Introduction

Arsenic contamination of groundwater is a problemworldwide, and the Terai Basin in the southern plains ofNepal is no exception. The Department of Water Supplyand Sanitation of Nepal, with assistance from the WorldHealth Organization (WHO), surveyed three districts ofeastern Terai for the first time in 1999, and found thatarsenic concentration exceeded the 10 ppb guidelinerecommended by WHO. Other agencies working withdrinking water supply such as the Nepal Red CrossSociety, the Rural Water Supply and Sanitation SupportProject, Nepal Water for Health, UNICEF, and localNGOs then started testing for arsenic in groundwater,and found positive results in most places. Most of thework carried out so far has concentrated on quantitativetests of arsenic, short-term mitigation activities, andhealth impacts. Of the 18,635 tube wells tested, 23.7%have arsenic contents above the WHO limits of 10 ppb,and 7.4% were above the Nepal interim standard of50 ppb (Shrestha et al. 2004).

The aim of this study was to evaluate geological andgeochemical conditions of the Terai basin relevant toarsenic contamination. Core sediments samples from awestern Terai district of Nepal (Fig. 1) were collected byboring and analyzed for major oxides and trace elementsby X-ray fluorescence (XRF). Physical and chemicalproperties of water samples were determined with fieldkits as well as laboratory tests. The chemistry of sedi-ments and water from a contaminated area in Nawal-parasi was compared with data from an uncontaminatedarea in Bhairahawa. The results show that the chemistryof sediments is similar, but water chemistry variesdepending upon the grain size of sediments and the or-ganic matter concentration.

Terai Basin

The southern plains of Nepal known as the Terai areremarkably flat, with an average altitude of 115 m abovemean sea level, and stretch from east to west along the

Jaya K. Gurung

Hiroaki Ishiga

Mohan S. Khadka

Geological and geochemical examinationof arsenic contamination in groundwaterin the Holocene Terai Basin, Nepal

Received: 15 February 2005Accepted: 2 August 2005Published online: 27 September 2005� Springer-Verlag 2005

Abstract Geological and geochemi-cal study has been carried out toinvestigate arsenic contamination ingroundwater in Nawalparasi, thewestern Terai district of Nepal. Thework carried out includes analysesof core sediments, provenance studyby rare earth elements analyses, 14Cdating, and water analyses. Resultsshowed that distribution of the ma-jor and trace elements are nothomogeneous in different grain sizesediments. Geochemical characteris-tics and sediment assemblages of thearsenic contaminated (Nawalparasi)and uncontaminated (Bhairahawa)

area have been compared. Geo-chemical compositions of sedimentsfrom both the areas are similar;however, water chemistry and sedi-mentary facies vary significantly.Extraction test of sediment samplesshowed significant leaching of ar-senic and iron. Chemical reductionand contribution from organic mat-ter could be a plausible explanationfor the arsenic release in groundwa-ter from the Terai sediments.

Keywords Arsenic Æ Teraisediments Æ Geochemistry Æ Nepal

Environ Geol (2005) 49: 98–113DOI 10.1007/s00254-005-0063-6 ORIGINAL ARTICLE

J. K. Gurung (&) Æ H. IshigaDepartment of Geoscience,Shimane University,Matsue 690-8504, JapanE-mail: s049709@matsu.shimane-u.ac.jpTel.: +81-852-326078Fax: +81-852-326469

M. S. KhadkaDepartment of Irrigation, HMG/Nepal,2055 Jawalakhel, Nepal

southern border with India (Fig. 1). Although the Teraiconstitutes less than one-fifth of Nepal’s total area, itcontains over half the total arable land. Terai produces60% of the national food grain crop, and is home to47% of the total Nepalese population of 23 million.

Groundwater is the main source of water for drinkingand irrigation in the Terai. The local population hasused groundwater from historical times, primarily fromstreams, ponds, hand-dug wells, and more recently fromtube wells. Over 90% of the Terai population drawsgroundwater from tube wells for drinking, householduse, and irrigation. An estimated 800,000 shallow tubewells and over 3,000 deep tube wells exist in the Teraiplain. Demand for water has increased steadily withincreasing population and the accompanying increase inagriculture activity.

Geologically, the Terai Plain is an active forelandbasin consisting of Quaternary sediments that includemolasse sediments along with gravel, sand, silt, andclay (Sharma 1995). Many rivers in Terai flow fromnorth to south. The major rivers originate from thehigh Himalayas. Minor rivers emanate from the nearbySiwalik Hills, and deposit sediments in the form of a

fan along the flank of the Terai basin. Fine sedimentsand organic material are deposited in inter-fan low-lands in wetlands and swamps (Sharma 1995). Oldersediments were buried by younger materials, forming athick pile of sediments which have often been erodedand reworked (UNDP 1989). The large volume of riverflux and the basin configuration creates a variety ofriver morphologies, including meandering and braidedsystems.

High monsoon precipitation (1,800–2,000 mm) andyear-round snow-fed river systems recharge the Teraisediments, giving them high potential for groundwaterresources. Shallow aquifers (<50 m) are generallyunconfined or semi-confined, whereas the deep aquifers(>50 m) are mostly confined by impervious clay layers.Shallow aquifers are not continuous horizontally, arelens-shaped, and behave as perched aquifers. Regionalgroundwater flow is from north to south (flow gradient2 m per km; UNDP 1989). The aquifer system is highlysensitive to precipitation, especially the shallow aquifers.However, most rainfall is lost as runoff. Only 10% of theaverage rainfall of 1,600 mm infiltrates and rechargesshallow aquifers at Nawalparasi (UNDP 1989).

Fig. 1 Location of the Terai Basin, Nepal in the Ganges–Bramhaputra–Meghna watershed areas

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Study areas

Nawalparasi is situated in the western Terai district ofNepal. Samples were collected from three villages(Kunwar, Goini, and Manari) in the eastern Nawal-parasi bazaar of the Nawalparasi district, between27�31¢ to 27�32¢N and 83�40¢ to 83�43¢E (Fig. 2). Sam-pling was also carried out at Bhairahawa, 25 km west ofNawalparasi. Sediment samples at Bhairahawa werecollected from the compound of the Bhairahawa Lum-bini Groundwater Irrigation Project, located at27�31¢43¢¢N and 83�41¢29¢¢E.

Methodology

Sample collection

Sediment samples were taken using an indigenousmanual drilling percussion method, locally known as‘‘dhikuli’’. This method consists of a set of wooden

pillars, drilling iron pipes, and a steel chain. A 40 mmdiameter iron pipe open at both ends is used as thedrilling tool. The drilling pipe is hoisted by a steel chainattached to the pillars, and downward percussion iscreated manually. One end of the drilling pipe is used asa drilling chisel and the upper mouth is kept closed andopened periodically by hand. Sediments cuttings fromthe bottom of the pipe are pushed up by the sludge andcollected. Two hundred grams of sample from eachlithological unit was collected and packed in plasticbags. Water samples at Nawalparasi were also collectedfrom hand pumps around the sediment sampling sitesand along an E–W traverse (Fig. 2). Pumps were runbefore sample collection to remove all standing water inthe tube wells. We did not filter the water samples be-cause they were being directly used by villagers fordrinking. Filtration was also avoided to minimize thepossible oxidation of arsenic during the filtration pro-cess. No suspended particles were found to have settledat the bottom of the sample bottles. Temperature, pH,oxidation–reduction potential (ORP), chemical oxygen

A

B

Fig. 2 Location of boring operation and tubewells studied, Nawalparasi, Western Terai Basin, Nepal

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demand (COD) and electrical conductivity (EC) werestudied at the time of collection, using portable field kits.Each sample was also tested for arsenic with a HironakaKit. In this kit, water is treated with reagents that pro-duce arsine gas (AsH3), which reacts with bromide paper(HgBr2). The stain thus produced ranges from yellow tobrown, showing low to high concentrations of As (Hi-ronaka 2000). The detection limit of the kit is 0.01 ppm.At each site, 200 ml water samples were collected andacidified with 1 N HCl, and taken to the laboratory forfurther examination.

Laboratory analysis

XRF analysis

Fifty grams of each sediment sample was dried in an ovenat 110�C for 24 h. The dried samples were crushed in anagate mortar and pestle for 10 min to ensure the particlesize was <63 lm. Powdered samples were compressedinto briquettes using a force of 200 kN for 60 s. Thebriquettes were then analyzed for selected major oxides(Fe2O3, TiO2, CaO and P2O5), total sulfur (TS), andtrace elements (As, Pb, Zn, Cu, Ni, Cr, V, U, Th, Sc, Y,Nb, Sr, Zr, Br, and I), using a RIX-2000 XRF spec-trometer (Rigaku Denki Co. Ltd.) at Shimane Univer-sity. The concentrations of the elements were determinedby the pressed powder method (Ogasawara 1987).Average error for these elements is less than ±10%.

Instrumental neutron activation analysis (INAA)

Rare earth elements (REE: La, Ce, Sm, Eu, Gd, Tb, Yband Lu), and other trace elements (Ta, Sb, Hf, Cs, Co,Rb, and Ba) in selected samples were analyzed by INAAat the Kyoto University Research Reactor Institute. Themethod used is described by Musashino (1990). Averageerrors for these elements are less than ±10%, and theresults are acceptable when compared with values for theGeological Survey of Japan Standard JA-2 as given byPotts et al. (1992). The Eu anomaly (Eu/Eu*) was cal-culated by the formula given by Condie (1993): Eu/Eu*= EuN/(SmN · GdN)

1/2 ; where Gd= (Sm · Tb2)1/3;

and N= chondrite normalized concentrations.

Atomic absorption spectrophotometer (AAS) analysis

Water samples were analyzed for As and Fe at theShimane University, using an AAS (Shimadzu AA-660G) with a graphite furnace atomizer (GFA-4B).Calibration of the AAS was made using a blank solution(1N HNO3,0.01 ml/L) and a standard arsenic solution(1,000 ppb). The detection limit of the AAS used is lessthan 1 ppb.

Elution analysis

Samples were treated with alkali and acid solutions toextract arsenic from organic matter and iron oxide hosts,respectively. The procedure used followed the bottomsediment test method as set by the Association forEnvironment Agency of Japan (1995). Ten grams of thesample was separately treated with 50 ml of 1 N HCland 0.5 N NaOH solutions. Both were agitated forabout 30 min in an electric shaker (250 rpm) at roomtemperature (20�C). Samples were centrifuged at3,000 rpm for 5 min. The supernatant liquid thus ob-tained was analyzed by AAS for arsenic and iron con-tents. The eluted amount was corrected for the volumeof solution to derive rock mass equivalent values (elutedvol.·10 ml/50 ml). Elution rate was calculated using theformula given by the Association for Environmen-tal Measurement and Analysis in Japan (1995): ER=(Celuted/Cbulk)* 100%, where Celuted is the eluted con-centration, and Cbulk is the bulk concentration in the soilsample.

Carbon dating

14C dating of molluscan shells and wood fragments wasdone at the Geoscience Laboratory, Nagoya Japanusing accelerator mass spectrometry (AMS). Known agereference materials were analyzed to verify the accuracyof the results. The calibration was done using the latestcalibration data (Stuiver et al. 1998).

Analytical results

Subsurface lithology and geological cross-section

Lithologies at all three sites at Nawalparasi are similar,being predominantly silt, clay, black clay, and fine sand(Fig. 3). The uppermost layer is yellowish brown silt.Gray clay underlies the top silt bed, and is characterizedby concretions, molluscan shells, and wood fragments.A sticky black clay bed rich in carbonate concretions,organic matter, and small gastropod shells occurs at adepth of 12–15 m. The thickness of this bed varies from0.5 to 1 m. Very fine sand beds were found at all sites,alternating with clay beds of variable thickness (5 cm–1 m). Fine sand is generally gray or is locally bicolored(gray and yellowish brown). Clayey silts and silt layersoccur interbedded with clay beds and fine sand beds. Thegeological cross-section clearly shows that the black claybed found in all three boreholes at Nawalparasi can becorrelated. Most existing tube wells tap water fromaquifers below the black clay (Fig. 3a, b). Lithology ofthe Bhairahawa cores differs slightly from those at Na-walparasi. At Bhairahawa a 4 m thick gravel bed is

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Fig. 3 Geological cross-sections of the study area, a along E-W and b along N-S direction, X–Y and A–B respectively, from the locationmap (Fig. 2)

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intercalated within fine sand and clay beds. The gravelbed consists of angular cobbles, pebbles, and coarsesand. The sandstone grains are stained by iron oxides. Ablack clay strip of about 0.5 m thick is observed at adepth of 20 m. Unlike Nawalparasi, no concretions arepresent at Bhairahawa.

Sediment geochemistry

Core ND-1 (Kunwar)

Twenty-six core samples from Kunwar village (ND-1)were analyzed by XRF. The average concentration ofarsenic for the sediments is 9 ppm (Table 1).

However, the distribution of arsenic is not uniformamong sediment types. In general, arsenic concentra-tions are greater in the finer grained lithotypes (blackclay and clay) than in coarser materials (fine sand andsilt). The average As concentration in clay and blackclay is 10 ppm, and in silt and fine sand is 6 ppm. Thehighest arsenic concentration (16 ppm) is observed inthe clay layer at a 14 m depth (NP-1-15). Averagetotal iron as Fe2O3 is 4.7 wt.%. Vertical distributionof iron is similar to that of arsenic, with higher con-centrations in clay and lower concentrations in siltsand fine sands (Fig. 4). Fine sand contains 3 wt.%

Fe2O3 on average, while clay and black clay averages6.8 wt.%. TS averages 0.03 wt.% overall, but blackclay has higher concentrations of sulfur (0.06 wt.%)compared to fine sand and silt (0.03%). Calcium oxide(CaO) is exceptionally enriched in the clay, silt, andfine sand beds (16 wt.%, n=6) between 14 and 19 m.Vanadium is enriched in black clay (163 ppm) andclay (107 ppm) relative to coarser samples such as silt(41 ppm) and sand (54 ppm). Similarly, other elementsincluding Zn, Cu, Th, Sc, Zr, and V have variableconcentrations in differing lithotypes and are generallymore abundant in clay and black clay than in finesand and silt (Fig. 4). The average Th/Sc ratio is 1.3.Bromine is <1 ppm and iodine averages 27 ppm.

Core ND-3 (Manari)

Twenty samples were analyzed from the Manari villagecore (ND-3, Fig. 2). Results are similar to those for ND-1. Average arsenic concentration is 9 ppm, but again thedistribution is not uniform (Table 2, Fig. 5). The highestvalue (31 ppm) was found in black clay at a 20 m depth(NP-3-18). Sandy samples average 5 ppm As, clay8 ppm, and black clays 15 ppm. Total iron follows atrend similar to As, with higher concentrations in fine-grained samples and an overall average of 5 wt.%(Fig. 5). The black clay samples have the highest average

Table 1 Major and trace elements analyses, core ND-1, Nawalparasi Nepal

Sample ID Depth (m) Code Major oxides (wt%) TS (wt%) Trace element (ppm)

Fe2O3 TiO2 CaO P2O5 As Pb Zn Cu Ni Cr V U Th Sc Y Nb Sr Zr Br I

NP-1-0 0 S 2.30 0.67 0.80 0.16 0.04 5 18 43 19 18 48 44 3 12 3 23 10 26 312 1.0 28NP-1-1 1 S 4.21 0.66 2.48 0.14 0.02 10 22 44 13 26 54 69 3 15 8 27 11 34 261 0.6 26NP-1-2 2 C 5.50 0.83 0.82 0.10 0.02 9 24 67 23 36 68 116 5 18 10 37 14 40 220 ND 24NP-1-3 3 C 7.52 0.90 0.83 0.08 0.02 11 30 95 30 52 95 154 7 26 15 48 18 51 161 0.2 28NP-1-4 4 C 6.50 0.83 1.45 0.07 0.02 10 26 75 24 42 79 130 5 21 13 40 15 42 195 ND 29NP-1-5 5 SC 3.57 0.66 0.64 0.07 0.02 7 19 36 13 26 71 61 3 14 5 24 10 23 303 ND 27NP-1-6 6 FS 3.04 0.64 0.65 0.08 0.02 8 17 38 13 25 74 61 3 13 5 24 10 22 274 ND 26NP-1-7 7 S 1.02 0.39 0.61 0.06 0.02 2 11 16 6 15 96 11 2 6 ND 11 5 7 214 0.3 27NP-1-8 8 FS 0.88 0.37 0.66 0.05 0.02 2 11 16 3 14 74 5 2 6 1 10 4 7 216 0.2 22NP-1-9 9 SC 3.15 0.67 1.58 0.08 0.03 6 20 39 14 25 59 66 3 12 7 24 9 26 260 0.1 27NP-1-10 10 C 3.66 0.71 1.00 0.10 0.03 7 20 51 29 33 61 75 4 15 8 28 11 30 265 0.2 28NP-1-11 11 SC 4.75 0.74 2.71 0.10 0.02 7 21 53 17 27 54 90 4 17 9 33 13 45 257 0.5 28NP-1-12 12 C 6.28 0.91 0.85 0.07 0.02 10 28 83 29 53 91 142 6 23 15 44 17 45 190 ND 24NP-1-13 12.8 BC 8.34 0.79 2.12 0.01 0.06 10 25 99 48 63 109 163 5 22 18 40 13 108 147 1.8 27NP-1-14 14 C 6.67 0.79 4.00 0.09 0.04 9 22 76 32 51 84 131 5 17 14 34 13 217 135 1.1 30NP-1-15 14.3 C 3.75 0.46 21.9 0.09 0.05 16 14 41 20 14 38 54 3 12 22 21 7 217 95 0.3 29NP-1-16 15 FS 2.28 0.38 30.3 0.10 0.04 8 14 38 15 8 27 26 2 8 22 18 7 253 97 0.1 24NP-1-17 16 FS 5.05 0.65 16.0 0.11 0.03 5 21 63 26 18 46 82 4 15 19 30 12 254 144 0.2 29NP-1-18 17 C 5.13 0.71 14.8 0.12 0.04 12 22 75 50 40 52 93 5 14 20 34 13 294 112 0.1 33NP-1-19 19 FS 4.89 0.79 9.16 0.12 0.02 9 21 61 22 30 65 97 5 14 15 34 13 283 148 ND 29NP-1-20 19.2 C 5.60 0.88 1.30 0.08 0.04 9 22 77 30 52 92 122 5 18 14 36 14 125 204 0.5 29NP-1-21 20 C 5.23 0.76 3.36 0.08 0.02 15 20 60 23 42 75 106 4 16 13 31 11 229 191 0.2 27NP-1-22 21 C 3.50 0.65 5.68 0.05 0.02 7 18 36 14 27 45 66 3 13 12 23 9 75 239 0.3 25NP-1-23 23 C 6.84 0.85 1.59 0.08 0.02 11 23 75 27 43 78 131 5 20 12 38 14 63 185 0.1 26NP-1-24 24 C 6.24 0.83 3.08 0.08 0.02 11 22 78 29 44 79 124 5 22 14 40 15 69 176 ND 30NP-1-25 26 C 7.23 0.86 1.19 0.08 0.02 10 23 82 33 50 91 154 6 22 14 42 15 65 178 ND 29

Lithostratigraphy code; S Silt, C Clay, SC Silty clay, FS Fine sand, BC Black clay, ND not detected

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value (6.6 wt.%), and the lowest values are observed infine sands (3 wt.%). CaO is enriched (8.6 wt.%) from 17to 18 m depth in the fine sand layer compared to theoverall average (2.6 wt.%). Vanadium content is high inthe black clays (140 ppm) compared to sands (55 ppm).The trace elements Zn, Cu, V, Th, Sc are generally moreabundant in clays and black clays than in silts and finesands. The Th/Sc ratio averages 1.2. Average TS contentis 0.05 wt.%, but the black clay is enriched (0.13 wt.%)relative to clay, silt, and fine sand (0.02 wt.%). Two highvalues (0.31 and 0.19 wt.%) occur in samples from theblack clay layer at a depth of 18 and 19 m (NP-3-18,NP-3-19).

Core ND-4 (Bhairahawa)

Major oxide and trace element concentrations in theBhairahawa core (ND-4) are very similar to those ofNawalparasi samples (Fig. 6, Table 3). Average arsenicconcentration is 8 ppm, with the highest value of18 ppm in the clay bed at a depth of 19 m. Fe2O3

averages 4.8 wt.%, and is higher in clay (5.2 wt.%) than

in fine sand (3.8 wt.%). Calcium concentration increasesabruptly below 9 m, rising from <1% to 3–9 wt.%(Table 3). Concentrations of trace elements (Zn, Ni, Cu,Th, Pb) are similar to those at Nawalparasi.

REE analysis

REE and other elements (Ta, Sb, Hf, Cs, Co, Rb, Ba)were analyzed in five samples from Nawalparasi (ND-3).The REE data were normalized to chondrite (Taylorand McLennan 1985) and compared with the uppercontinental crust (UCC). The general pattern of thesediments is similar to UCC (Fig. 7, Table 4). Negativeeuropium (Eu/Eu*) anomaly is observed in all samples(average Eu/Eu* 0.59), but the depth of the anomaliesvaries (Eu/Eu* range 0.49–0.67). All samples are en-riched in light rare earths relative to the middle andheavy REE. Average LaN/SmN(3.9), GdN/YbN(1.5), andLaN/YbN(9.5) are similar to UCC values of 4.1, 1.3, and9.1, respectively. The average

PREE measured is

slightly greater (P

REE=127) than that of UCC(P

REEUCC 106, Taylor and McLennan 1985).

Fig. 4 Vertical profile from Kunwar Village, ND-1, Nawalparasi, Nepal

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Fig. 5 Vertical profile from Manari Village, ND-3, Nawalparasi, Nepal

Table 2 Major and trace elements analyses, core ND-3, Nawalparasi Nepal

Sample ID Depth (m) Code Major oxides (wt%) TS (wt%) Trace element (ppm)

Fe2O3 TiO2 CaO P2O5 As Pb Zn Cu Ni Cr V U Th Sc Y Nb Sr Zr Br I

NP-3-0 0 S 4.75 0.73 2.32 0.20 0.06 9 22 71 24 31 57 85 5 17 12 33 12 46 212 1.3 30NP-3-1 1 S 3.36 0.70 0.83 0.05 0.02 6 18 41 13 28 57 65 4 14 9 26 11 28 288 0.5 23NP-3-2 2 C 5.25 0.79 1.53 0.07 0.02 9 23 61 22 33 60 106 5 18 13 34 13 39 226 0.2 30NP-3-3 3 C 5.08 0.70 3.40 0.08 0.02 11 21 59 20 33 60 101 4 16 14 32 12 43 197 ND 24NP-3-4 4 C 7.35 0.88 1.12 0.08 0.02 11 28 86 31 46 88 150 6 22 17 43 16 48 181 ND 26NP-3-5 5 C 4.62 0.72 2.06 0.09 0.02 8 22 56 19 32 54 92 4 17 11 33 13 42 237 ND 26NP-3-6 6 S 2.59 0.72 0.70 0.05 0.02 5 18 33 12 26 58 56 3 15 8 27 11 23 391 0.1 27NP-3-7 7 S 5.03 0.78 0.75 0.03 0.02 8 22 47 21 33 60 95 4 16 11 30 12 29 299 ND 30NP-3-8 8 C 4.80 0.83 0.82 0.09 0.02 7 23 57 18 39 69 100 4 17 12 33 13 37 259 ND 27NP-3-10 10 S 5.30 0.71 2.91 0.10 0.02 5 20 50 15 25 58 78 4 15 12 29 11 42 219 0.2 25NP-3-11 11 C 3.66 0.68 3.15 0.09 0.02 6 20 49 16 29 54 73 4 15 11 28 11 45 239 ND 23NP-3-12 12 C 4.88 0.66 5.20 0.10 0.02 5 19 62 36 25 51 82 4 15 13 29 12 59 200 0.2 30NP-3-13 13 BC 7.87 0.78 1.12 0.08 0.04 10 27 104 41 66 108 178 7 25 22 47 15 58 124 0.7 33NP-3-14 14 BC 7.98 0.86 1.73 0.09 0.06 9 23 100 40 65 112 161 6 21 19 42 15 86 159 0.9 29NP-3-15 15 BC 7.33 0.88 2.05 0.08 0.05 10 22 87 41 60 107 153 6 19 18 45 15 99 168 0.9 28NP-3-16 17 FS 2.96 0.47 7.44 0.08 0.02 4 23 36 8 16 74 54 4 8 14 21 6 294 122 0.3 27NP-3-17 18 FS 3.04 0.63 9.94 0.16 0.02 3 20 39 6 13 68 57 3 17 16 30 11 291 169 0.4 29NP-3-18 20 BC 5.32 0.77 1.42 0.09 0.31 31 19 63 23 47 83 106 4 14 13 30 12 93 231 0.2 31NP-3-19 21 BC 4.80 0.78 1.19 0.08 0.19 16 17 54 22 45 84 100 4 15 12 28 12 77 253 0.7 28NP-3-20 22 S 4.74 0.68 3.06 0.09 0.02 6 18 42 17 31 64 80 3 13 11 25 10 87 293 0.6 27

Lithostratigraphy code; S Silt, C Clay, SC Silty clay, FS Fine sand, BC Black clay, ND not detected

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Water analysis

The average concentration of arsenic observed in thewater samples was 0.41 ppm (n=12), but contents var-ied locally from <0.01 to 0.74 ppm (Fig. 2). The sample

with the lowest As content (<0.01 ppm) had the highestORP value (109 mV). Samples with negative ORP hadhigh arsenic concentrations (Table 5). Average ironcontent was 5.3 ppm, ranging from 0.3 to 19.5 ppm. Nocorrelation with arsenic is evident. The pH of all water

Fig. 6 Vertical profile from Bhairahawa, ND-4, Rupandehi, Nepal

Table 3 Major and trace elements analyses, core ND-4, Bhairahawa Nepal

Sample ID Depth (m) Code Major oxides (wt%) TS (wt%) Trace element (ppm)

Fe2O3 TiO2 CaO P2O5 As Pb Zn Cu Ni Cr V U Th Sc Y Nb Sr Zr Br I

NP-4-0 0 S 3.39 0.59 0.78 0.10 0.05 5 16 40 11 30 72 52 2 8 4 18 9 18 212 0.8 29NP-4-1 0.9 SC 2.16 0.73 0.70 0.04 0.02 4 15 28 8 26 59 60 3 12 5 23 12 21 280 0.6 27NP-4-2 1.5 C 4.46 0.80 0.69 0.04 0.02 7 21 44 13 36 78 85 4 15 10 27 12 23 250 0.5 29NP-4-3 1.8 C 4.66 0.73 0.70 0.03 0.02 9 18 42 14 31 77 88 3 13 7 24 11 22 267 0.1 29NP-4-4 2.7 FS 3.06 0.56 0.63 0.01 0.02 6 15 30 8 27 64 52 2 8 5 16 8 14 217 0.4 28NP-4-5 3.6 FS 3.87 0.57 0.61 0.10 0.02 8 18 31 8 26 77 57 2 8 5 17 8 17 154 ND 30NP-4-7 9.7 FS 6.29 0.82 6.10 0.11 0.02 7 23 76 30 45 80 126 5 18 18 37 15 69 159 0.6 28NP-4-8 11.5 C 6.69 0.82 6.04 0.10 0.02 13 24 73 26 42 75 124 5 18 19 36 16 81 157 0.4 30NP-4-9 13 C 5.35 0.81 5.70 0.11 0.02 8 22 66 24 43 72 105 4 16 16 34 15 67 181 0.1 31NP-4-10 16.7 FS 2.18 0.41 8.07 0.11 0.02 4 12 26 4 20 58 30 2 5 11 13 5 35 117 0.7 29NP-4-12 19.1 C 5.32 0.81 4.54 0.11 0.02 18 20 64 22 42 72 105 4 17 14 33 16 73 205 ND 26NP-4-13 19.7 C 8.36 0.90 4.26 0.14 0.02 11 21 91 37 54 96 162 5 19 20 39 16 82 151 0.3 33NP-4-14 20.3 BC 7.40 0.89 3.64 0.13 0.03 8 24 83 32 58 97 149 5 19 17 38 16 85 169 ND 29NP-4-15 22.4 C 5.23 0.73 8.89 0.09 0.02 8 20 66 22 36 67 100 4 17 18 34 14 131 150 0.2 27

Lithostratigraphy code; S Silt, C Clay, SC Silty clay, FS Fine sand, BC Black clay, ND not detected

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samples was relatively neutral. COD averaged 1.85 ppm,ranging from 0 to 6 ppm. The average phosphate mea-sured was 0.53 ppm, with the highest value of 1.5 ppm(NP-3). ORP in all samples was low, with all negativeexcept two samples. Average EC was 788 lS/cm, with amaximum value of 1,278 lS/cm (NP-3).

Elution analysis

Ten samples (silt, silty clay, clay, black clay, and sand)were subjected to elution tests for arsenic and iron. In

acid solutions ER for arsenic ranges from 0.02 to0.44% compared to 0.07–0.61% in alkaline solutions.Arsenic release from black clay in NaOH was>60 ppb, but in HCl averaged only 14 ppb (Table 6).The overall arsenic elution rate was greater in theNaOH treatment (0.25%) than that by HCl (0.14%).Also, clay and black clay had a higher elution rate(0.41%) than silt and fine sands (0.11%) (Fig. 8). Feeluted by acid ranged from 0.01 to 6.2 ppm, comparedto 0.01–1.09 ppm for the NaOH treatment. In contrastto arsenic, average iron elution was higher in HCl(0.005%) than in NaOH (0.001%).

Table 4 REE analyses of selected samples, ND-3, Nawalparasi Nepal

REE Clay (NP-3-5) Black clay (NP-3-8) Clay (NP-3-13) Black clay (NP-3-18) Black clay (NP-3-19) UCC

La 39.1 40.3 47.8 33.4 35.1 30.0Ce 82.1 79.8 95.7 67.2 52.2 64.0Sm 6.0 6.6 7.1 6.0 5.9 4.5Eu 1.2 1.2 1.4 1.0 0.8 0.9Gd 1.6 1.7 1.7 1.5 1.2 3.8Tb 0.8 0.8 0.9 0.8 0.5 0.6Yb 3.0 3.0 2.7 2.6 2.5 2.2Lu 0.4 0.4 0.4 0.4 0.4 0.3P

134 134 158 113 99 106Eu/Eu* 0.67 0.62 0.66 0.54 0.49 0.64Th/Sc 1.6 1.4 1.1 1.1 1.1 0.97LaN/SmN 4.1 3.9 4.3 3.5 3.7 4.1GdN/YbN 1.4 1.5 1.7 1.6 1.3 1.3LaN/YbN 8.8 9.0 11.8 8.7 9.5 9.1Other trace elementsTa 1.3 1.2 1.1 1.1 0.9 2.2Sb 1.4 1.4 2.0 1.0 1.0 0.2Hf 8.2 8.8 3.3 7.8 7.0 5.8Cs 6.8 6.6 13.6 6.2 4.6 3.7Co 10.6 10.1 17.4 9.9 7.5 10Rb 157 150 276 136 107 112Ba 456 424 619 336 313 550

Eu/Eu*=EuN/(SmN·GdN)1/2 and Gd= (SmN· TbN

2 ) 1/3, Where N= Chondrite normalized concentrations Chondrite, UCC (Taylor andMcLennan 1985)

1

10

100

La Ce Sm Eu Gd Tb Yb Lu

Clay

Black Clay

Clay

Black Clay

Black Clay

UCC

Sed

imen

ts/C

hond

rite

REE

Fig. 7 Rare earth element(REE) patterns for sediments,Nawalparasi, Terai basin,Nepal. Chondrite normalizingfactors from Taylor andMcLennan (1985)

107

Discussion

Stratigraphy and 14C dating

The stratigraphy and depositional history of Quaternarysediments in the Terai basin is not well known. Someauthors (e.g. Sinha and Friend 1994; Sharma 1995) be-lieve that sediment deposition on the Terai basin is af-fected by climate change in Late Quaternary times.Studies of As contamination of groundwater of theBengal delta (McArthur et al. 2001; Ishiga et al. 2000;many others) have demonstrated the geological controland found that high concentration of As is restricted tothe Holocene sediments rich in organic matter. In thisstudy we attempt to correlate the stratigraphy of Ban-gladesh sediments with the Terai Basin sediments basedon carbon-14 dating. Wood fragments from a 4 m depthin ND-2 give an age of 3,340±70 year BP, and shells

from 11.2 m depth are dated at 12,680±40 year BP. Theblack clay beds at 12–15 m depth at all three boring sitesare rich in organic matter and are correlated (Fig. 3a, b)on the basis of textures and stratigraphic position. Thedepositional environment for all black clay beds may besimilar, and these beds can be regarded as markers. Peatbeds from shallow depths in Bangladesh also yield 14Cages from 4,000 to 12,000 BP (Monsur 1995). The blackclay bed dated at 12,680 year BP in the present studymay be correlative with the peat beds of Bangladesh andso represents the Holocene climate.

Sediment provenance

Provenance of the aquifer sediments is relevant to trac-ing the source of arsenic. There are two possible sourcesfor the Terai sediments, the Siwalik hill and the higherHimalayas (Kansakar 2004). Sediments carried from the

Table 6 Arsenic (As) and Iron (Fe) elution from sediments, Nawalparasi, Nepal

Sample ID Code As in soil (ppm) As ppb eluted(rock massequivalent)

As elution rate%

Fe in soil (ppm) Fe ppm eluted(rock massequivalent)

Fe elution rate% (in 10)5)

HCl NaOH HCl NaOH HCl NaOH HCl NaOH

NP-1-0 S 4.5 4.76 6.56 30.9 5.43 23000 6.18 1.09 2680.0 480.00NP-1-3 C 10.5 45.8 41.60 9.50 1.10 75000 1..90 0.22 260.0 0.29NP-1-5 SC 7.0 8.12 4.96 5.51 0.82 36000 1.10 0.16 300.0 0.45NP-1-7 FS 2.1 2.36 2.10 5.89 0.16 10000 1.18 0.03 1180.0 40.00NP-1-13 BC 10.0 3.04 63.20 0.16 0.18 83000 0.03 0.04 4.0 4.00NP-3-3 C 11.0 3.20 19.10 0.07 0.22 51000 0.01 0.04 2.0 0.08NP-3-6 SC 5.0 8.40 3.76 0.51 0.33 26000 0.10 0.07 40.0 0.25NP-3-13 BC 10.0 32.4 60.80 0.95 0.21 79000 0.19 0.04 20.0 5.20NP-3-17 FS 3.0 2.72 3.00 0.18 0.04 30000 0.04 0.01 20.0 2.80NP-3-18 BC 31.0 5.80 69.40 1.04 0.16 53000 0.21 0.03 40.0 6.00

Lithostratigraphy code; S Silt, C Clay, SC Silty clay, FS Fine sand, BC Black clayRock mass equivalent values= (Eluted vol. · 10 ml/50 ml)

Table 5 Results from water analyses, Nawalparasi Nepal

Sample ID As conc. (ppm) Fe conc. (ppm) pH EC (lS/cm) PO43) (ppm) COD (ppm) ORP (mv)

AAS Field kit AAS Field kit

NP-1 0.35 0.35 4.5 3 7.2 890 1.0 6 10NP-2 0.43 0.40 2.7 2 7.1 857 0.7 4 )6NP-3 <0.01 0.15 4.1 3 7.1 1,278 1.5 2 )12NP-4 0.43 0.40 4.3 4 7.3 697 0.3 2 )32NP-5 0.74 0.50 2.0 1 7.5 656 0.2 2 –NP-6 0.27 0.30 2.6 7 7.0 1,087 0.2 2 )10NP-7 0.24 0.30 2.9 3 7.2 733 0.5 1 )20NP-8 0.73 0.40 4.3 3 7.4 693 0.4 1 )45NP-9 0.29 0.35 7.5 3 7.0 693 0.5 3 )20NP-10 0.46 0.30 1.9 4 7.2 732 0.3 ND )10NP-11 0.31 0.30 19.5 4 7.3 697 0.4 2 )30NP-12 0.26 0.35 6.1 3 7.2 691 0.6 ND )30NP-13 0.41 0.40 12.1 4 7.4 607 0.3 1 )10NP-14 <0.01 0.02 0.3 0.2 7.2 724 ND ND 109

ND not detected

108

Siwalik hills by the minor rivers release more arsenicthan those carried by major rivers from the higherHimalayas (Kansakar 2004). However, it seems unlikelythat large-scale difference in the sediment source canexplain the local spatial variation of arsenic concentra-tions. REE and other charged cation elements like Th,Sc, Hf, and Zr are highly immobile in most geologicalprocesses, and thus can be used for provenance studies(Taylor and McLennan 1985; Bhatia and Crook 1986;Roser 2000). REE patterns for all samples in this studyare similar (Fig. 7), suggestive of homogeneous sourcecomposition or efficient mixing. Th/Sc ratios (average1.2) higher than UCC (0.97; Taylor and McLennan1985) indicate a felsic source. Furthermore, all samplesare enriched in incompatible elements, such as Zr

(210 ppm), La (106 ppm), Ce (79 ppm), compared toUCC levels of 190, 30, and 64 ppm, respectively (Taylorand McLennan 1985). This enrichment of incompatibleelements is also indicative of a felsic source. Sedimentshosting As-contaminated aquifers are therefore proba-bly homogeneous mixtures of different types of rocks,with a felsic source. Textural and spatial facies variationcould possibly cause variable redox conditions that inturn may affect arsenic concentrations in the ground-water.

Geochemistry of sediments

The average concentration of arsenic in the sedimentsamples analyzed in this study is 9 ppm (Tables 1–3),

0.01

0.1

1

1 1 0 100

ER

%

As in soil (ppm)

104 105 10610-7

10-6

10-5

0.0001

0.001

0.01

0.1

Fe in soil (ppm)

0.01

0.1

1

1 1 0 100

ER

%

As elution (ppm)

10-7

10-6

10-5

0.0001

0.001

0.01

0.1

0.001 0.01 0.1 1 10Fe elution (ppm)

SiltClaySilty ClaySandBlack Clay

SiltClaySilty claysandBlack Clay

HCl NaOH

Fig. 8 Arsenic (As) and Iron (Fe) elution in HCl and NaOH solution, Nawalparasi, Nepal

109

similar to the general level seen in unconsolidatedsediments, typically 3–10 ppm (Smedley and Kinni-burgh 2002). This value is similar to the As content ofsediments in the distal Bengal fan, Bangladesh(7.5 ppm; Ali et al. 2003). Core samples from severelypolluted regions of Bangladesh also have similar Ascontents. For example, in Samta village (SW Bangla-desh), where more than 90% of the tube well watershave As>0.05 ppm, As contents of clayey sedimentsrange from 7 to 20 ppm, and those of sandy sedimentsrange from 0.07 to 3.6 ppm (Tanabe et al. 2001). Thedrinking water limit of arsenic recommended by WHOis very low (10 ppb level) compared to the averageavailability of arsenic in natural sediments (parts permillion level). Consequently, mobilization of only asmall fraction of the As present in the sedimentswould be sufficient to exceed the WHO limit. Geenet al. (2003) have estimated that the loss of only0.1 lg/g of As from the solid phase can enrich thegroundwater by 200 lg/L. If the aquifer sediment isthe primary source of the dissolved arsenic, the releaseand transport mechanisms are important aspects to beconsidered.

Discussion of sediment sources as the potentialorigin of dissolved As follows. Peaty sediments oftenhave high arsenic contents, and thus are potentialsources of arsenic contamination in groundwater(Yamazaki et al. 2003b). However, the peat-like blackclay samples from Nawalparasi do not have excep-tional arsenic contents, averaging only 12 ppm (maxi-mum 31 ppm, NP-18). This is considerably less than inpeat core samples reported from Bangladesh (average42 ppm, highest 111 ppm, Yamazaki et al. 2003a).However, As concentrations in Nawalparasi ground-water are significant (0.41 ppm, Table 5) despite therelatively low levels in the black clays within theaquifers. Therefore, the sole contribution of arsenic inNawalparasi groundwater from the black clay seemsunlikely.

Iron oxide is abundant in the study areas. The roleof iron oxides is crucial for the oxy-hydroxide (FeO-OH) dissolution model for arsenic release (Nicksonet al. 2000; McArthur et al. 2001). The averageFe2O3value in this study is 4.8 wt.%, similar to thevalue in UCC (4.5 wt.%, Taylor and McLennan 1985),but concentrations are higher in the finer grained sed-iments than in the sands (Figs. 5, 6, 7). The overallcorrelation between iron and arsenic is poor (r2 =0.2);however, it is not necessary to have a good correlationfor arsenic to be carried by iron oxides (McArthuret al. 2001). The role of organic matter is indispensablefor the release of arsenic from FeOOH in the reductiontheory (McArthur et al. 2001), and percentage releaseof arsenic increases significantly in peat samples(Yamazaki et al. 2003b). Under reducing conditions,organisms such as bacteria (e.g., Clostridium sp.) play

roles in the dissolution of oxy-hydroxide releasingarsenic (Akai et al. 2003). Organic matter is ubiquitousin the clay and fine sand layers at Nawalparasi. Highercontents of trace elements such as Ni and V suggestassociation with organic matter (Tribovillard et al.1994), and high values of these elements (Ni=35 ppm,V=119 ppm) occur in Nawalparasi. In the Holoceneperiod the Terai foreland basin was characterized bynumerous river systems with active floodplains, aban-doned channels, levese, splays/deltas, lakes, and mar-shes (Sinha and Friend 1994), creating favorableenvironments for the deposition and preservation oforganic matter. Similar fluvial environments exist in thestudy areas today.

Phosphate (PO43)) measured was low (average

0.53 ppm) (highest 1.5 ppm, NP-3) suggesting that thepotential anion exchange with arsenic may not be sig-nificant. However, phosphate and its effects in arsenicmobilization must be considered, in that phosphatefertilizers are increasingly being used in the area.

Anomalously high concentrations of CaO from 15 to19 m depth (18.5 wt.% in ND-1 and 8.7 wt.% in ND-3)are due at least in part to abundant shells at that level. InNawalparasi, the aquifer yielding high arsenic ground-water contains higher calcium and carbon (Williamset al. 2004). Potential dissolution of this calcium mayraise the pH locally, making the environment morealkaline. Alkaline conditions are favorable for thedesorption of arsenic from oxides (Smedley and Kinni-burgh 2002) and also from organic matter (Torres andIshiga 2003). Carbonate layers do give potential for Asrelease if dissolved.

Comparison between Nawalparasi and Bhairahawa

The study areas in Nawalparasi and Bhairahawa rep-resent As-contaminated and As-free, areas respectively.The sediments at both sites are Quaternary in age.

Water samples from Nawalparasi contained anaverage arsenic content of 0.41 ppm, whereas at Bhai-rahawa arsenic was not detected. XRF analyses showedthat sediment elemental concentrations at both sites aresimilar (Table 7). Although As concentrations at Na-walparasi are significant, As was not detected in Bhai-rahawa despite similar sediment chemistry andgeomorphology. The core logs show that Nawalparasisediments are dominantly fine grained, representingover-bank facies, whereas those at Bhairahawa representa coarser channel fill association. Two factors, lithofa-cies change and groundwater movement, could thus beresponsible for the variation of As content in the waters.

Coarser sediments near the surface generally acquirean aerobic condition, and As is adsorbed onto oxideminerals. The adsorbed As is released in high pHconditions (especially above pH 8.5, Smedley and

110

Kinniburgh 2002). The neutral pH measured (7.1) in thestudy areas suggests that such desorption in high pHcondition may not be substantial. As-bearing mineralssuch as sulfides (FeAsS, AsS, As2S3) commonly releasearsenic under aerobic conditions. However, Akai et al.(2004) observed that sulfides in similar Holocene sedi-ments from Bangladesh contained less As (0.1–0.4%).Although we have not analyzed the sulfide mineralogy inthis study, arsenic released from sulfide oxidation is as-sumed to be less, based on the similar geochemistry andstratigraphy of the Terai Basin and Bengal delta.Additionally, the bulk TS content of the sedimentsanalyzed here is low (0.04%) and the correlation of TSwith As is not significant (r=0.45%, n=60). Thesefeatures suggest that arsenic contents in the Terai Basingroundwater may not be related to the concentration insulfides. In case of arsenic release from the sulfides, thepH could be lowered by SO4

2) from sulfides (range 2.3–5.9; Lengke and Tempel 2005), but the pH measured inthe study area is neutral. This further suggests that Asrelease from sulfide oxidation is negligible.

In contrast to the Bhairahawa channel fill associa-tion, the finer sediments of the Nawalparasi over-bankdeposits are less permeable. Moreover, the Nawalparasisediments are commonly rich in organic matter, whichundergoes natural decomposition, thus consuming dis-solved oxygen and enhancing reduction. Such oxygendeficient environments thus contribute to elevated ar-senic abundance in groundwater if the arsenic is sorbed

into organic matter or onto iron oxides. Groundwatermovement is naturally greater in the coarser sediments,and so may dilute the dissolved arsenic, whereas in thefiner sediments dilution is less, due to slower ground-water movement. Nawalparasi and Bhairahawa maytherefore represent examples of the influence of coarserand finer sedimentary facies.

Conditions for arsenic release

The conditions of arsenic release vary with the type ofhost matter, its mineralogy, and Eh and pH (Smedleyand Kinniburgh 2002; McArthur et al. 2001; Nicksonet al. 2000; many others). In non-mining areas, reductivedesorption – dissolution and oxidizing desorption arethe two main processes of As release. Smedley andKinniburgh (2002) described some physical (e.g., Eh,pH, DOC) and chemical (e.g., Fe, Mn, NH4, SO4)indicators for each process of arsenic release. Based onthose indicators, it is most likely that As release in thisstudy area is dominated by the desorption – dissolutionprocess. The reducing environment at Nawalparasi(negative ORP) and presence of organic matter mayhave favored the desorption of As.

There is no clear consensus on how As is bound insediments, and thus there is no common elution tech-nique of arsenic so far. Arsenic elution from organicmatter is high in alkaline solution, and is high from iron

Table 7 Comparison of results between Nawalparasi and Bhairahawa, Nepal

District Nawalparasi Bhairahawa (Rupandehi)Geology Quaternary sediments Quaternary sedimentsLithology Clay, silt, fine sand, organic matter Gravel, coarse sand, silt, clayWater Parameters Average (n=14) Range Average Range

pH 7 7.0–7.5 7.1As 0.41 ppm <0.01–0.74 ND Not detectedFeT 5 ppm 0.3–19.5 ND by field kitCOD 2 ppm 0–6 NDPO4

3) 0.5 ppm 0–1.5 NDArsenic in soil (ppm) Average (n=46) Range Average (n=14) RangeAs 9 2–31 8 4–18Major Oxides (wt%)Fe2O3 5 0.9–8.3 5 2–8TiO2 0.7 0.4–0.9 0.7 0.4–0.9CaO 3.9 0.6–30.3 4 0.6–8.1P2O5 0.09 ND–0.2 0.09 0.03–0.14Trace Elements (ppm)Pb 21 11–30 19 12–24Zn 59 16–104 54 28–91Cu 23 3–50 19 4–37V 94 5–178 93 30–162Th 16 6–26 14 5–19Sc 13 ND–22 12 4–20Zr 210 95–391 191 117–280Total sulfur (wt%)TS 0.04 0.02–0.31 0.02 0.02–0.05

ND not detected

111

oxides in acidic solutions (Torres and Ishiga 2003; Itoet al. 2001). In this study significant amounts of As wereeluted in both alkaline and acid solutions (Table 6). Thehighest As elution (69.4 ppb, NP-3–18) in black clay inalkali solution is likely due to the As hosted in organicmatter. In contrast, significant Fe (1.1 ppm) was elutedin HCl compared to 0.1 ppb in alkali. This result dem-onstrates that arsenic adsorbed on organic matter andiron oxides may be released during desorption.

Summary and conclusion

The geology of the Terai Basin of Nepal and the Bengaldelta of Bangaldesh is similar, based on geochemistry,stratigraphy, and carbon-14 age. Average arsenic con-tent of the Terai sediments is within the range of normalsediments (9 ppm), but the distribution is not homoge-neous. Abundances are greater in finer sediments such asblack clay (maximum 31 ppm) than in coarser sediments(silt and fine sand, 3 ppm). Fe2O3 is abundant in sedi-ments (4.8 wt.%), but the correlation between arsenicand iron is not significant. Concentrations of Zn, Ni, Cu,

Th, Sc, Pb, Y, Nb are also greater in the fine-grainedsediments. REE patterns of all types of sediments of theTerai Basin are similar and are comparable to theaverage UCC. The sediments represent homogeneousmixtures of a wide range of parent rocks of felsic com-position. Significant As leaching rates indicate that theTerai sediments have high potential for arsenic release,and that pH and redox conditions play pivotal roles.The As-bearing Nawalparasi aquifers are reducing, asindicated by slightly negative ORP, whereas the As-freeBhairahawa aquifers are oxidized, as indicated by thelow COD. Chemical reduction is a plausible mechanismfor the release of arsenic in groundwater in the TeraiBasin. The distribution of arsenic is therefore related tovariable redox conditions, which are likely controlled bylithofacies of the sediments and organic matter contents.

Acknowledgements We would like to thank the MonbukagakushoJapan for financial support and the Department of Irrigation(HMG) Nepal for logistical support during the field survey for thisstudy. Thanks are also due to F. Ahmed and P.D. Ulak for theirhelp with field work, to Mst. H. Bibi for her guidance on wateranalysis, and to Dr B.P. Roser for helpful comments on the man-uscript.

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