis is abundant enough to drive FeOOH
reduction to completion. Lesser amounts of peaty sediment occur elsewhere in the Song Hong delta (Tanabe
et al., 2003) making it likely that toxic concentrations of
As are widespread in the area’s ground water.
9. Temporal trends in As concentrations
9.1. Reasons why the composition of ground water may
change with time
All ground waters change their composition with
time: the important question is, at what rate? In the
Bengal Basin, some reasons for real change are clear
from the chemical stratification of the ground water:
water, and hence As, may move by bulk flow, both
vertically and horizontally, and so redistribute As.
With vertical gradients in As concentration of up to
200 lg L1 m1 (at FP in JAM), a small migration of
the As front will severely poison aquifer levels that are
presently safe to exploit. Additionally, seasonal fluctuations of the water table might affect the As concentration of wells screened near the water table, although
this has not been observed over a monsoonal cycle
elsewhere (van Geen et al., 2003a). In the longer term,
redox reactions may deplete the store of As, or FeOOH, or redox driver, in the sediments and so cause
the water composition to change with time. One thing
that is unlikely to cause change is a change in the rate
of ground water flow. Microbial reduction of FeOOH
J.M. McArthur et al. / Applied Geochemistry 19 (2004) 1255–1293
occurs on a timescale of hours to days (e.g. Dowdle
et al., 1996; Jones et al., 2000; Zobrist et al., 2000), as
do in-situ redox reactions in the Ganges Plain (Harvey
et al., 2002). Such rates are rapid in comparison to
rates of ground water movement in the Ganges Plain,
which are typically in the range of a few centimetres
per day or less (Ravenscroft et al., 2001; Harvey et al.,
2002), and so are likely to be de-coupled from rates of
ground-water flow.
Some reasons for apparent change with time in
ground water composition include:
(1) improving well-location as understanding of As pollution improves;
(2) poor or inconsistent techniques for sampling, sample
storage, or analysis;
(3) inappropriate purging volume;
(4) misidentification of the well;
(5) transcription errors.
9.2. Long-term changes in the Bengal Basin
There are insufficient time-series data to demonstrate
or disprove a long-term change in water composition
with time in the Bengal Basin, but tentative conclusions
1285
on long-term trends in composition can be inferred from
data on the ages and As concentrations of wells documented in regional surveys (e.g. DPHE, 1999, 2001; van
Geen et al., 2003a). For the shallow aquifer of Bangladesh, the proportion of wells falling above various
threshold concentrations of As, plotted as a function of
well age (Table 6; Fig. 18), suggests that, for the first ten
years of existence, the exceedance for most threshold
values of As increases as well-age increases, irrespective
of region or threshold. A similar effect is not seen in data
for Na (Fig. 18), an element that is largely conservative
in solution, unaffected by redox transformations, and so
is used as a comparator. For ages greater than 10 a, the
large uncertainties in the proportion of wells exceeding a
given threshold, which arise as a result of the paucity of
older wells, makes uncertain the validity of an apparent
increase in concentrations towards older ages. Similar
deductions were made for a data set of 6 000 wells from
the region of Araihazar (van Geen et al., 2003a), but
were based on linear regression to highly skewed data
(without log-transformation) and so may not be robust.
The trend to 10 a seen in the DPHE data probably does
not reflect the fact that an increasing awareness of the
As problem might have lead drillers to site wells more
Fig. 16. Comparison of the distribution of As in ground waters around Hanoi (after Berg et al., 2001) with the distribution of peat
deposits (after BGS, 1996). The inner box shows the extent of the BGS map, and the lightly stippled area is where peat is >5 m thick.
There is some indication that As enrichment is greatest where the peat is found, and further indication that the enrichment is greatest
where the peat is thickest, to the immediate south of Hanoi.
1286
J.M. McArthur et al. / Applied Geochemistry 19 (2004) 1255–1293
JAM Area, West Bengal
Hanoi Area, Viet Nam
Faridpur Area, Bangladesh
Mn mg L
-1
4
3
2
1
0
0
5
10
15
0
20
5
Fe mg L-1
10
15
20
0
5
Fe mg L-1
10
15
20
Fe mg L-1
PO4 mg L
-1
12
10
8
6
4
2
0
0
5
10
15
SO4 mg L
-1
NH4 mg L
0
20
5
-1
10
15
NH4 mg L
0
20
5
-1
10
15
NH4 mg L
20
-1
40
30
20
10
0
0
200
400
600
HCO3 mg L
800
0
200
-1
400
600
HCO3 mg L
-1
800
0
200
400
600
HCO3 mg L
800
-1
Fig. 17. Comparison of the concentrations of Fe, Mn, PO4 , NH4 , SO4 , and HCO3 in wells tapping the As-affected shallow aquifers of
Bangladesh (data from DPHE, 1999), West Bengal (this work) and the Song Hong (Red River) Basin around Hanoi (data from BGS,
1996, which did not report concentrations of As). All 3 areas contain waters rich in Fe, As, PO4 , and NH4 , which shows the influence of
much reduction of FeOOH and organic-matter degradation.
carefully than before – perhaps on the basis of sediment
colour. Two considerations suggest that the trends may
be real. Firstly, sampling was competed by 1998, when
the As problem was not widely known about in rural
areas. Secondly, there is a distinct difference in trend
between redox-sensitive As and the more conservative
tracer Na (Cl was not analysed for the national survey of
Bangladesh, so could not be modelled).
In the deep aquifers of the Bengal Basin, the establishment of anoxic conditions in the deep aquifer, as a
consequence of the deposition of overlying, organic-rich,
Holocene aquifers, has been sufficient to mobilise some
Fe into groundwater in the deep aquifer (Table 3;
DPHE, 1999), but has been insufficient to reduce the
entire pool of FeOOH, which may now act as a sorptive
buffer to prevent P or As from accumulating in solution.
If FeOOH in the deep aquifer of the Bengal Basin does
host sorbed As (i.e. the deep aquifer has not been flushed
of it – see earlier), the potential exists for downwardmigrating organic matter to eventually reduce all its
FeOOH and so mobilize As. The data in Table 7,
compiled from DPHE (1999), suggest that this is not yet
happening. In the most As-polluted enriched areas of
Bangladesh, deep wells have remained free of As for up
to 16 a.
9.3. Short-term trends in JAM
In order to examine whether wells in the JAM area
changed over the short time encompassed by the present
study, the wells were sampled on 3 separate occasions
over 18 months; in February, 2002, November, 2002,
and March, 2003. In the most precise data viz. electrical
conductivity, Ca, Mg, and Cl, some differences are seen
in water composition between samplings. Changes are
small and best illustrated by the correlation in relative
J.M. McArthur et al. / Applied Geochemistry 19 (2004) 1255–1293
0.523
0.597
0.45
0.184
0.036
0.921
0.963
0.968
0.899
0.665
24
55
66
22
7
Depth <100 m: As
25
0.173
50
0.139
100
0.076
200
0.002
400
)0.019
0.587
1.406
0.645
0.204
0.041
0.944
0.985
0.982
0.921
0.736
34
143
120
28
9
Barisal: As
25
50
100
200
400
)0.135
)0.093
)0.044
)0.014
0.001
0.51
0.415
0.425
0.346
0.1
0.797
0.786
0.858
0.899
0.925
11
10
16
22
29
Khulna: As
25
50
100
)0.222
)0.333
)0.318
0.535
0.457
0.26
0.713
0.682
0.581
8
7
6
Chittagong: As
25
)0.199
100
)0.121
0.661
0.687
0.895
0.951
23
49
Rajshahi: As
25
0.103
0.772
9
0.416
0.329
0.233
0.15
0.568
0.505
0.391
0.451
4
4
3
4
change between these determinands (Fig. 19). The correlation between Ca and Mg might be an analytical artefact – despite the authors’ belief in its reality–because
both were measured on the same solution, with the same
ICP-AES, at the same time. The measurements of Cl
and electrical conductivity were made independently of
Ca; Cl by ion chromatography in the laboratory, and
electrical conductivity by conductivity meter in the field.
The correlation between Ca and these determinands
must therefore be real. No changes were seen in con-
0.8
0.6
0
0.2
0.4
0.6
0.8
As > 50 µg L -1
a = 0.139
b = 0.146
c = 0.985
All wells
L -1
0.4
0.6
0.8
Na > 60 mg
a = - 0.199
b = 0.329
c = 0.505
0.2
Khulna: As ¼ 200, 400; no fit.
Chittagong: As ¼ 50, 200 400; no fit.
Rajshahi: As ¼ 50, 100, 200, 400; no fit.
t90 ¼ time in years to reach 90% of the plateau value.
Wells < 100m deep
0
1
All wells: Na mg L
40
)0.048
60
)0.199
100
)0.281
200
)0.182
0.4
All Wells: As lg L1
25
0.159
50
0.122
100
0.068
200
)0.005
400
)0.025
)0.001
t90
-1
0.2
c
As > 50 µg L
a = 0.122
b = 0.597
c = 0.963
0
b
Proportion over threshold
a
Proportion over threshold
Threshold
value
All wells:
Proportion over threshold
Table 6
Non-linear models for the proportion of wells exceeding given
threshold concentration of As, as a function of age. The data
are fitted with the function P ¼ b ðb aÞct , where P is the
proportion exceeding a given threshold, a is the intercept, b is
the plateau value, c is a fitted constant and t is time in years.
Data from DPHE (2001)
1287
0
10
20
30
Well age in years
Fig. 18. Trend of As concentration in wells with well age,
represented by a model showing the proportion of wells above a
threshold concentration as a function of time. Fitted numerically by the function [proportion over threshold] ¼ bðb aÞct ,
where a is intercept, b is plateau value, c is a constant and t is
time in years.
J.M. McArthur et al. / Applied Geochemistry 19 (2004) 1255–1293
centrations of As: the analytical repeatability for As
analysis was around 10%, so the magnitude of any
changes must be less than this amount; the data do not
rule out changes in As concentration, but merely constrain the rate of change. Indeed, the piezometer profiles
suggest that the depths at which ground water is enriched will increase as organic matter moves downwards
through the aquifer, consuming FeOOH, although
subsequent migration of the As released may be slowed
by adsorption to such phases elsewhere in the aquifer.
4
Ca % difference
1288
2
0
-2
-4
-5
-4
-3
-2
-1
0
1
2
3
4
5
Mg % difference
9.4. Changes induced by irrigation pumping in the Bengal
basin
Ca % difference
4
2
0
-2
-4
-5
-4
-3
-2
-1
0
1
2
3
4
5
25
30
Measured EC % difference
4
Ca % difference
An important issue is whether pumping for irrigation
is spreading As through the shallow aquifers of the
Bengal Basin, or is flushing them of As by inducing
additional recharge. The authors believe the latter is the
case. This contention may be best illustrated by site FP
in the JAM area (Fig. 5). The piezometer profile shows
that As concentration increases steeply between 31.2 and
37.0 m depth (or, given that there are only two control
points, somewhere between). Given the large amount of
organic matter at 29.5 m depth, the increase should
occur across this unit and give higher concentrations of
As at 31.2 m. That it does not suggests that the entire
profile has been translocated downwards by between
about 2 and 9 m by abstraction of water from shallow
and deep aquifers. Such movement would also have
lowered As concentrations above 19.2 m depth (Fig. 5)
by drawing in new water to give the present profile.
Water above 19.2 m depth has concentrations of Ca and
Na quite different to those in deeper waters, adding
support to this suggestion.
Perhaps more convincingly, the depth profiles of
Harvey et al. (2002), for groundwater in, Bangladesh,
show that concentrations of dissolved constituents derived from differing sources (e.g. As from reduction of
FeOOH; dissolved organic C, NH3 and CH4 from organic matter degradation; Ca from mineral weathering;
HCO3 from multiple sources) all increase with depth to a
peak around 35–40 m depth. Such a concordance of
profiles chemically different species can be induced only
by flushing, not by reactions in the aquifer. In this
context, it is interesting to compare the profiles in
Munshiganj and JAM.
2
0
-2
-4
-15
-10
-5
0
5
10
15
20
Cl % difference
Fig. 19. Percentage difference in Ca concentration in wells,
sampled in February 2002 and March 2003, plotted against the
% change in Mg concentration (both measured by ICP-AES),
the % change in EC (field measurement) and the % change in Cl
concentration measured by ion chromatography.
In JAM, As concentrations reach a maximum at
depth in FP (Fig. 5) because of the OM-rich layer at 29.5
m depth. At AP, however (Fig. 4), the maximum is not
Table 7
Summary of As concentrations, depth, and age, for deep wells in As-affected areas of Bangladesh. Data from DPHE (2001)
Area
No. of
deep wells
Age range
in 1998
Range of
As lg L1
Depth range (m)
% of shallow wells
with >50 g L1 As
Chandpur
Lakshmipur
Noakhali
4
6
6
2–3
3–10
5–15
0–6
0–8
2–10
221–269
183–318
246–292
98
64
79
J.M. McArthur et al. / Applied Geochemistry 19 (2004) 1255–1293
developed strongly (if data from the upper aquitard are
excluded). The progression from a ‘flat’ profile, similar
to that seen at AP, to the ‘peaked’ profile seen in
Munshiganj can be achieved by depth-dependent flushing, which would lower the concentration of all species
in the upper part of the aquifer (Fig. 20). Harvey et al.
(2002) propose or imply, instead, that the shape of many
element profiles results from redox reactions that are
driven by organic matter from surface sources; this is
unlikely, as dissolved species move down, not up, concentration gradients, and organic matter cannot move
downwards and increase in concentration as it is being
consumed by reaction. The suggestion of a surface
source of organic matter is also at odds with the distribution of 14 C-ages, as discussed by van Geen et al.
(2003b), who point out that Harvey et al. (2002) report
measurable amounts of 14 C to 19 m depth but not below.
A maximum in dissolved As concentration around
20–40 m depth is common across the Bengal Basin
(Karim et al., 1997; DPHE, 1999; McArthur et al.,
2001), but not universal: for example, in Araihazar,
Bangladesh, it appears to be around 15 m depth (Fig. 6
of van Geen et al., 2003a), whilst in the region of Sylhet,
it is around 60 m and may even be bi-modal, although as
a result of differential subsidence, not pumping (Ravenscroft et al., 2001). The explanation that shallow
maxima in As concentrations are caused by flushing of
shallow levels in the aquifers seems reasonable because
natural flow, and abstraction, particularly for irrigation,
are confined to shallow depths – natural flow because of
the lack of topography and the lack of natural outlets at
deeper levels, the irrigation flow because shallow irrigation wells are at shallow depths in order to minimize
the cost of installation. Ground water dating by Dowling et al. (2002) confirms that young water may be
found to depths of 30 m in the Bengal Basin. If correct,
the authors’ explanation for peaked profiles of As shows
that the extensive development of ground water for irrigation is reducing the As problem in the ground water
across the Bengal Basin.
1289
ated by resorption to residual FeOOH, frequent high
concentrations of As (>100 lg L1 ) require the accumulation of organic matter in amounts sufficient to exhaust the entire store of FeOOH in a location that is in
hydraulic continuity with aquifer sands. This condition
is most easily met on the margins of buried peat basins,
where organic matter accumulates in low percentage
abundance in moderately permeable confining units or
in the aquifer sands themselves, or where significant
contributions from human and animal organic loading
exist locally.
As first proposed by P. Ravenscroft, as author of
DPHE (1999, V1–3, 5, 6), by Ahmed and Ravenscroft
(2000) and Ravenscroft et al. (2001), and since repeated
by Smedley and Kinniburgh (2002), the global rise in
sea-level between 18 and 6 ka, created settings worldwide for As-enriched aquifers to form. The large volume
of accommodation space that resulted is now filled with
recent sediments. The rates of sea level rise and sediment
supply have been the primary controls on wetland formation, where organic-rich muds and alluvial or deltaic
sands are closely juxtaposed. The problem of As in
ground water will therefore be most prevalent in depositional areas where climate was hot and wet during the
10. Wider considerations
The process of FeOOH-reduction is not site specific:
it can explain the As problem in many sedimentary basins, both coastal and continental, because Fe oxides
and sorbed As are ubiquitous components of clastic
sediments. Whether the reduction of FeOOH will push
As concentrations to hazardous levels depends on how
near the process goes to completion. That, in turn, is
governed by the balance between the amount of FeOOH
and the amount and reactivity of organic matter available to reduce it. Whilst concentrations of up to a few
tens of lg L1 of what the authors have called kinetic As
may prove common where concentrations are moder-
Fig. 20. Development of a peaked profile in As concentration
as a result of abstraction for irrigation and public supply.
Heavy pumping of the shallow aquifer for irrigation would
enhance the rate of renewal of water in the aquifer to the depth
of pumping. From an initial profile, A, similar to that seen in
JAM at site AP, would develop a peaked profile, B, similar to
that seen in Munshiganj by Harvey et al. (2002), by rapid renewal of water in the upper aquifer (shaded area between A and
B) to a degree that diminishes with depth. Pumping from the
deep aquifer (not shown) beneath the lower aquitard would
induce slow translocation downward of the entire profile B.
Localized OM-rich sediment shown as black stringers.
1290
J.M. McArthur et al. / Applied Geochemistry 19 (2004) 1255–1293
climax of nearshore sediment deposition around 4–8 ka
and where there was limited chemical weathering in the
upper catchment. A set of globally applicable parameters–the depth of pre-Holocene incision, the relative
rates of sea level rise and alluvial aggradation, the Holocene climatic record, and the geology of the upper
catchment–therefore provide a general model for predicting the possible occurrence of As in coastal alluvial
basins. Inland basins lack the drainage control imposed
by global sea-level, so inland basins will be subject to
local base-level controls and application of the outline
model to such areas may be difficult. In a global context,
the physical and demographic characteristics of the
Bengal Basin represent an extreme, in which the extreme
topography of the hinterland, its weathering under extremes of climate, and its crystalline geology, have
conspired to exacerbate the pollution by minimising
chemical weathering and production of FeOOH. Other
areas that might be expected to suffer from the As
problem, although perhaps to a lesser degree, include the
deltas of tropical rivers such as the Mekong, Irrawaddy,
and Chao Phraya rivers, and the coastal plains of Java
and Sumatra.
11. Conclusions
In the Bengal Basin, As is released to solution by
reductive dissolution of FeOOH and release of its sorbed
As to ground water. In the study area near Barasat in
southern West Bengal, and by implication elsewhere in
the Bengal Basin and worldwide in alluvial aquifers, the
reduction is driven by natural organic matter buried in
sediments. In JAM, the organic matter exists in moderate percentage abundance within the upper confining
units of the aquifers and/or in discrete organic-rich units
within aquifer sands. The organic matter accumulated in
peat basins; the portion that drives reduction in the
aquifer is that which accumulated on the margins of the
basin where the sediments are sufficiently permeable to
allow infiltration to the aquifer of the soluble organic
products of fermentative degradation of the organic
matter. Within the JAM area, which is typical of much
of the southern Bengal Basin, surface sources of organic
matter do not contribute to the reduction of FeOOH
and so do not result in ground water containing high
concentrations of As. In areas of the Bengal Basin where
dug latrines penetrate the upper confining layer, sewage
in hydraulic continuity with the aquifer may locally
promote additional reduction of FeOOH and add to the
As problem. Unless populations reach an urban density,
however, this process will not exacerbate the regional As
problem but will operate at the scale of individual wells.
Where the reduction of FeOOH goes to completion,
As concentrations often exceed 100 lg L1 . Where reduction of FeOOH is incomplete, concentrations of ki-
netic As in solution are moderated (to <100 lg L1 ) by
resorption to residual FeOOH. The extent of As mobilisation will be controlled by the balance between the
quantity and biodegradability of organic matter, and the
size of the FeOOH buffer available to sorb As. Reduction of FeOOH in the Bengal Basin commonly goes to
completion because the sediments contain abundant
natural organic matter to drive reduction but little FeOOH, owing to the immaturity of the sediment, which is
produced and transported rapidly from a crystalline
Himalayan source to its place of accumulation in the
anoxic environment of the Bengal Basin.
Acknowledgements
We are indebted to the inhabitants of Joypur,
Ardivok, and Moyna for allowing us to sample their
wells. Particular thanks are due to Ali Ahmed Fakir,
for his assistance over several years with logistics, and
to him, Mrs. D. Chandra Ghatak, and Dual Ch. Shee,
for allowing us to install piezometers on their land. We
thank A. Osborn for assistance with analysis, much of
which was done in the Wolfson Laboratory for Environmental Geochemistry (UCL-Birkbeck, University of
London). Additional analysis was done using the
NERC ICP-AES Facility at RHUL, with the permission of its Director, Dr. J.N. Walsh. Thanks are due
Sam Tonkin for providing Fig. 1. This work was
supported by the Royal Society, the British Council via
a LINK Scheme grant to Delhi University and a new
investigators grant to KHE (NERC Grant No. GR8/
04292). A.C. thanks Drs. S. Peddle and R. Taylor for
their support. We thank Laurent Charlet, Doris
St€
uben, and an anonymous reviewer, for their helpful
comments about our work.
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