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Occurrence of Arsenic-contaminatedGroundwaterin Alluvial Aquifers fromDelta Plains, Eastern India:Options for Safe DrinkingWater SupplyProsun Bhattacharya , Debashis Chatterjee &Gunnar JacksPublished online: 21 Jul 2010.
To cite this article: Prosun Bhattacharya , Debashis Chatterjee & GunnarJacks (1997) Occurrence of Arsenic-contaminatedGroundwater in AlluvialAquifers from Delta Plains, Eastern India: Options for Safe Drinking WaterSupply, International Journal of Water Resources Development, 13:1, 79-92,DOI: 10.1080/07900629749944
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W ater Resources D evelopm ent, Vol. 13, N o. 1, 79 ± 92, 1997
Occurrence of Arsenic-contaminatedGroundwater in
A lluvial A quifers from Delta Plains, Eastern India:
Options for Safe Drinking Water Supply
PROSUN BH ATTACHARYA1, DEBASH IS CH ATTERJEE2 & GUN NARJACKS1
1Division of Land and W ater Resources, Department of C ivil and Environmental Engineering,
Royal Institute of Technology, S-100 44 Stockholm , Sweden; 2Department of Chemistry,
U nivers ity of Kalyani, Kalyani 741 235, W est Bengal, India
ABSTRACT Arsenic contam ination in groundwater used for drinking purposes has been
envisaged as a problem of global concern. Exploitation of groundwater contam inated
w ith arsenic within the delta plains in West Bengal has caused adverse health effects
am ong the population within a span of 8 ± 10 years. The sources of arsenic in natural
w ater are a function of the local geology, hydrology and geochemical characteristics of
the aquifers. The retention and m obility of different arsenic species are sensitive to
varying redox conditions. The delta plains in West Bengal are characterized by a series
of m eander belts form ed by the ¯ uvial processes com prising different cycles of complete
or truncated ® ning upward sequences (sand ± silt ± clay). The arseniferous groundwater
belts are m ainly located in the upper delta plain and in abandoned m eander channels.
M ineralogical investigations have established that arsenic in the silty clay as well as in
the sandy layers occurs as coatings on mineral grains. C layey sediments intercalated
w ith sandy aquifers at depths between 20 and 80 m are reported as a major source of
arsenic in groundw ater. Integrated knowledge on geological, hydrological and geochem -
ical characteristics of the m ulti-level aquifer system of the upper delta plain is therefore
necessary in predicting the origin, occurrence and m obility of arsenic in groundwater in
W est Bengal. This would also provide a basis for develop ing suitable low-cost techniques
for safe drinking water supply in the region.
Introduction
Arsenic (As) contamination of natural origin in groundwater has been envisaged
as a worldwide problem. Several accounts of the presence of arsenic at elevated
levels have been reported from various parts of the USA such as Arizona,
California, Montana, Nevada, Oklahoma and Washington in relation to issues on
arsenic in groundwater (Robertson, 1986, 1989; Moncure et al., 1992; Schlottmann
& Breit, 1992; Frost et al., 1993). Similar cases have also been reported from many
other countries including Chile, Bangladesh and the Taiwan province of China.
Global perspectives on the problem of arsenic occurrences in groundwater and
treatment strategies have recently been reviewed by Hering & Elimelech (1995).
In India, the problem of the occurrence of high As in groundwater has been
0790-0627/97/010079± 14 $7.00 Ó 1997 Journals Oxford Ltd
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80 P. Bhattacharya et al.
detected for a decade in some areas of West Bengal located in the Indo-
Gangetic delta plains affecting the districts of Nadia, Murshidabad, Malda,
Barddhaman, North and South 24-Paraganas. The mid-1970s saw a large-scale
exploitation of groundwater, resources for irrigation purposes. As a conse-
quence of the war in Bangladesh in 1971, the exodus of 80± 90 million people
from Bangladesh increased the number of settlements in the bordering dis-
tricts of West Bengal, thereby signi® cantly increasing the demand for water.
Adverse health effects due to the consumption of groundwater with excess
arsenic content were manifested among the population within a time span of
8± 10 years (Goriar et al., 1984; Chakraborty et al., 1987; Guha Mazumder et al.,
1988; Das et al., 1994).
The source of As in groundwater as well as in surface water is most often
leaching of geological materials, inputs from geothermal sources, mining
wastes and land ® lls (Welch et al., 1988; Korte & Fernando, 1991). Uncon-
trolled anthropogenic activities such as smelting of metal ores, use of arseni-
cal pesticides and wood preservative agents may release arsenic directly to
the environment (Bhattacharya et al., 1995c). Occurrence of arsenic in natural
water depends on the local geology, hydrology and geochemical characteris-
tics of the aquifer materials. Furthermore organic content in sediments as well
as the land-use pattern may also be important factors controlling the natural
mobility of arsenic in alluvial aquifers.
In spite of the reported occurrence of high As in groundwater in West
Bengal, the people of the area are solely dependent on the groundwater
resources in this region. The research carried out so far has concerned the
quantitative determ ination of the level of arsenic in groundwater and epi-
demiological studies among the population in the infested zone. Geochemical
investigations pertaining to the occurrence of As in groundwater in West
Bengal should receive priority to evaluate overall quality of the groundwater
and quanti® cation of geochemical processes that control groundw ater chem-
istry. The examination of the present nature of hydrological conditions in the
aquifer system is essential to understand the impacts of groundwater devel-
opment.
The chemistry of the solid phase, i.e. soils , minerals and underly ing
bedrocks together considered as aquifer materials, and their interaction with
the aqueous phase play a key role in controlling the retention and/or
mobility of As under different redox conditions within the subsurface en-
vironment (Bhattacharya et al., 1995a,b). The occurrence and origin of As in
groundwater depend on several factors such as adsorption ± desorption as well
as precipitation ± dissolution of unstable As minerals, subsurface redox con-
dition, grain size of aquifer materials, ion-exchange capacity of the aquifer,
mineralogy of the aquifer, organic contents, biological activity and aquifer
characteristics (Robertson, 1989). The present contribution therefore aims to
highlight the need for proper understanding of the problem in order to pre-
dict the possible primary mechanisms responsible for retention or mobility of
various As species through adsorption/desorption phenomena in groundwa-
ter under the prevailing redox condition. The redox conditions are likely to
play a key role in determining the mobilization of arsenic (Robertson, 1986).
Any phenomenon affecting the redox conditions such as the pumping rate
and the land-use pattern is of interest in revealing the mechanisms behind
the excess arsenic content in groundwater.
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O ccurrence of Arsenic-contam inatedGroundwater in Aquifers 81
Figure 1. Map of West Bengal, showing the areas with high arsenic in
groundwater in the delta plains of Eastern India.
Natural Arsenic Contamination in the Indo-Gangetic Delta Plains
Geographical D istribution
High arsenic in groundwater has been encountered in Nadia, Murshidabad,
Malda, Barddhaman and the North and South 24-Paraganas districts of West
Bengal, restric ted between latitudes 21°30 9 and 27°10 9 N and longitudes 86° and
90°E (Figure 1). The total affected area covers around 34 000 km2, representing
nearly 39% of the total area of the state. The geographical extent of the
arsenic-infested area is about 450 km from the district of Malda in the north to
the 24-Paraganas district in the south, affecting about 35% of the total population
of the state. Table 1 summarizes the present status of geographical distribution,
demography and epidemiological impacts of elevated As concentration in six
districts of the state. Analytical data on elevated As concentration of tubewell
water samples (June± July 1995) from Nadia district indicate that 143 villages are
affected out of 221 situated in the arsenic-prone area (Table 2).
Epidem iological Impacts
Approximately 175 cases of arsenical dermatosis were reported during 1983± 84,
from the bordering districts of Nadia, Murshidabad and Malda, while a few
cases were also reported from the districts of North and South 24-Paraganas and
Barddhaman (Guha Mazumder et al., 1988). Symptoms such as hyperkeratosis
and hyperpigmentation in palms and soles, and non-cirrhotic portal ® brosis
were clinically observed among the affected population. Epidemiological studies
have shown evidence of arsenical dermatosis and hepatomagaly among nearly
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82 P. Bhattacharya et al.
Ta
ble
1.
Geo
gra
ph
ica
ld
istr
ibu
tio
n,d
em
og
rap
hic
pa
ram
ete
rsan
dep
idem
iolo
gic
al
imp
act
so
fa
rsen
icco
nta
min
ate
dg
rou
nd
wa
ter
fro
mtu
bew
ell
sin
Ben
ga
lD
elt
aP
lain
s(d
ata
inco
rpo
rate
du
pto
July
19
95
)
Ars
en
ic-a
ffect
ed
24-P
arg
an
as
24
-Parg
an
as
dis
tric
ts(S
ou
th)
(No
rth
)M
ald
aB
ard
dh
am
an
Mu
rsh
ida
ba
dN
ad
iaT
ota
l
To
tal
are
a(*
10
3k
m2)
10
.02
4.1
33
.73
7.0
25.5
23
.93
34
.34
To
tal
po
pu
lati
on
(*10
6)
5.7
27
.28
2.6
56
.05
4.7
43
.85
30
.3
To
tal
nu
mb
er
of
blo
ck
s3
022
15
34
26
14
144
To
tal
nu
mb
er
of
ars
en
ic-
aff
ecte
db
lock
s3
52
29
13
41
Perc
en
tag
eo
fa
rsen
ic
occ
urr
en
ces
inth
eb
lock
s1
041
33
63
57
6.5
Ð
To
tal
nu
mb
er
of
aff
ect
ed
vil
lag
es
54
50
75
20
92
14
34
34
Ra
ng
eo
fars
en
ic
co
ncen
trati
on
in
gro
un
dw
ate
r(i
nmg
/l)
10±
46
01
0±
490
10±
410
10
±4
50
10
±5
60
10
±5
90
Ð
Ra
ng
eo
fd
ep
tho
fa
rsen
ic-
co
nta
min
ate
dw
ell
s1
2±
16
01
2±
125
12
±8
52
0±9
512
±8
512
±1
50
Ð
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O ccurrence of Arsenic-contam inatedGroundwater in Aquifers 83
Ta
ble
2.
An
aly
tica
ld
ata
on
As
con
cen
tra
tio
ns
ing
rou
nd
wa
ter
ex
tra
cted
fro
mth
eall
uv
ial
aq
uif
ers
of
Ben
ga
lD
elt
aP
lain
s
at
dif
fere
nt
blo
cks
of
Na
dia
Dis
tric
t,W
est
Ben
gal
No
.o
fh
am
lets
No
.o
fsa
mp
les
No
.o
fsa
mp
les
wit
hR
an
ge
of
As
No
.o
fsu
bd
ivis
ion
saff
ect
ed
wit
hA
san
aly
sed
fro
mA
sele
va
ted
lev
el
of
As
co
ncen
trati
on
Blo
ck
co
vere
din
the
stu
dy
(,
50
mg/
l)aff
ect
ed
vil
lag
es
(,
50
mg/
l)(m
g/
l)
Kari
mp
ur
I3
21
16
10
±1
85
Kari
mp
ur
II26
13
24
15
7±1
88
Teh
att
aI
33
36
34
10
±4
85
Teh
att
aII
83
28
810
±1
85
Kali
gan
j6
53
36
10
±3
00
Nak
ash
ipa
ra10
66
610
±2
40
Nab
ad
wip
86
36
20
10
±1
75
Han
sk
ha
li5
36
520
±7
0
Kri
sh
na
gan
j9
33
310
±9
0
Hari
ng
ha
ta11
47
50
±7
0
Ch
ak
da
h14
10
10
10
10
±2
20
Sa
nti
pu
r15
71
79
10
±5
7
Ch
ap
ra1
14
410
±6
0
To
tal
119
66
22
11
43
Ð
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84 P. Bhattacharya et al.
92.5% of the population exposed to arsenic in the concentration of 0.2± 2.0 mg/l
in contrast with about 6.25% of the population with # 0.05 mg/l in drinking
water (Goriar et al., 1984; Chakraborty et al., 1987; Guha Mazumder et al., 1988).
Laboratory investigations of the urine, nails, hair and skin-scales as well as the
analysis of biopsy samples collected from a cross-section of the affected people
of the area revealed signi® cant concentrations of As (Chatterjee et al., 1995; Das
et al., 1995). The evidence of bio-accumulation of As has also been observed
among cattle (Das et al., 1995) due to the consumption of 40± 50 l/day of
arsenic-contaminated groundwater.
Physiographic and Geomorphologic Fram ework
The major physiographic and geomorphic domains in the state of West Bengal
and their geographic locations are presen ted in Table 3. The area encircled by the
rivers Padma and Bhagirathi and the Bay of Bengal in the south represents the
Gangetic delta. The delta plain comprises a thick succession of sediments
deposited by the Ganga± Brahmaputra river systems with a typical southward
gradient. The delta plain is typically of moribund character and formed due to
silting of the old river levees. The Ganga has shifted eastwards from its original
course and is branched into two distributaries, Bhagirathi± Hooghly and Padma±
Meghna. The causes of shifting of the Ganga and the meandering behaviour of
the river have not been properly understood. Among the possible explanations
for the changes in the river courses are alluviation at the heads of successive
main spillways, response to neotectonism and eustatic sea-level changes. Secular
sw ing in the course of the Teesta towards the east is a recent manifestation
(PHED, 1991).
The upper delta plain (UDP) with a gentle southerly slope is characterized by
a series of meander belts formed by the ¯ uvial processes as a response to
varying hydrodynamic conditions . The wavelengths and amplitudes of the
various segments of the meander belts vary widely and are often characterized
by detached loops of ox-bow lakes and alluvial ridges. Other geomorphic forms
are levees and swamps in between inter-distributary levees. The basin-® lled
deposits are ¯ uvial deposits and comprise stacks of different cycles of upwards
® ning sequences (sand ± silt± clay). Such cyclic sedimentation (symmetrical and
asymmetrical) in the form of festoon-bedding are found with coarse to medium
sand, ® ne sand, clay and silt respectively . The arseniferous belts are located in
the upper delta plain and in the abandoned meander channels as well as scrolls
(Figure 2). The plain between the moribund delta in the north and the Sun-
darbans (the coastal part) is considered as the lower delta plain (LDP). The rivers
¯ ow with a gentle slope towards the south. The Sundarbans is the currently
active delta and is covered with tidal mangrove forests.
Sedimentological Characteristics
A thick pile of ¯ uvial sediments pertaining to the Quaternary age constitutes the
Bengal delta plains . The UDP comprises a composite sequence of meandering
riverine deposits of the Proto-Padma Meander Belt (PPMB) with a NNE± SSW
trend. The belt merges with the lateritic piedmont plain towards the north-east.
The PPMB is transected by four younger meander belts of the Padma
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O ccurrence of Arsenic-contam inatedGroundwater in Aquifers 85
Table 3. Major physiographic and geomorphic domains of West Bengal, Eastern
India
Geographic location Physiographic domain Geomorphic domain
Darjeeling and northern parts of Mountainous terrain Himalayan and sub-
Jalpaiguri district H imalayan ranges
Cooch Bihar, Jalpaiguri and northern Sub-montane terai Sub-montane terai
part of West Dinajpur
Malda and West Dinajpur Para delta Terrace of older lateritic
alluvium
Birbhum, Bankura, Purulia and part Laterite upland Laterite piedmont plain
of M idnapur
Murshidabad, Nadia and parts of Gangetic delta Upper delta plain of
Barddhaman and North 24-Parganas meander belt
Barddhaman, Hooghly and parts of Gangetic delta Marginal fan and valley
Midnapur margin fan
Calcutta, Howrah, South 24-Parganas Lower Gangetic delta Lower delta plain
and parts of North 24-Parganas
Figure 2. Diagrammatic sketch showing the deposition of mutually truncating
® ning upward sequences in a typical cross-section of the arseniferous alluvial
sediments of the UDP.
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86 P. Bhattacharya et al.
Figure 3. Section showing the disposition of the various aquifers along a
NNW ± SSE transect of the UDP. N ote: The sediments are classi® ed into three units
among which Unit 2 is arseniferous.
river (PMB) towards the south-east of the upland plain. The River Bhagirathi
also indicates development of three consecutive meander belts in the western
part of the PPMB. These meander belts are discontinuous in nature and often
preserved as festoons or show truncated character. Upward-® ning cycles of
various thicknesses constitute the meander belts (Saha & Chakraborty, 1995).
Abrupt changes in lithology could be explained in terms of ¯ uctuations in the
hydrodynamic conditions and the result of the erosion of the older sediments as
well as the deposition of younger sediments. The sediments are in general sand,
silt and clay in the younger meander belt sequences, while the older ones are
more sandy and underlain by gravel beds that indicate the existence of high-en-
ergy streams during their formation (PHED, 1991).
The oldest gravel beds of large dimensions were formed possibly during the
rapidly rising stage of the Flandrian transgress ion. The extensive clay beds of the
Tertiary period are observed in the subsurface at depths of about 150 m and
show unconformable relation to the younger ¯ uvial cycles. No strong evidence
of older Quaternary deposits was found at this depth possibly due to erosion
during the falling stage of the sea level (PHED, 1991).
H ydrogeologicalCharacteristics
The northern part of the Bengal basin is characterized by extensive near surface
aquifers of uncon® ned nature. The lithology is primarily dominated by interca-
lations of sand, silt and clays of Quaternary age. Subsurface ridges of basements
demarcate the hydrological boundary to the lower and deeper parts of the basin
in the south. The basin is, however, open towards the south-east into the
Tertiary formations of Bangladesh. Con® ned aquifers occur at depths of about
300 m, with possibilities of zonal inter-connection with the upper group of
uncon® ned aquifers especially along the tectonic troughs through which the
major rivers ¯ ow.
Groundwater occurs under uncon ® ned condition particularly in the Nadia,
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O ccurrence of Arsenic-contam inatedGroundwater in Aquifers 87
Figure 4. Simpli® ed lithological column together with the disposition of arsenic-
contaminated aquifers at a borehole site in Nadia District, West Bengal, India.
Murshidabad and Malda districts and in semi-con® ned condition in Bard-
dhaman, and the North and South 24-Paraganas districts. Thus the aquifers
change gradually from open to semi-con® ned character towards the south
(Figure 3). The closed aquifers are genera lly inter-connected with the upper
groups of open aquifers.
Fluvial sand and gravel are the principal deposits forming the major aquifers.
The recharge areas are located in upland and sub-mountain fronts. The deposits
in the aquifers are ® ne, medium and coarse sand with gravel. Clayey intercala-
tions are darker in colour, possibly re¯ ecting elevated contents of organic matter.
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Sandy clay mixed with kankar and coarse, medium to ® ne sand characterize the
open hydrological system s. The semi-con® ned systems are deposits with ® ne
white sand with clayey intercalations and medium to coarse sand with gravels
towards the bottom. Fine-grained deposits are also accumulated in the bottom of
the aquifer overlain by a pure ® ne white sand. The lithological succession of a
well site in Nadia district is given in Figure 4.
The aquifers in the lower delta plain and coastal tracts of Midnapur districts
normally lie deeper ( . 200 m) under a blanket of a widespread aquiclude in the
near surface zone (Saha & Chakraborty, 1995). The groundwater in the western
part of the basin is restric ted to localized zones found both in con® ned and
uncon® ned conditions. The eastern margin of the groundwater basin is formed
from the fractured and weathered parts of the old rocks and in minor channel-
® lled sediments of some streams.
M ineralogical and Geochemical Constraints
Investigations have revealed that within the meander belt the As contaminated
groundwater is mainly con® ned to the intermediate aquifer (20± 80 m), while the
occurrence of As in the shallow and deep aquifers (90± 150 m) is quite limited.
The absence of impervious clay partings between the intermediate and deeper
aquifers seems to play an important role for the occurrence of As. The analytical
data indicate that the major ions are calcium, magnesium and bicarbonate with
elevated contents of iron, phosphate and arsenic. Contents of sulphate, chloride
and ¯ uoride are low. Distinct trends of increasing arsenic have been docu-
mented during pumping, suggesting a release of As ¯ owing in from distant
sources (PHED, 1991; Chatterjee et al., 1995; Das et al., 1995).
Mineralogical investigations by SEM and EDX of aquifer materials have
established that arsenic occurs in the silty clay as well as in the sandy layers as
coating on mineral grains. The impersisten t clay horizons separating the shallow
and intermediate aquifers have yielded a relatively high arsenic content with
occasional distinct grains of arsenopyrite observed (PHED, 1991).
Arsenic in groundwater is con® ned to the meander belt zone of the UDP
comprising Late Quaternary sediments. Groundwater extracted from older sed-
iments of Barind and Ilambazar Formations in the area west of the UDP do not
indicate As occurrence. Clayey sediments intercalated within the sandy aquifers
at depths between 20 and 80 m might act as a major source of As in groundwa-
ter. It has been inferred that the sediments were transported from the source
terrains located in the Chhotanagpur± Rajmahal high lands in eastern Bihar and
deposited by sluggish meandering streams in the Bengal ¯ ood plain under
reducing conditions.
Alternatives for Safe Drinking Water Supply
Groundwater contamination is a priority environm ental issue particularly in the
context of a safe drinking water supply for the semi-urban and rural population
in developing countries such as India, China, Chile and Bangladesh. Occurrence
of arsenic in groundwater can be attributed to natural sources or be induced by
anthropogenic activities or through a combined effect.
Several options for the supply of safe drinking water were suggested by the
joint investigators to the planners and a few of them were adopted during
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O ccurrence of Arsenic-contam inatedGroundwater in Aquifers 89
subsequent action plans (PHED, 1991, 1993). Extraction of water from deeper
aquifers (150± 300 m) has so far remained the most viable alternative to safeguard
the drinking water supply, as long as concentrations of arsenic remained below
the permissible limit ( , 50 m g/l). This expensive operation has not been success-
ful as elevated levels of arsenic have been noted in recent groundwater samples.
This could be due to leaching and downward movement of soluble arsenic from
the overly ing sediments resulting from forced extraction of groundwater. Recent
observations suggest that the problem of high As contents in groundwater has
affected aquifers at several depths as well as spread to newer areas, emerging as
the greatest natural arsenic calamity in the world (PHED, 1993).
Chemical treatment using coagulation, ¯ occulation, sedimentation, ® ltration
and disinfection is considered as another suitable option for the removal of As
from groundwater for drinking purposes (Bellack, 1971; Gulledge & O’Connor,
1973; Shen, 1973; Gupta & Cheng , 1978; Sorg & Legsdon, 1978; Hathway &
Rubel, 1987; Harper & Kingham, 1992; Brewstar, 1994; Cheng et al., 1994;
Edwards, 1994; Scott et al., 1994; Hsia et al., 1994). In Chile, a population of nearly
200 000 is served by a full-scale conventional treatment plant for the removal of
As. This experience suggests that for source water with high As concentrations,
more stringent standards for As ( , 20 m g/l), now being considered, could not be
met by conventional coagulation (Sancha, 1995). The problem of chemical
treatment of groundwater in West Bengal is dif® cult to solve by conventional
practice owing to the occurrence of As in variable oxidation states, As(III) and
As(V) and distinct variations in their ratio. The redox speciation of As has
signi® cant implications for the ef® ciency of treatment processes (Hering &
Elimelech, 1995). Transformation of As species from a lower to higher oxidation
state can be achieved by using a suitable oxidizing agent before coagulation
(Mazumdar et al., 1993). Effectivity and maintenance of such high-cost, full-scale
treatment plants are not viable alternatives for municipal water supply schemes
in rural and semi-urban areas in developing countries like India with poor
infrastructural facilities. Another major aspect of the applicability of such pro-
cessing plants concerns the safe disposal of sludges containing high As.
Action plans suggested by state planners during 1993± 94 recommended
distribution of surface water from distant sources in selected areas of Malda
district, amendment of groundwater in domestic as well as tubewells by at-
tached ® lter units or dugwells ® tted with hand-pumps (PHED, 1993). The merits
of these options for safeguarding the supply of drinking water in the arsenic-in-
fested zone are yet to be established. The economy of long-range transportation
of surface water, further handling and disposal of used ® lter units and bacterio-
logical contamination of water from the dugwells are the possible lacunae of
these options. Assessment of the technical feasibility , social acceptability and
cost-bene® t analysis of these options would therefore be important while imple-
menting these techniques for future planning of safe drinking water supply in
rural and semi-urban areas of West Bengal.
Discussion
None of the proposed remedies has been proved to solve the problem for the
rural and semi-urban drinking water supply. Moreover the mechanism s of As
mobilization are not unequivocally explained. The possibility of localizing
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90 P. Bhattacharya et al.
groundwater of acceptable quality depends on an integrated knowledge of the
geology, hydrogeology and hydrochemistry of the aquifers and their setting.
Even remedies such as rein ® ltration of ironous, As-containing groundwater
must be based on a sound interpretation of the mechanism involved in the
mobilization of the As.
The development of the problem is likely to be closely associated with the
redox conditions in the aquifers (Robertson, 1986, 1989). Currently it is not
known whether the arsenic concentrations in the groundwater have always been
high in the second aquifer or whether the concentrations have increased as a
consequence of groundwater development. The bedrock source of the arsenic is
inferred to be a source rock in eastern Bihar in the Chhotanagpur± Rajmahal hills.
The arsenic contained in pyrite or arsenopyrite has been deposited in the
sediments and partly redistributed there. The part contained in clays may have
remained in its initial form while in the sandy sediments it has oxidized and
been adsorbed onto ferric coatings on the sand grains. We can infer that arsenic
is mobilized mainly by two processes:
(1) oxidation of pyrite and/or arsenopyrite in clayey intercalations;
(2) reduction of ferrous coatings on sand grains releasing arsenic and adsorbed
phosphate.
Most of the arseniferous groundwater is high in ferrous iron and phosphate as
well. This seems to support the second mechanism. Moreover the generally low
level of sulphate contents in the groundwater indicates that the oxidation of
pyrite may not be the source of the arsenic. Thus anaerobic conditions leading
to reduction of ferric iron seems to be the most plausible mechanism for the
formation of the observed hydrochemical conditions in the UDP.
An important issue is whether groundwater extraction has affected quality.
The experiences from the pumping of the deeper aquifers when the arsenic
content increased with time indicate that the pumping rate may in¯ uence the
quality. The land-use pattern may also affect the water quality by creating more
or less anaerobic soil conditions. Wetland cultivation such as paddy ® elds may
give a shift to more reducing conditions. Drainage of swamps may act in the
other direction, increasing the oxygen diffusion into the ground.
The in¯ uence of land-use changes and pumping rates may be revealed by
time-series analysis of accumulated data. The measurement of redox potentials
in pumped groundwater is a means of de® ning the redox conditions prevalent
in the aquifers. The measured potential could be correlated with the potential
inferred from the ferrous/ferric and arsenate/arsenite redox couples (Holm &
Curtiss , 1989).
A simple batch test for sediments to test the potential for arsenic mobilization
could be helpful. Enclosing sediment and water along with an easily degradable
organic compound such as glucose may be a possible model system for the
simulation of arsenic mobilization under reducing conditions.
Laterite could be a possible ® lter medium for some of the As contaminated
groundwaters, notably those in which the arsenate dominates over the arsenite.
The laterite could be manipulated in a number of ways to achieve good physical
form and maximum adsorption capacity.
Rein ® ltration of ironous groundwater high in As may be an option if per-
meable surface sediments are present. In particular, arsenate may be removed by
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O ccurrence of Arsenic-contam inatedGroundwater in Aquifers 91
adsorption of the ferric precipitates. The removal and safe disposal of the ® lter
bed must be undertaken.
A cknowledgements
The authors would like to thank Jon Petter Gustafsson and Sune Nordqvist for
several stimulating discussions and comments on the preliminary drafts of this
manuscript. One of the co-authors (DC) would like to thank KTH for providing
the travel grants and all the facilities at the Division of Land and Water
Resources.
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