Article
pubs.acs.org/est
Sedimentological Control on Mn, and Other Trace Elements, In
Groundwater of the Bengal Delta
J. M. McArthur,†,* P. K. Sikdar,‡ B. Nath,§ N. Grassineau,∥ J. D. Marshall,⊥ and D. M. Banerjee#
†
Department of Earth Sciences, University College London, Gower Street, London WC1E 6BT, United Kingdom
Department of Environment Management, Indian Institute of Social Welfare and Business Management, College Square, Kolkata
700073
§
School of Environmental Systems Engineering, The University of Western Australia, MO15, 35 Stirling Highway, Crawley, WA
6009, Australia
∥
Department of Geology, RHUL, Egham, Surrey TW20 0EX, United Kingdom
⊥
School of Environmental Science, University of Liverpool, Brownlow Street, Liverpool L69 3GP
#
Department of Geology, Delhi University, Chattra Marg, Delhi 110007
‡
S Supporting Information
*
ABSTRACT: To reveal what controls the concentration and distribution of
possibly hazardous (Mn, U, Se, Cd, Bi, Pb) and nonhazardous (Fe, V, Mo,
PO4) trace elements in groundwater of the Bengal delta, we mapped their
concentrations in shallow groundwater (<60 mbgl) across 102 km2 of West
Bengal. Only Mn is a potential threat to health, with 55% of well water
exceeding 0.3 mg/L, the current Indian limit for drinking water in the absence
of an alternate source, and 75% exceeding the desirable limit of 0.1 mg/L.
Concentrations of V are <3 μg/L. Concentrations of U, Se, Pb, Ni, Bi, and Cd,
are below WHO guideline values.
The distributions of Fe, Mn, As, V, Mo, U, PO4, and δ18O in groundwater
reflect subsurface sedimentology and sources of water. Areas of less negative
δ18O reveal recharge by sources of evaporated water. Concentrations of Fe, As,
Mo, and PO4 are high in palaeo-channel groundwaters and low in palaeointerfluvial groundwaters. Concentrations of U, V, and Mn, are low in palaeochannel groundwaters and high in palaeo-interfluvial groundwaters. Concentrations of Fe and Mn are highest (18 and 6 mg/L
respectively) at dual reduction-fronts that form strip interfaces at depth around the edges of palaeo-interfluvial aquifers. The
fronts form as focused recharge carries dissolved organic carbon into the aquifer margins, which comprise brown, iron-oxide
bearing, sand. At the Mn-reduction front, concentrations of V and Mo reach peak concentrations of 3 μg/L. At the Fe-reduction
front, concentrations of PO4 and As reach concentrations 3 mg/L and 150 μg/L respectively. Many groundwaters contain >10
mg/L of Cl, showing that they are contaminated by Cl of anthropogenic origin and that organic matter from in situ sanitation
may contribute to driving reduction.
■
manganese normally found in drinking-water”.10 This is plainly
not the case for groundwater consumed by many of the 240
million inhabitants of the Bengal delta (Ganges-BrahamaputraMeghna River system of Bangladesh and West Bengal). In
Bangladesh, 46% of shallow groundwaters (arbitrarily taken as
<120 m deep) have >0.4 mg/L of Mn, and 5% have >2 mg/L
(n = 3313; data from refs 11 and 12), with a maximum of 10
mg/L. For southern West Bengal, the figures are 47%, 11%, and
6 mg/L, respectively (527 wells <60 m deep; this work). Using
the national standards of 0.1 mg/L for India and Bangladesh
increases the percentage of non-compliant groundwater wells,
INTRODUCTION
High concentrations of trace elements in potable water and
irrigation water are problematic for reasons of aesthetics,
practicality, or because of their impact on health, and
agricultural sustainability. In the case of Mn, its ingestion may
adversely affect human health,1−3 with the young and unborn
being most at risk.2,4−8 To mitigate the hazard from ingestion
of Mn, the Bureau of Indian Standards Acceptable Limit for
drinking water (IS 10500, 1991) is 0.1 mg/L, with 0.3 mg/L
being permissible in the absence of an alternate source. The
World Health Organization’s guideline value (GV) for Mn in
drinking water was set at 0.4 mg/L prior to mid-2011. That
limit may have been too high.9 Surprisingly, the fourth edition
of the World Health Organization’s Guidelines for Drinking
Water Quality10 has no guideline value for Mn in drinking
water because 0.4 mg/L of Mn is “well above concentrations of
© 2011 American Chemical Society
Received:
Revised:
Accepted:
Published:
669
August 2, 2011
December 7, 2011
December 8, 2011
December 8, 2011
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as does the use of the Indian limit of 0.3 mg/L Mn where no
alternate source of water is available.
Excess Mn in soil is also a problem worldwide, albeit mainly
for acid soils.13,14 To mitigate the hazard to crops from Mn in
irrigation waters, concentrations <0.2 mg/L are recommended.15 Across the Bengal delta, irrigation in the dry season,
much of it using shallow groundwater, provides a third annual
crop that is vital to rural livelihoods. In Bangladesh, 56% of
shallow groundwaters contain >0.2 mg/L of Mn;11,12 for West
Bengal, the figure is 62% (this report). The threat to agriculture
from Mn in irrigation water is clear.
Given the potential hazards posed by Mn, and possibly other
trace elements such as U,16−18 in groundwater, an improved
understanding is needed of their source, transport, and fate. In
Bangladesh and West Bengal, where As in groundwater is a
major hazard to health of consumers,19,20 the distribution of Aspollution is influenced strongly by the subsurface distribution of
palaeo-interfluvial and palaeo-channel aquifers.21,22 We show
here that a similar control is exerted on the concentration and
distribution of Fe, PO4, Mo, Mn, U, and V, in groundwater in
the shallow aquifers of southern West Bengal. The concept we
develop, of sedimentological control of groundwater composition, has applicability across the Bengal delta and in deltaic
aquifers worldwide.
Figure 1. (a) Distribution of Mn in groundwaters of the study area.
The base map is the 1 in. toposheet of West Bengal circa 1975.
Arrowed red square is study area at Moyna of refs 21 and 29.
Distribution of palaeo-interfluvial aquifer (areas of yellow), and palaeochannel aquifers (uncoloured areas) are updated from ref 22.
Coordinates to WGS84. (b) wells used for the line of profile in
Figure 2.
■
GEOLOGICAL SETTING
The Bengal Basin. The detailed sedimentology of the
Bengal delta is documented elsewhere,11,12,23−26 so a synopsis
only is given here. In the period 125 ka to 20 ka, falling sea
level, coupled with weathering and erosion, created a coastal
region of low-relief palaeo-interfluves cut by major rivers and
their lesser tributaries. A laterite,24 also described as a clay-rich,
red, palaeosol (the Last Glacial Maximum Palaeosol, or LGMP,
of ref 21), formed on the exposed palaeo-interfluvial surfaces:
underlying palaeo-interfluvial sands were thoroughly flushed by
oxic water, turning them oxidized and brownthe oxidized
facies of ref 24. These brown sands comprise the Late
Pleistocene palaeo-interfluvial shallow aquifers of today. As sea
level rose after the Last Glacial Maximum at 20 ka, the incised
valleys were infilled mainly by gray sands; these palaeo-channel
sands constitute today’s shallow palaeo-channel aquifers. More
recent alluvium capped the old topography with a variety of
sediments. Where the capping was fine-grained, it formed an
upper aquitard to the palaeo-channel aquifers that impeded the
ingress of oxygen and allowed anoxic conditions to develop.
The Study Area. The study area (Figure 1) occupies 1022
km of southern West Bengal, India, between 88° 41′ and 88°
49′ North, and 22° 29′ and 22° 35′ East. It has been reported on
previously with respect to As-pollution.22 The region extends
northeast from the town of Barasat. The mineralogy of
sediments in the Bengal delta is described elsewhere.27,28
Across the region drilling and local knowledge attests to the
presence of a clay aquiclude at depth-to-top of around 60 to 65
m. The clay is around 30 m thick, and separates sands below it,
that form a deep aquifer, from sands above it, that form shallow
aquifers.22 This work deals exclusively with the shallow aquifers.
Both palaeo-interf luvial and palaeo-channel aquifers21,22 occur
above the basal clay aquiclude. The palaeo-interf luvial aquifers
comprise pre-LGM, brown, oxidized, sands capped by the
impermeable LGMP,24,26,29 the depth to which in our area
ranges from 16 to 24 mbgl. The LGMP is typically between 1.6
and 6 m in thickness, depending on location.22 In our area, it is
commonly overlain by silts, peats, and clays. The LGMP
prevents vertical recharge to palaeo-interfluvial aquifers, which
are therefore recharged at their margins by lateral flow inward
from surrounding palaeo-channels. This recharging groundwater is rich in dissolved Fe and As.21,22,29
The palaeo-channel aquifers comprise post-LGM gray sands
some 20−50 m thick that infill palaeo-channels. In our area,
these aquifers are capped by a silty aquitard between 5 and 12
m thick. The aquitard permits recharge but maintains
semiconfined conditions in the palaeo-channel aquifers. The
palaeo-channels are incised to different levels in different places.
In our area, deep palaeo-channels cut through the full thickness
of the late Pleistocene brown sands, but not through the
underlying clay aquiclude (but may do so elsewhere). Shallow
palaeo-channels cut to shallower levels, typically around 30 mbgl
in our area, leaving a basal unit of brown Pleistocene sand
underlying the post-LGM gray sands.
Groundwaters are anoxic, with a pH of 7 ± 0.5 and
alkalinities typically of 400−600 mg/L.29 Palaeo-channel
groundwaters have gone beyond the stage of complete Feoxyhydroxide reduction, and so are rich in Fe, PO4, and As, but
lack Mn. Palaeo-interfluvial groundwaters are typically poised at
Mn-reduction, so they contain dissolved Mn, but little Fe, As,
or PO4.
Across much of the western Bengal delta, the natural
direction of groundwater flow is from north to south, except
where locally perturbed by interaction with rivers. Since 1970,
increasing pumping of shallow groundwater for irrigation has
locally disturbed the natural flow, but the effect is poorly
quantified. In Moyna (Figure 1), where the density of irrigation
wells is high, pumping reverses the flow direction during some
of the year, so that it is north northeastward.29 Eastward of
Moyna, the density of irrigation wells is lower.
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Article
MATERIALS AND METHODS
We analyzed 527 groundwaters from domestic and irrigation
wells that were collected in February 2009 and February 2010
for a previous study of As-pollution.22 Depths of wells (where
known) were mostly between 20 and 60 m, with 13 less than 20
m deep. Wells tapping water from beneath the regional, basal
aquiclude, are identifiable in the field (deep wells) and are not
considered here. Samples acidified in the field to be 1% with
respect to nitric acid were analyzed by ICP-AES for Mn, Fe,
and PO4 (527 samples). Analysis was by ICP-MS for trace
elements (325 samples). Unacidified samples were analyzed for
Cl by ion chromatography. Oxygen isotope data were obtained
on 0.22 μm-filtered unacidified samples using a Picarro WSCRDS laser instrument for which analytical precision is better
than 0.08‰. The measured concentrations are in Supporting
Information Table S1. Subsurface sedimentology is documented for 43 logged boreholes of ref 22, and by those authors’
mapping of palaeo-interfluves and palaeo-channels.
of Mn in groundwater are mostly <0.1 mg/L in the palaeochannel aquifers. They are mostly 0.2 to 0.4 mg/L in
groundwater from the interior of the palaeo-interfluvial aquifer.
Concentrations of Mn in groundwater are highest (up to 6 mg/
L) at the margins of the palaeo-interfluvial aquifer where it is
being recharged through the interface at which gray palaeochannel sands abut brown palaeo-interfluvial sands. This
distribution of Mn in groundwater is explained below.
Manganese at Palaeo-Interfluvial Margins. The palaeo-interfluvial aquifer is capped by the impermeable LGMP,
so it can be recharged only by lateral inflow at depth from the
flanking palaeo-channels (Figure 2). This subsurface flow into
the palaeo-interfluvial aquifers creates dual Fe- and Mnreduction fronts that appear to be ubiquitous around the
margin of the palaeo-interfluvial aquifers (Figures 1,2). In the
field, the Mn-reduction front is revealed by intense black stain
on well-completions from precipitation of manganese oxide,
and the Fe-reduction front is revealed by intense red stain from
the precipitation of iron oxide.22 The fronts generates extreme
concentrations of Mn and Fe in groundwater, with the Mnfront occurring downflow of the Fe-reduction front (Figure 2).
These reduction fronts have parallels in water-supply schemes
involving both river-bank infiltration,30−32 where they are
driven by DOC in river water, and aquifer recharge and
recovery,33 where they are driven by DOC in injected
wastewater.
The reduction fronts are developed more strongly at the
northern margin of the palaeo-interfluvial aquifer than at the
southern margin (Figure 2). At the northern margin,
concentrations of Fe reach 18 mg/L and those of Mn reach
6 mg/L. At the southern margin, concentrations reach 3.5 mg/
L for Fe and 1.5 mg/L for Mn. This asymmetry results from the
combining of radially inward flow, induced by pumping from
the palaeo-interfluvial aquifer, with the regional, natural, flow of
groundwater to the south. At the northern margin of the
palaeo-interfluve, these flows (and so DOC supply) reinforce
each other. At the southern margin of the palaeo-interfluve,
they oppose each other, making flow sluggish.
The distribution of δ18O values in groundwater along the line
of traverse (Figure 2) supports this explanation. They are least
negative beneath the conurbations (Bira, Kharki) that straddle
the palaeo-interfluvial margins. Southward along the traverse
from both localities, but most noticeably from Bira, the
groundwater becomes isotopically more negative (Figure 2).
The isotopic enrichments of δ18O result from focused recharge
of partly evaporated water. One source is irrigation return-water
derived locally, and from southward flow of infiltrated returnwater above the palaeo-interfluve, which rolls over the LGMP
margin to recharge the palaeo-interfluvial aquifer. Another is
domestic wastewater evaporated during the dry season in the
conurbations of Bira and Kharki. Such water includes that in
urine, which is also isotopically enriched in δ18O by around
10‰ compared to ingested water.34
The asymmetry of the profile of δ18O is consistent with
groundwater flow being predominantly to the south, with
recharge to the palaeo-interfluvial aquifer being derived
predominantly from the north. The tailing of δ18O to less
enriched values southward from Kharki, and the weak redox
fronts in its vicinity, suggests that, at least locally, irrigation
pumping has only marginally impaired the natural southerly
flow of groundwater at the southern margin of the palaeointerfluve. The effect on flow may be seasonal and related to
pumping times (and their lags), as it is in Moyna in the west of
■
MANGANESE IN GROUNDWATER
The distribution of Mn in groundwater across the study area is
shown in Figure 1 (data in Supporting Information Table S1).
In Figure 2, we profile concentrations of Mn, Fe, δ18O, and Cl,
Figure 2. (a) Concentrations of Mn, Fe, δ18O, and Cl, in groundwaters
along the line of profile shown on Figure 1a, matched to (b) a
schematic sedimentological model (not to scale) based on drilling and
mapping of the color of well-completions along the same line of profile
(ref 22 for details: updated groundwater flow here shows stronger
southerly flow than in ref 22). Highest concentrations of Mn in
groundwater are found at Mn-reduction fronts the margins of palaeointerfluvial aquifers, with the northern margin being enriched over the
southern margin. Dotted guide-lines on (a) arbitrarily drawn through
samples with the highest concentrations of Mn. The palaeosol is
typically between 1.5 and 6 m in thickness.22,29.
along a N−S traverse through the east of our study area and
show how the profiles relate to sedimentology. Concentrations
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our field-area.29 Elsewhere, the impact of irrigation pumping
might be more severe.35,36
The Redox Driver. In sediments, microbial metabolism of
organic carbon drives reduction of both Mn-oxides37−39 and
Fe-oxyhydroxides.40−43 Because our Mn-reduction fronts are
downflow of the Fe-reduction fronts (Figure 2), Mn-reduction
may be driven by dissolved Fe(II).44−46 Nevertheless, both
fronts ultimately result from metabolism of DOC. Possible
sources of DOC include: natural solid organic matter in
aquitards and aquifers;47,48 waste organic-matter in in situ
sanitation;29 degrading hydrocarbons49 that may be derived
from diesel fuel. To these may be added DOC in rivers entering
aquifers via river-bank infiltration32 and cow dung, which is
used to thicken drilling fluid when drilling in areas where caving
occurs. We discuss these sources in turn below.
River-bank infiltration is too small-scale and localized in our
area to be an influence on the reduction we document (Figure
1). Nor, on mass balance grounds, can spilt fuel drive it. Cow
dung adds organic carbon directly into the aquifer, but at
present its effect cannot be quantified. Any dissolved organic
carbon infiltrating from ponds does not noticeably drive
reduction of either Fe- or Mn-oxyhydroxides.50−52
By elimination, DOC at the reduction front is supplied in
three sources of recharge to the palaeo-interfluvial aquifers.
First, groundwater flowing laterally from palaeo-channels
(Figure 2) carries DOC derived from aquitard diffusion, or
from organic-rich sediments intercalated within the palaeochannel sands. Second, groundwater flowing laterally above the
impermeable LGMP reaches the nearest palaeo-interfluvial
margin, where it rolls over the edge to contribute focused
recharge. This water likely contains DOC derived from the peat
and peaty sediment that is abundant in units that overly the
palaeosol.22,29 Peat sensu stricto, and peaty sediment, is also
abundant elsewhere in the Bengal delta.11,12,23,26,47,53 Finally,
some DOC may derive from sullage and sewage water. We
draw this inference from the fact that Cl concentrations in
many of our groundwaters exceed the natural maximum of 10
mg/L (Figure 3), particularly in palaeo-channels. The Cl is not
derived from saline connate water which, in our area, is found
only in groundwaters deeper than those we have sampled. It
follows that the high Cl represents anthropogenic contamination.
Manganese in Palaeo-Interfluvial Interiors. In the
interiors of the palaeo-interfluvial aquifers, the groundwaters
contain mostly 0.2−0.4 mg/L of Mn (Figure 1; Supporting
Information Table S1). Concentrations are higher around
Moyna, at the western end of the palaeo-interfluvial aquifer
where it thins and pinches out. At this tapered western tip, the
concept of margin and interior become indivisible, and
concentrations of Mn are higher than in palaeo-interfluvial
interiors further east, which are remote from reduction fronts.
Reduction of Mn in palaeo-interfluvial interiors probably has
not been driven by residual sedimentary organic matter in the
aquifer. These aquifers were thoroughly flushed and oxidized
during the period 125 ka to 20 ka.11,47,54 Such flushing would
have destroyed any organic matter present. The Mn in palaeointerfluvial interiors may have resulted from migration inward
of Mn from marginal reduction fronts. Such migration would
necessarily have been historical, because migration of more
mobile anthropogenic Cl into the palaeo-interfluves is limited
(Figures 2 and 3) and because the asymmetric traverse profile
of δ18O bears no relation to that of Mn, which is flat (Figure 2).
Most likely, the Mn in palaeo-interfluvial groundwater results
Figure 3. Distribution of Cl in groundwaters of the study area. High Cl
occurs mostly around the palaeo-interfluvial margin and in palaeochannel aquifers. Base map and aquifer distribution as in Figure 1. The
upper limit to natural concentrations of Cl in groundwater in the
Bengal delta is 10 mg/L.
from reduction of MnO2 driven by recalcitrant DOC that
escaped reaction at palaeo-interfluvial margins and was carried
into the aquifer over historical time by natural flow. Being
recalcitrant, it has been metabolized slowly and so has not
driven much reduction.
■
OTHER TRACE ELEMENTS: AS, U, MO, PO4, V
Trace Elements at Palaeo-Interfluvial Margins. When
reduction fronts move through aquifers, elements released from
aquifer materials can resorb downflow, only to be released
again, at higher concentrations, as the moving redox front
sweeps through these downflow regions: the aquifer is, in
essence, zone-refined. Species that are conservative in solution,
such as Cl, lead the plume and travel at the same rate as the
groundwater. Migration distances of other species depend on
their propensity to sorb, or form insoluble products such as
carbonates.32,55
Profiles of trace elements along our line of traverse (Figure
4) show that V, Mo, PO4, and As have also been generated at
either the Mn- or the Fe-reduction fronts. As with Mn and Fe,
the fronts have developed to different degrees at the northern
and southern margins of the palaeo-interfluve. At the northern
edge, the front for V leads the Mn front; at the southern margin
it is hardly resolved from background. The Mo front coincides
with the Mn front, but Mo concentrations are higher in the
northern front. As Mo sorbs strongly to Fe-oxides, resorption
downflow to Mn-oxides is indicated. The As and PO4 fronts
appear coincident with the Fe-reduction front at the northern
margin, but are not discernible at the southern margin. Denser
sampling would better resolve detail in these fronts.
Trace Elements in Aquifer Interiors. Concentrations of
Mo, As, and PO4 are high in the Fe-rich palaeo-channel waters
(Figure 4), from supply by the reductive dissolution of some
sedimentary iron oxyhydroxides. In palaeo-interfluvial groundwaters, the concentrations of As, PO4 or Mo are low because
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Figure 4. Concentrations of U, V, Mn, As, PO4, Mo, and Cl, along the
line of profile shown on Figure 1. Highest concentrations of Mn, Fe,
Mo, and V, occur at Fe- and Mn-reduction fronts at the northern
margin of the palaeo-interfluvial aquifer. Highest concentrations of As,
and PO4, occur in palaeo-channel groundwaters. Dotted guide-lines
arbitrarily drawn through the samples with the highest concentrations
of Mn.
Figure 5. (a) Distribution of U in groundwaters of the study area. (b)
Distribution of V in groundwaters of the study area. The base map and
aquifer distribution for both are as in Figure 1.
the waters are insufficiently reducing for Fe-reduction to
liberate them to solution. Any entering the palaeo-interfluvial
sediments in recharge are sorbed by, or reacts with, palaeointerfluvial marginal sands downflow of the reduction fronts.
The sink for Fe may be reduction of Mn-oxides,44−46 or
formation of magnetite or green rust.56,57 Magnetite in
sediments from Ariahazar, Bangladesh, is detrital.58 Analysis
by SEM and EDAX microprobe of magnetic separates from
many of our cores also reveals only detrital magnetite. If
magnetite is forming at palaeo-interfluvial margins, it is too
minor a component to be seen easily and further research is
needed to identify this sink.
Palaeo-interfluvial groundwater contains 5−15 μg/L U and
1−2 μg/L V (Figures 4 and 5). Palaeo-channel groundwaters
contain none. Uranium solubility is sensitive to redox;17,59
under reducing conditions it is immobilized as insoluble U(IV).
Our data show it to be soluble under conditions of Mnreduction. Others have shown U to be soluble under even more
reducing conditions.59 As is the case eleswhere,60 the U in our
Mn-reducing, palaeo-interfluvial, groundwater may be stabilized
through the formation of a Ca−U−CO3 complex in these
water, which contain much Ca (∼ 100 mg/L) and bicarbonate
(400−600 mg/L).
■
IMPLICATIONS FOR HEALTH AND AGRICULTURE
Drinking Water. concentrations of Mn between 0.2 and
0.4 mg/L occur widely across southern West Bengal.
Concentrations of Mn >2 mg/L occur in our area only at the
margins of palaeo-interfluves at reduction fronts forming stripinterfaces between palaeo-channel and palaeo-interfluvial
aquifers. High concentrations of Mn in groundwater elsewhere
in the Bengal delta11,12 may also result from reduction at the
interface of pre-LGM brown sand and post-LGM gray sand.
The limited width of the strip interfaces we document restricts
their impact on water quality and explains why it is uncommon
to find a well that taps groundwater containing >2 mg/L of Mn.
Migration of extreme concentrations of Mn into palaeo673
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palaeo-interfluvial aquifers were exceptionally easy to locate
using the color differences between palaeo-channel wells, which
yield Fe-rich water that stains red, and palaeo-interfluvial wells,
which yield Mn-rich water that stains black. The reason for that
ease is now clear. It is because the margins of palaeo-interfluvial
aquifers are the locus of focused Fe- and Mn-reduction fronts,
so wells at the palaeo-interfluvial margins tap water that is
especially rich in either Mn or Fe. The juxtaposition of the
reduction fronts heightens the contrast between the two colors
of stain developed from each type of water. Marginal wells
tapping the Mn-front stain more strongly black than do wells in
the palaeo-interfluvial interiors where Mn concentrations are
lower. Marginal wells tapping the Fe-front stain more strongly
red that do most of those in palaeo-channel settings. The
intensity of stain can, therefore, provide confirmation that a
well is located at the Mn-reduction front or Fe-reduction front,
which, today, means at the palaeo-interfluvial margin.
interfluvial interiors has yet to occur, and is limited by sorption
of Mn to, or its reaction with, marginal sediments of the palaeointerfluvial aquifer. Concentrations of Mn in groundwater from
palaeo-interfluvial interiors will therefore remain low for
decades-to-centuries, as in the case of As.22 When retardation
factors for Mn migration are known, this conclusion can be
refined.
Data of others6,11,12 show that Mn is widely distributed in
groundwater of the Bengal delta at concentrations that may be
hazardous in the light of modern epidemiological study.1−8
Nevertheless, in 2011, soon after these studies appear in print,
WHO10 abandoned its Guideline Value for Mn in Drinking
Water. Its stated reason for doing is that 0.4 mg/L of Mn is
“well above concentrations of manganese normally found in
drinking-water”.10 This statement is plainly wrong. This report,
and further studies of Mn in groundwater, should provide the
evidence needed to reverse that decision, if only as a
precautionary measure to protect human health while
epidemiological studies advance to the point of being definitive
on this risk.
Concentrations of U in the groundwater of our area do not
exceed the WHO GV of 30 μg/L (and rarely exceed the
previous GV of 15 μg/L). Concentrations >5 μg/L are found
confined to palaeo-interfluvial groundwaters. Concentrations of
V are typically 1−2 μg/L in palaeo-interfluvial groundwater,
and nil in palaeo-channel groundwater. Concentrations of Sb,
Se, Cd, Pb, and Bi, (Supporting Information Table S1) are
mostly undetectable (<1 ppb) by ICP-MS under the conditions
used for analysis, or occur at concentrations (Supporting
Information Table S1) that are less that the health-related
guideline values of WHO10 (4th edition).
Irrigation Water. Manganese in irrigation water may be
“toxic to a number of crops at a few-tenths to a few mg/L, but
usually only in acid soils”15 (see also ref 14). In our study area,
around half the irrigation wells tap the shallow palaeointerfluvial aquifer. Its groundwaters typically contain 0.2−0.4
mg/L of Mn, so its long-term use for irrigation might adversely
affect Mn-intolerant crops. The risk is greatest in acidic
soils;13,14 some soils in the Bengal delta are acidic in the dry
season, with acidity increasing with the length of time under
cultivation.61 The reducing conditions of paddy soils when
flooded may also mobilize Mn by reduction of Mn-oxides in the
soil.62
The risk to dry-season rice,63 the main irrigated crop, posed
by Mn is lessened, during the period of growth in waterlogged
soils, by the Fe- and Mn-plaque that rice forms around its roots
by aeration.14,64,65 The plaque protects roots by sorbing and
precipitating many trace elements, thereby reducing the
potential toxicity of Mn and other trace elements to rice. As
palaeo-interfluvial groundwater typically contains 0.2−0.4 mg/
L Mn, except at its narrow margins, the potential for harm may
be small, except at that margins. Paddy-field soils away from
roots are reduced in the wet season, but are oxidized in the
ripening period, during which fields are allowed to dry for
harvesting. The oxidation generates acidity through ferrolysis.66
Whether a problem of Mn toxicity may be present in the dryland, ripening, phase of rice production in the dry-season crop
is, therefore, a possibility, but one that is beyond the scope of
this paper to address.
Well Color. The color of stain on well completions, pumps,
and domestic utensils, can be used to screen wells for the
presence or absence of As-pollution and to locate palaeointerfluvial aquifers.22 Those authors noted that the margins of
■
WIDER IMPLICATIONS
The palaeo-interfluvial and palaeo-channel aquifers of the
Bengal Basin that we have studied were created as a result of
eustatic changes in sea-level since 125 ka. The changes created
similar coastal landforms around the world, so the sediment
architecture seen in the Bengal delta may be found in other
modern deltas (for some examples, see ref 21). That
architecture will affect groundwater composition as it does in
the Bengal delta. In other deltas, extreme concentrations of Mn
(>2 mg/L) and Fe (>15 mg/L?) in groundwater may mark the
presence of strip-interfaces at the margins of buried palaeointerfluves. If so, palaeo-interfluvial margins will be identifiable
using the criteria of well-color that has proven successful in the
Bengal delta.22 Despite the operation of regional variations
governed by geological and climatic setting, the context of
palaeo-channels and palaeo-interfluves may provide a unifying
concept to assist development of deltaic aquifers and explain
the distribution of Fe, Mn, Mo, V, PO4, U, and other dissolved
constituents and pollutants, in deltaic aquifers worldwide.
■
ASSOCIATED CONTENT
S Supporting Information
*
Table S1 shows concentrations of dissolved species in well
water and well depth and includes GPS coordinates to WGS84
datum. This material is available free of charge via the Internet
at http://pubs.acs.org.
■
■
AUTHOR INFORMATION
Corresponding Author
*Phone 0044 (0)20 7679 2376; e-mail: j.mcarthur@ucl.ac.uk.
ACKNOWLEDGMENTS
This work was funded by the Department of Earth Science at
UCL, by the University of Western Australia (B.N.) and by
NERC grant NE/G/016879/1 (J.M.McA. and P.K.S.), and the
University of Liverpool. We thank J.D. Ball, of the Lifer
Laboratory at Liverpool University, for the isotopic analysis,
Elliot Harper for separating and identifying magnetite from
sediments cored in West Bengal, and Jim Davy for driving the
EDAX-SEM.
■
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