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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.

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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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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

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min

ate

dw

ell

s1

2±

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2±

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±8

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0±9

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±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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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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