Applied
Geochemistry
Applied Geochemistry 20 (2005) 55–68
www.elsevier.com/locate/apgeochem
Arsenic and other drinking water quality issues,
Muzaffargarh District, Pakistan
R.T. Nickson
a
a,*
, J.M. McArthur b, B. Shrestha a, T.O. Kyaw-Myint a,
D. Lowry c
Water and Environmental Sanitation Section, UNICEF Pakistan, Saudi Pak Tower, Islamabad, Pakistan
b
Geological Sciences, University College London, Gower Street, London WC1E 6BT, UK
c
Department of Geology, Royal Holloway University of London, Egham, Surrey TW20 0EX, UK
Received 11 April 2003; accepted 5 June 2004
Editorial handling by R. Fuge
Abstract
In 49 samples of groundwater, sampled in Muzaffargarh District of south-western Punjab, central Pakistan, concentrations of As exceeded the World Health Organisation provisional guideline value, and United States Environmental
Protection Agency (USEPA) Maximum Contaminant Level (MCL), of 10 lg L1 in 58% of samples and reached up to
906 lg L1. In this semi-arid region canal irrigation has lead to widespread water-logging, and evaporative concentration of salts has the potential to raise As concentrations in shallow groundwater well above 10 lg L1. In fact, in rural
areas, concentrations stay below 25 lg L1 because As in the oxic shallow groundwater, and in recharging water, is
sorbed to aquifer sediments. In some urban areas, however, shallow groundwater is found to contain elevated levels
of As. The spatial distribution of As-rich shallow groundwater indicates either direct contamination with industrial
or agricultural chemicals, or some other anthropogenic influence. Geochemical evidence suggests that pollutant organics from unconfined sewage and other sources drives reduction of hydrous ferric oxide (HFO) releasing sorbed As to
shallow groundwater. The situation is slightly less clear for seven wells sampled which tap deeper groundwater, all of
which were found with >50 lg L1 As. Here As concentrations seem to increase with depth and differing geochemical
signatures are seen, suggesting that As concentrations in older groundwater may be governed by different processes.
Other data on parameters of potential concern in drinking water are discussed briefly at the end of the paper.
Ó 2004 Elsevier Ltd. All rights reserved.
1. Introduction
It is estimated that approximately one third of the
worldÕs population use groundwater for drinking
*
Corresponding author. Present address: SEPA, Graesser
House, Fodderty Way, Dingwall Business Park, Dingwall IV15
9XB, UK.
E-mail address: ross.nickson@sepa.org.uk (R.T. Nickson).
(UNEP, 1999). During the 1990s, naturally occurring
As was found to be widespread in groundwater in the
USA, Argentina, Taiwan, China, Hungary, Vietnam,
and the Ganges Plain (Smedley and Kinniburgh,
2002). The reduction of the World Health Organisation
(WHO) provisional guideline value for As concentration
in drinking water from 50 lg L1 to a provisional 10
lg L1 in 1993 (WHO, 1993), and the reduction in
2002 of the USEPA Maximum Admissible Concentration (MAC) to 10 lg L1, has been made in response
0883-2927/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.apgeochem.2004.06.004
56
R.T. Nickson et al. / Applied Geochemistry 20 (2005) 55–68
to growing concern about this poisonous carcinogen and
has raised awareness of the dangers of As in drinking
water. If groundwater is to remain a mainstay of public
water supply worldwide, there is an urgent need for better understanding of the mechanisms of natural As
enrichment in groundwater.
In view of the health concerns outlined above, and
alerted by the magnitude of the problem afflicting nearby Bangladesh and West Bengal, the Public Health
Engineering Department (PHED) and the Local Government and Rural Development Department
(LGRDD) of Pakistan, in conjunction with UNICEF,
recently undertook a survey of As concentration in
groundwater from drinking water supply wells in Pakistan (Shrestha, 2002). That survey revealed hot spots
of As enrichment in parts of the Indus alluvial basin.
The survey identified Muzaffargarh District as one enriched in As at concentrations in the low hundreds of
lg L1 range. The authors sampled groundwaters in
the area to investigate the distribution and associations
of As enrichment with the aim of elucidating the reasons
for this enrichment.
During the investigation, the authors also found As
Ôcold spotsÕ. These were areas where evaporative concentration of groundwater might have been expected to result in high concentrations of As in groundwater, but
where concentrations were, in fact, below levels of concern. It is inferred that such wells tap groundwater from
which As has been removed by sorption to oxic aquifer
sediments. Given the widespread use of irrigation, and
the potential danger signalled by the recent reductions
in drinking water guideline values and MACs to values
approaching the As concentrations found in rivers and
lakes (Cullen and Reimer, 1989; Smedley and Kinniburgh, 2002), the power of oxic aquifers to remove As
from aquifer recharge (and returning irrigation water)
deserves further investigation.
2. Geography, geology and geomorphology of the study
area
The district and town of Muzaffargarh are in central
Pakistan at the south-western edge of the Punjab
(Fig. 1). The District lies upon the Thal Doab, an area
that lies between the Indus River (to the west) and the
Chenab River (to the east), which join some 100 km
south of Muzaffargarh town. The study area is triangular in shape and situated towards the southern end of the
Thal Doab where it abuts the Chenab River. The area is
bounded by Muzaffargarh town in the south, and the
settlements of Rangpur in the NE and Chowk Sarwar
Shaheed in the north (Fig. 1). The area comprises abandoned floodplain terraces of the Chenab River, which
are covered at inland sites (site 14 and northwards to
Chowk Sarwar Shaheed; site 33 and westward to same)
by windblown sands of the Thal Desert. The hydrology,
hydrogeology, climate and aquifer sediments of the Punjab have been described in great detail by Greenman
et al. (1967), from whose work the account of the southern end of Thal Doab is summarised in the following 3
paragraphs.
The area is underlain by >350 m of Quaternary and
older alluvial sediments derived from the Indus and
Chenab Rivers (Fig. 1). The sediments are very transmissive fine-to-coarse sands with little intercalated silt
or clay. The climate is semi-arid, with annual rainfall
of about 150 mm a1 and a mean annual air temperature of about 28 °C. Soils are permeable and lack organic matter. Recharge from rainfall is insignificant.
In pre-irrigation times, the margins of Thal Doab
would have contained abundant water of good quality
as a result of annual recharge from the Chenab and Indus rivers during monsoonal flooding. The regional
groundwater flow in the lower Thal Doab in 1960
was to the SE, obliquely across the long axis of the
Doab, owing to the influence of the nearby Indus River
at a higher elevation to the west.
Recharge from the rivers has diminished since canal
irrigation began in the 17th century. Since the rivers became wholly controlled by numerous dams constructed
in the last century, recharge has been greatly supplemented by loss from irrigation canals and by irrigation
water applied to the land. The irrigation has raised water
levels regionally so that water-logging and salinisation is
now common. In the present study area (Fig. 1), which
lies at the south-western end of the Punjab, and thus is
at its driest end, the impact of this agricultural development by 1960 was to raise the water table less than 3 m
(compared to 30 m elsewhere) and by less at its margins
where water level is controlled by river level. In the lower
Thal Doab, development of irrigation has been extensive
since Greenman et al.Õs (1967) work was completed
(around 1960) and the impacts on local groundwater
quality must have increased.
Groundwater quality at depths >30 m deteriorates
away from the Thal Doab margins and reaches >4000
mg L1 total dissolved solids in the Thal DoabÕs interior
as a result of long-term evaporative concentration of
salts in pre-irrigation times (Greenman et al., 1967).
Prior to the introduction of irrigation, a component of
local flow occurred from the Indus and Chenab Rivers
into evaporative sumps in the interior of the Thal Doab
that caused salinity to increase away from the rivers.
One such sump is centred by Greenman et al. (1967)
some 13 km WSW of Muzaffargarh town and the
authors have sampled the margins of that area as well
as less saline areas bordering the Chenab River. In much
of the Punjab, irrigation and leakage of water from irrigation canals has disturbed the original groundwater
flow: recharge from return irrigation water and leakage
from irrigation canals has raised water levels and created
R.T. Nickson et al. / Applied Geochemistry 20 (2005) 55–68
57
Fig. 1. Location of the District and town of Muzaffargarh, south-western Punjab, Pakistan, showing location of sample sites. Large
inset to the right shows location of study area in Pakistan. Smaller inset at the foot of the figure shows detail of wells sampled in
Muzaffargarh Town.
a shallow aquifer body that is different in composition to
older, pre-irrigation, water, which still exists at depth in
the aquifer. As irrigation has increased the elevation of
water tables, the local pre-irrigation flow has diminished
or reversed in direction, and it is likely that in lower Thal
Doab a component of evaporated irrigation water now
contributes base flow to the Chenab River during its
low stage in the dry-season.
3. Methods
Water from the Chenab River, Taunsa Panjnad
Link Canal, and wells, was collected in July 2001
(Fig. 1; Table 1). Of the 49 wells sampled in the study
area, 42 tap water at <30 m depth, of which five were
electrically pumped water-supply wells, and 37 were
hand-pumped tubewells. The other seven tap water of
>30 m depth, all of which were electrically pumped
water-supply wells. One sample was also collected of
Chenab River water. All wells were purged of at least
one well volume before sampling. On collection, wellhead measurements were made of As, alkalinity, pH,
dissolved O2, and temperature (Table 1). Arsenic concentrations were measured in the field using the
MERCKÓ field-test kit, alkalinity was measured by
titration, pH and temperature using a Whatman PHA
260 pH meter, and DO2 using a Jenway DO probe.
Samples were collected in polyethylene bottles cleaned
with 50% HNO3. Samples for anion analysis were unfiltered and unacidified. For cation and As analysis, the
samples were unfiltered and acidified with HNO3 to
pH < 2. Samples were left unfiltered in order that an
accurate value could be obtained for As that would
be consumed by well users who do not filter their water
before use. In order to assess particulate concentrations
to the dissolved load, the concentration of Al was
measured and turbidity recorded. All but 4 samples
were crystal clear and without turbidity and so they
were taken to be unaffected by particulate matter
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R.T. Nickson et al. / Applied Geochemistry 20 (2005) 55–68
Table 1
Sample location and hydrochemical data
R.T. Nickson et al. / Applied Geochemistry 20 (2005) 55–68
59
Table 1 (continued )
Note. SDPW is ÔSmall diameter electrically-pumped wellÕ, LDPW is ÔLarge diameter electrically-pumped wellÕ and HP is ÔHandpumped wellÕ.
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R.T. Nickson et al. / Applied Geochemistry 20 (2005) 55–68
contributing to dissolved concentrations. For three
samples (6, 36 and 50; Table 1), turbidity was zero,
but several grains of coarse sediment were present; nevertheless, dissolved Al concentrations after acidification
were less than 50 lg L1, so contributions to dissolved
constituents from particulate matter are not thought to
be important. Mass-balance calculations show that contributions to As from flocs of hydrous ferric oxyhydroxides (HFO) are insignificant, as the maximum
concentration of dissolved Fe in samples is 3 mg L1,
which would contribute 3 lg L1 of As were it to come
wholly from particulate Fe-floc containing 1000 ppm
As, which is a high figure compared to other estimates
of the amount of As in sedimentary HFO (e.g. 500 ppm
by Nickson et al., 2000). Organic particulate matter,
which is a potential carrier of As, was not assessed in
the samples although it is not thought that this is likely
to be significant.
Samples were analysed using ion chromatography
(major anions, NH4), inductively coupled plasmaatomic emission spectrometry (major cations, Al) and
furnace-atomic absorption spectrometry (As), and colorimetrically (H4SiO4) according to standard procedures
using laboratory standards and National Institute of
Standards and Technology (NIST) Water Reference
Material 1643d as control. Blanks and duplicates taken
every five sample sites allow an assessment of the reliability of data. The data are given in Table 1. Ionic
balances,
calculated
as
100*(cations anions)/
(cations + anions), were ±7% with two outliers of
9%, and 13%, for which no explanation is forthcoming; the mean balance is 0.3%.
Canal end-member and is likely affected by ionexchange.
4.2. Arsenic
Of 49 wells sampled, 21 contain <10 lg L1 of As
and 21 contain >50 lg L1. Wells containing <50 lg L1
of As are found both within and outside of urban areas,
but wells containing >50 lg L1 are found only within
urban areas. Seven wells tap groundwater deeper than
30 m and 42 tap groundwater shallower than 30 m.
All wells more than 30 m deep are in urban areas and
all contain >50 lg L1.
An important observation is that As is not conservative in solution (Fig. 3); many waters contain between 10
and 100 times less As than expected from evaporative
concentration of end-members (water from the TPLC
or Chenab River), whilst others contain much more.
In particular the majority of the samples taken from
wells tapping deeper groundwater are enriched in As
(solid symbols, Fig. 3).
The authors focus on As in the discussion that
follows and consider the genesis of waters that contain
unexpectedly high levels of As and those with unexpected low levels. In doing so the As/Cl values measured
in the TPLC (0.98) and Chenab River (0.25) during
June 2001, when the river flow was at its peak are used
as end members. The authors are unable to say what
the range of such values might be through the seasons,
but suggest that for older (pre-irrigation) water the value of 0.25 might be appropriate because most recharge
would have occurred when the river stage in the past
was high.
4. Results
5. Discussion
4.1. Major ions
Natural enrichment of groundwater by As can arise
in several ways (for a review, see Welch et al., 2000),
viz. hydrothermal volcanism, oxidation of arsenical sulphide minerals (see also Schreiber et al., 2000), reduction
of FeOOH and release of its sorbed load to groundwater
(see Matisoff et al., 1982; Korte and Fernando, 1991; Nimick, 1998; Nickson et al., 1998, 2000; McArthur et al.,
2001; Ravenscroft et al., 2001), desorption of As from
mineral sorption sites in response to increase of pH
(Robertson, 1989), and evaporative concentration (see
also Nicolli et al., 1989). This last mechanism might pose
a threat to groundwater from As in any area where natural evaporation over long periods has caused solute
concentrations in shallow groundwater to increase
(Welch et al., 2000) or where return irrigation flows affect the irrigation source itself. Mitigating the effects of
evaporative concentration under oxic conditions will
be sorption of As to soils (e.g. Jones et al., 1999) and
aquifer sediments (Nimick, 1998).
Decades of natural recharge and evaporation, induced irrigation recharge and evaporative concentration
due to water-logging, have concentrated dissolved constituents in groundwaters in the Muzzafargarh area.
Conservative behaviour of Na, Cl, and SO4 are apparent
(Fig. 2), with samples plotting along evaporative trends
from end-members represented by either Chenab River
or the Taunsa Panjnad Link Canal (TPLC hereinafter;
Fig. 2). Calcium concentrations are less than those predicted by evaporative trends (Fig. 2). This can be explained by precipitation of calcite on evaporation:
samples highest in HCO3 are amongst the lowest in
Ca. Small departures from the evaporation trends may
also be accounted for by mineral weathering, gain or
loss of less soluble salts (gypsum) and ion-exchange
reactions: e.g. Well 36, with 8 mg L1 of Cl, has a lower
Ca/Cl and a higher Na/Cl than the Taunsa Panjnad Link
R.T. Nickson et al. / Applied Geochemistry 20 (2005) 55–68
61
1000
10000
100
Ca mg L
Na mg L
-1
-1
1000
100
10
1
10
1
1
10
100
1000
Cl mg L
(a)
1
10000
-1
10
1000
100
Cl mg L
(b)
1000
10000
1000
10000
-1
10000
SO4 mg L-1
Mg mg L-1
1000
100
10
100
1
10
1
1
10
100
1000
10000
Cl mg L-1
(c)
1
10
0
500
(d)
100
Cl mg L-1
60
1000
NO3 mg L
HCO3 mg L
-1
-1
50
30
20
10
0
100
1
(e)
40
10
100
Cl mg L
-1
1000
10000
(f)
1000
Cl mg L
1500
2000
-1
Fig. 2. Evaporative trends in the major ion composition of groundwater from Muzaffargarh District. Open symbols are wells less than
30 m depth, closed symbols are wells >30 m depth. Circles are samples from the rural area including and to the north of samples 15 and
31, triangles are samples from the Muzaffargarh area, squares are samples from Multan. Large open circle for Chenab River water
(sample 28) and double circle for sample 20, which is taken to be representative of the water recharging from the Taunsa Panjnad Link
Canal.
5.1. Groundwater with unexpectedly low levels of arsenic
Based on the As/Cl value of the TPLC (0.98) and
Chenab River (0.25) endmembers, the concentration of
As expected in the most saline sample (Sample 52, from
Chaha Thadi Wallah) as a result of evaporative concentration is up to 1.5 mg L1 (using the TPLC) or 375
lg L1 using the Chenab River as end members. This
sample actually contains 9 lg L1 of As. The authors ascribe the low concentration of As in this well water, and
others with low As/Cl values, to sorption of As onto
aquifer sediments during recharge and groundwater
flow. In oxic recharge water, the As will be present as
As(V), which sorbs strongly to HFO (Mok and Wai,
1994), a phase ubiquitous in alluvial aquifers. In geologically young alluvial aquifers, which generally contain
high levels of organic matter, HFO is routinely consumed as an oxidising agent in the process of decay of
this organic matter. Dissolved O2 and NO3, however,
are more thermodynamically favourable oxidising
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R.T. Nickson et al. / Applied Geochemistry 20 (2005) 55–68
not progressed to the stage of HFO reduction when
As(V) is reduced to As(III), the oxidation state of As
is likely to be As(V), except where local organic pollution occurs (see below). That oxic aquifer sediment sorbs
As from recharging river water was also the conclusion
reached by Nimick (1998) after studying As in shallow
groundwater of the Madison River valley, in Montana,
where recharge is largely from the arsenical Madison
River, which rises in the hydrothermal area of Yellowstone National Park.
1000
As µg L-1
100
10
1
0
1
10
100
Cl mg L-1
1000
10000
Fig. 3. Relation between concentrations of As and Cl in
Muzaffargarh District. Symbols as in Fig. 2.
agents in this process and are consumed preferentially
(Stumm and Morgan, 1981). Given that many of the
shallow groundwaters contain NO3 (Fig. 5), and so have
5.2. Groundwater with unexpectedly high levels of arsenic
The spatial pattern of As distribution in the study
area can be seen in Fig. 4. It can be seem that low As
groundwater (containing <50 lg L1 of As) is found
both within and outside of urban areas, but high As
groundwater (containing >50 lg L1) is only found
within urban areas. These observations would seem to
Fig. 4. Concentrations of As in Muzaffargarh District and its relation to wells, rivers and irrigation canals.
R.T. Nickson et al. / Applied Geochemistry 20 (2005) 55–68
suggest that the enrichment of groundwater with As is in
some way linked to urban areas.
The authors cannot entirely discount agricultural/industrial pollution as a source of As, as there is no available data on the use of arsenical pesticides or industrial
chemicals in the area. It is thought such sources of pollution are unlikely, given the location of As enrichment
in urban areas, and the fact that surface applications of
As, for example from arsenical cattle dip or arsenical
pesticides, have rarely been shown to migrate to depth
(Welch et al., 2000): even in areas where extensive use
has been made of arsenical agricultural chemicals, little
groundwater pollution by As is found (Hudak, 2000).
Furthermore, the As concentrations in the wells bear
no relation to pH (Fig. 6), so that desorption, induced
by pH change, as a mechanism for enrichment is discounted. Finally, and for several reasons, it seems unlikely that oxidation of sedimentary sulphides would be
causing the enrichment. This mechanism would not explain the fact that enrichment is seen only in urban
areas. Furthermore, the region has had ample time for
such sulphides to be destroyed by oxidation and flushing
or sorption prior to human invasion, as the regionÕs soils
are organic-poor and the groundwaters naturally oxic
(Greenman et al., 1967). Almost by elimination, it is
concluded that where As enrichment of shallow groundwater occurs, it arises from local reductive dissolution of
HFO and release of sorbed As to groundwater the following equation:
8FeOOH þ CH3 COOH þ 14H2 CO3
! 8Fe2þ þ 16HCO
3 þ 12H2 O
ð1Þ
Reduction of HFO is common in nature and has been
invoked previously to explain the presence of As in anoxic groundwaters (Gulens et al., 1979; Matisoff et al.,
1982; Korte, 1991; Korte and Fernando, 1991; Bhattacharya et al., 1997; Nickson et al., 1998, 2000; Nimick,
63
1998; McArthur et al., 2001; Ravenscroft et al., 2001;
refs. therein). Reduction of HFO (Eq. (1)) is a microbial
process: that is driven by microbial metabolism of organic matter, particularly acetate (see Nealson, 1997;
Lovley, 1997; Banfield et al., 1998; Chapelle, 2000; Lovley and Anderson, 2000) and is accompanied by microbial reduction of As(V) to As(III) (Zobrist et al., 2000;
but also Ahmann et al., 1997; Dowdle et al., 1996; Stolz
and Oremland, 1999). In the study region, pit latrines,
unlined sewage discharge channels, and open ponds used
for disposal of human and animal sewage, are in hydraulic continuity with the underlying unconfined shallow
aquifer which is tapped by the majority of shallow wells
sampled in the study area (38 out of 49 wells <10.6 m total depth, average total depth of these wells is 8.3 m) and
in areas of extensive human habitation must be contributing to groundwater a large dissolved load that includes
organic matter.
In some areas evidence that organic pollution is driving HFO reduction is very clear. Of the three samples taken in Multan, water from the two shallowest wells
(Wells 6 and 8, total depth, 9 and 27 m, respectively,
Fig. 4 and Table 1) contains high levels of Fe (2.7 and
2.1 mg L1) as predicted by Eq. (1), high levels of
NH4 (10 and 61 mg L1), a product of de-nitrification
under reducing conditions, and elevated PO4 (0.29 and
26 mg L1), which is also desorbed as a result of HFO
reduction. These samples have high As/Cl values (6.8,
4.5) and the highest As concentrations found (275 and
906 lg L1), well above those expected from evaporative
concentration of Chenab River water (10 and 50, respectively). These wells are clearly influenced by reduction of
HFO driven by organics. Water from Well 8 smelled of
fuel and it was suspected that reduction here may have
been driven by a plume of hydrocarbon pollution from
a spillage or a leaking fuel storage tank: of the samples,
it alone lacks SO4, which has clearly been removed by
300
200
100
As µg L-1
As µg L
-1
1000
100
10
1
0
0
20
40
60
-1
NO 3 mg L
Fig. 5. Relation of NO3 to As in groundwaters from Muzaffargarh District. Symbols as for previous figures. Plot excludes
well 8, which is an outlier affected by hydrocarbon pollution
and contains zero NO3 and 905 lg L1 of As.
0.1
6
7
pH
8
9
Fig. 6. Relation of pH to As in groundwaters from Muzaffargarh District. Symbols as for previous figures.
64
R.T. Nickson et al. / Applied Geochemistry 20 (2005) 55–68
SO4 reduction, also driven by pollutant organics in this
case hydrocarbons. From the conservative mixing relations shown in Fig. 2, the amount of SO4 removed is
around 300 mg L1. The Fe sulphides formed as a result
in the degrading hydrocarbon pollution plume that impacts Well 8 would have sequestered As and Fe (e.g.
Korte and Fernando, 1991; Rittle et al., 1995; Moore
et al., 1998). That any As or Fe remain in solution is
therefore interesting. Reduction of groundwater sampled in Well 6 may have been driven by the same source,
or organic matter contributed from uncontained sewage
discharge.
Geochemical indicators that groundwater is reducing
and HFO reduction is likely to be occurring can also be
seen in other shallow groundwaters with elevated concentrations of As in the Muzzafargarh Town area. Well
numbers 12, 40, 46, 48, 50, 55 and 58, for example, are
low in dissolved O2 (0.2–0.9 mg L1) and nitrate (five
out of seven with 0 mg L1 NO3, range 0–0.7 mg L1)
and relatively high in dissolved Fe (0.08–1.65 mg L1)
and PO4 (0.08–0.18 mg L1).
Interpretation of the data is made more confusing by
the power of the aquifer sediments to sorb As, a factor
that will introduce further uncertainty into any chemical
relationships in the waters, and by the mixed redox signals in some wells (e.g. Well 54 which contains NO3, Fe,
Mn and NH4). Where the waters contain NO3, this is
likely to suppress HFO reduction and keep As concentrations low. It is therefore not surprising that NO3
and As are generally seen to be mutually exclusive in
solution (Fig. 5). Samples from five wells (Nos. 21, 22,
23, 24 and 54) are the exceptions to this, no explanation
is forthcoming for this and it is possible that these wells
may be subject to surficial NO3 seepage into poorly constructed wellheads. Some positive correlation (although
a poor one, Fig. 7) can be seen between the presence
As µg L
-1
300
200
100
0
0
100
200
300
-1
PO 4 µg L
Fig. 7. Relation of PO4 to As in groundwaters from Muzaffargarh District. Symbols as for previous figures. Plot excludes
well 8, which is an outlier affected by hydrocarbon pollution
and contains zero NO3 and 905 lg L1 of As.
of PO4 in wells in moderately high amounts (>80
lg L1) and the concentration of As. Both are released
by HFO reduction (e.g. McArthur et al., 2001).
Where the water is rich in SO4 (as are all wells but
Well 8), microbial SO4 reduction may be occurring in
reducing microenvironments within a general regime of
NO3 reduction and Fe reduction (cf. Li and Peng,
2002). Such reducing microenvironments will act as
sinks for As and Fe and confuse the search for more
subtle manifestations of lower degrees of HFO reduction. Although there is no clear relationship, it can be
seen that some of the samples with the highest concentrations of As have relatively low SO4 (e.g. Wells 36,
48, 24 contain As 169, 147, 184 lg L1and SO4 59, 58,
113 mg L1 relative to an average of 295 mg L1).
Reduction of SO4 in solution is clearly occurring.
The genesis of As in deep wells (>30 m) is difficult to
interpret. All seven of the deep wells are in urban areas
and three serve textile mills, thereby increasing the possibility that they are affected by industrial pollution. In
the seven deep wells (>30 m) the maximum As concentration is 170 lg L1. All wells deeper than 30 m have
concentrations of As that exceed 60 lg L1 and there
is a suggestion that concentrations increase with depth
(Fig. 8(d)). In wells of differing depths that are closely
spaced (<50 m apart), the concentration of As is lower
in the shallower (Table 2). This fact suggests that the
As source is subsurface, and that As from surface pollution does not contribute to As in deep wells. Furthermore, at depth, organic pollution from sewage is less
likely than in shallow wells.
As with many shallow wells, the clear signs of HFO
reduction in deep wells are sparse; only wells 42 and
47 contain Fe in amounts (both 0.9 mg L1) compatible
with HFO reduction. These two wells are also saline
(Table 1) with As/Cl of 0.17 and 0.15, respectively, well
below the value of 0.25 obtainable by evaporation of
water from the Chenab River (As/Cl = 0.25). Their As
concentrations of 110 and 90 lg L1, respectively, are
explicable in terms of evaporative concentration in
which sorption has not removed all of the As, but as
both are anoxic and contain no NO3, >0.5 mg L1 of
Fe, and >0.1 mg L1 of Mn, additional As may have
been introduced into these wells by HFO reduction.
Two wells (22, 23) contain NO3 and As, which suggests
that in these wells mixed water is drawn from a large
screen interval. The two deepest wells, No. 36, from
Chowk Sarwar Shaheed (106 m), and No. 7 from Multan (88 m) contain no NO3, and Well 7 contains <1
mg/L of dissolved O2, but other indicators of anoxia
are absent from both wells: Fe and Mn are <0.1 mg L1
and Well 36 contained 3.8 mg L1 of dissolved O2,
although this may have been added by pump aeration.
These wells are also amongst the freshest sampled (Table
1), but were that to be a result of modern recharge, NO3
might be present.
R.T. Nickson et al. / Applied Geochemistry 20 (2005) 55–68
concentration. The difference between pre-irrigation and
later water is seen in vertical profiles of major constituents (Fig. 8). Concentrations of most constituents of
interest are low in deep wells (>30 m), which sample
pre-irrigation water. Concentrations are also low in
many shallow wells, which must sample modern water
recharged either directly from the Chenab River or indirectly from irrigation canals and return irrigation flow.
The composition of this recharge can be deduced from
the composition of the present Chenab River (Table
1), which was sampled in July at the peak of discharge,
and the composition of sample 20, which was taken
from a well screened between 6 and 8 m depth at a site
next to the TPLC (Fig. 1). Sample 20 is slightly fresher
than the sample of Chenab River water, although both
have low Electrical Conductivity (313 lS cm1 at 25
°C for sample 20, 532 lS cm1 at 25 °C for Chenab
River; Table 1).
Concentrations of NO3 in pre-irrigation water (deeper than 30m) were reported in Thal Doab by Greenman
et al. (1967) to be rarely above 3 mg L1: the present
Given the propensity of water in even unconfined
aquifers to become anoxic as water ages and moves deeper, and given the fact that most deep wells contain no
NO3 but do contain appreciable PO4, which would be
released by HFO reduction, the authors tentatively attribute the high As in deep wells to reduction of HFO. The
lack of, or low concentrations of, dissolved Fe might be
explained by its removal as Fe sulphides in micro reducing environments in the sediment, as the waters contain
tens of mg L1 of SO4. If the authors are correct about
HFO reduction, the process would have also reduced
strongly sorbed As(V) to weakly sorbed As(III), and
so a reason exists as to why As remains in solution in
the face of evidence for its removal elsewhere.
5.3. Other parameters of particular relevance in drinking
water
0
0
20
20
40
40
40
80
60
80
100
100
120
1000
Cl mg L
1500
-1
2000
40
NO 3 mg L
60
-1
0
(c)
0
20
20
20
40
40
40
60
80
100
100
0
100
200
As µg L
300
-1
80
1
2
Fe mg L
3
-1
0
(f)
20
20
20
40
40
40
60
80
100
100
120
120
0
1
2
NH 4 mg L-1
3
Depth, m
0
Depth, m
0
80
4
100
200
-
PO 4 µg L 1
300
400
60
80
100
120
0
(h)
1500
60
0
60
1000
SO 4 mg L-1
120
0
(e)
500
100
120
120
(d)
Depth, m
0
80
Depth, m
20
0
60
(g)
80
120
0
(b)
Depth, m
Depth, m
500
60
100
120
0
(a)
Depth, m
0
20
Depth, m
Depth, m
The effect of recent irrigation has been to emplace a
layer of younger groundwater over older groundwater
that is generally more saline through natural evaporative
60
65
50
100
150
Ca mg L-1
200
250
(i)
0.0
0.5
1.0
1.5
2.0
B mg L-1
Fig. 8. Depth relation of As and other constituents in groundwater from Muzaffargarh District wells. Symbols as in Fig. 2.
2.5
66
R.T. Nickson et al. / Applied Geochemistry 20 (2005) 55–68
Table 2
As concentrations in shallow and deep well water at three localities
Location
Shallow well
Muzaffargarh Centre,
UC City 35
Alipur Jhanubi Village
Chowk Sharwar
Shaheed Town
Deep well
1
Sample no.
Total depth (m)
As (lg L )
Sample no.
11
4.9
44
88
167
50
8.8
2.2
26
36
30
107
61
170
20
20
27
37
9
7.5
-1
10
B mg L
1
0.1
0.01
1
10
Lateral
distance (m)
45
data show NO3 to be below detection (0.2 mg L1) in
wells deeper than 40 m (Fig. 8). Nitrate concentrations
>4 mg L1, and up to 54 mg L1, are found only in
waters with <230 mg L1 of Cl (Fig. 2) and in shallow
wells (Fig. 8), suggesting that NO3 fertilizer is impacting
the shallow groundwater because of intensive irrigation
agriculture.
Concentrations of B range up to 1.9 mg L1 (Fig.
9), a concentration greater than the WHO guideline
value of 0.5 mg L1 (WHO, 1998), and also greater
than the tolerance of many crops to B in irrigation
water (Maas, 1986). A control on B concentrations
must be evaporative concentration. Many values of
B/Cl (Fig. 9) do plot along lines of evaporative concentration but around 30% of samples plot below the
evaporation lines, showing loss of B from solution.
Boron participates in sorption and exchange with mineral surfaces, and the process is sensitive to competing
ions, pH and ionic strength (Keren and Bingham,
1985; Goldberg et al., 1993; Ravenscroft and McArthur, 2004). As there is no relation between B concentration and either pH or HCO3 (figures not shown), it
is unlikely that these influence B concentrations, so the
authors postulate that B loss occurs by sorption in
some parts of the aquifer.
a
1
100
-1
1000
Total depth (m)
As (lg L )
6. Conclusions
In groundwater of the shallow Quaternary alluvial
sediments of Thal Doab, Punjab, As enrichment of shallow aquifers occurs in urban areas where pollutant
organics promote reduction of HFO, which releases
itÕs sorbed As to groundwater. In rural areas, which
are not impacted by such pollution, As concentrations
naturally remain 25 lg L1 despite strong evaporative
concentration of groundwater because of strong sorption of As from the oxic groundwater to aquifer sediments: potential concentrations of As up to 1500
lg L1 are therefore avoided. This sorption keeps the
oxic shallow aquifer As free in the absence of human
pollution. For the deep (>30 m) aquifer, the data would
seem to suggest that reduction of HFO by naturally-occurring organic matter is an important process and may
lead to concentrations of As up to 170 lg L1. This conclusion is tentative because all of the deep wells were in
urban areas and the possibility remains that HFO reduction is pollutant-driven. Arsenic concentrations appear
to increase with depth so it must be investigated whether
an increase in As with depth is confined to Muzaffargarh
District, or whether it is a general trend in the alluvial
aquifers of Pakistan, in order that long-term planning
of water resources can be undertaken.
The nature and extent of the human impact on As
pollution will have a profound influence on how national surveys for As (Nickson, 2001; Shrestha, 2002)
are interpreted. The knowledge that aquifer sediments
have a long-term capacity to sorb As and so remove it
from groundwater will also impact such interpretations.
Quantification of sorptive loss of As to aquifer sediments needs to be undertaken. Where As enrichment is
found in urban areas it is particularly important to
determine the nature of the pollutant organic matter
that drives HFO reduction: whether it is from unsewered
sanitation, waste dumps, or spilled hydrocarbon fuels
and, if a mixture, which has most impact where.
10000
Cl mg L
Fig. 9. Relation of B to Cl in groundwater from Muzaffargarh
District. Symbols as in Fig. 2. Dotted lines represent evaporative trends.
Acknowledgements
We thank Dr. Michael Wood, Department of Geography, Aberdeen University for assistance with digitising
R.T. Nickson et al. / Applied Geochemistry 20 (2005) 55–68
basemaps of Muzaffargarh and for the use of the UniversityÕs equipment for this purpose. Miss. Kieren Smith
and Mr. Rick Gard assisted greatly with this process.
Mr. A. Osborn is thanked for undertaking much of
the water analysis, using the facilities of the Wolfson
laboratory of the Department of Geological Sciences,
UCL, with permission of the Director, K. Hudson-Edwards. Cation analysis was done by JMMcA using the
NERC ICP-AES Facility at RHUL, with permission
of the Director, Dr. J.N. Walsh. We acknowledge the
invaluable assistance, under adverse field conditions, of
Mr. Tariq Kamran, of UNICEF Pakistan. Mr. Rana
Altaf Hussain, the Assistant Director Local Government, Muzaffargarh District greatly assisted the work
as did Mr. Ghulam Shabir and Mr. Safdar Bukhari.
We thank Dr. Nasreen Elahi, Project Officer of UNICEF Lahore, for her logistical support. Dr. George
Breit (USGS) provided invaluable satellite images, maps
and geological reports about Pakistan. Finally we thank
Dr. Karen Johannesson of the University of Texas at
Arlington and one anonymous reviewer for their accurate and constructive comments on the original
manuscript.
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