Geochemistry of aquifer sediments and arsenic-rich groundwaters from Kandal Province, Cambodia

Applied Geochemistry 23 (2008) 3029–3046

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Applied Geochemistry

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Geochemistry of aquifer sediments and arsenic-rich groundwaters fromKandal Province, Cambodia

Helen A.L. Rowland 1, Andrew G. Gault 2, Paul Lythgoe, David A. Polya *

School of Earth, Atmospheric and Environmental Sciences and Williamson Research Centre for Molecular Environmental Science,The University of Manchester, Manchester M13 9PL, UK

a r t i c l e i n f o a b s t r a c t

Article history:Available online 4 July 2008

0883-2927/$ - see front matter � 2008 Elsevier Ltddoi:10.1016/j.apgeochem.2008.06.011

* Corresponding author. Fax: +44 161 306 9361.E-mail address: [email protected] (D

1 Present address: Department of Swiss FederalScience and Technology, EAWAG, CH 8600 Dübendor

2 Present address: Department of Earth Sciences,140 Louis Pasteur, Ottawa, ON, Canada K1N 6N5.

Elevated As is well known to be present in aquifers utilised for drinking water and irriga-tion in West Bengal and Bangladesh. This problem has also more recently been discoveredin other parts of Asia, including Vietnam, Cambodia, Inner Mongolia and the Middle GangesPlain. Analysis of groundwaters in Kandal Province of Cambodia found waters with compa-rable geochemistry to the As-rich groundwaters of the West Bengali Delta. Similaritiesincluded high but heterogeneous As distributions, predominantly in the form As(III), highFe, moderate to high HCO�3 , circumneutral pH, low SO2�

4 and geochemical componentsindicative of reducing conditions. Good positive correlations between As, Fe, HCO�3 andNHþ4 , and dissolved organic C is consistent with As release predominantly via microbiallymediated reductive dissolution of As bearing Fe(III) oxides. Further evidence for such a pro-cess is found from correlations between As, Fe and organic matter from analysis of aquifersediments, by the presence of goethite in the finer fractions and from the association of Aswith amorphous, poorly crystalline and well crystallised hydrous Fe oxides. The presenceof several high As, but low Fe, wells implies that microbes could have a more direct rolein mediating As release via the direct utilisation of Fe(III) or As(V) as electron acceptors.The presence of elevated As in waters with short aquifer residence times (as indicatedby their geochemical signature) highlights the possible vulnerability of these aquifers tothe influx of surface derived waters, providing an additional source of labile organic C thatcould exacerbate As release by stimulating microbial activity.

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1. Introduction

Hazardous concentrations of naturally occurring As inground water utilised for drinking and irrigation are wellknown to be present in West Bengal and Bangladesh (Man-dal et al., 1996; DPHE, 1999; Smith et al., 2000; Bhattach-arya et al., 2001; Smedley and Kinniburgh, 2002; van Geenet al., 2003) and other parts of Asia including Vietnam(Berg et al., 2001), Inner Mongolia (Smedley et al., 2003),

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.A. Polya).Institute of Aquatic

f, Switzerland.University of Ottawa,

the Middle Ganges Plain (Chakraborti et al., 2003) andCambodia (Polya et al., 2003a, 2004, 2005; Buschmannet al., 2007; Berg et al., 2007; Rodriguez-Lado et al.,2008). Consumption of water with high levels of As canlead to a variety of chronic human illnesses including skinlesions and cancers both externally and internally (Mandalet al., 1996; Smith et al., 2000, 2002). In Bangladesh alone,it is predicted that more than 30 million people drinkwater containing >50 lg L�1 As, with over 6 million simi-larly afflicted in West Bengal (Smedley and Kinniburgh,2002). Unremediated, such exposure is predicted to resultin the development of millions of cases of arsenicosis andannually thousands of As-attributable excess deaths (Yuet al., 2003). Despite the extent of this crisis, modes of Asrelease are still widely contested within the literature,potentially making more challenging the development of

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3030 H.A.L. Rowland et al. / Applied Geochemistry 23 (2008) 3029–3046

strategies for positioning new low As wells for short-termwell-switching strategies (van Geen et al., 2003).

Research on causes of As mobilisation from these shal-low, reducing aquifers has mostly centred on West Bengaland Bangladesh as historically the problem was first iden-tified here. The recent discovery of elevated concentrationsof As in Cambodia, however, provides another region forstudy, with the country’s shallow aquifers showing highbut heterogeneous As distribution, found largely to thesouth and east of the country (along the Bassac and Me-kong rivers in the highly populated Kandal Province), withother smaller regions in the north also containing high Aswaters (Polya et al., 2005; Buschmann et al., 2007; Berget al., 2007). In Kandal Province, the high As waters areheld in thick low lying Quaternary deposits (Anderson,1978; Workman, 1972, 1977; ESCAP, 1993) with slowflushing rates that, in addition to the geochemistry of thewater, are typical characteristics of the As affected regionof West Bengal and Bangladesh and thought to be key inidentifying areas prone to As rich groundwater (Smedleyand Kinniburgh, 2002; Nordstrom, 2002; Charlet andPolya, 2006). Therefore, Cambodia provides an ideal ana-logue to determine if routes of As mobilisation postulatedwithin West Bengal and Bangladesh are unique to theseareas where there has been several decades of extensivehuman intervention through groundwater abstraction, orif the As-rich waters are indeed ubiquitous to such aquifersirrespective of groundwater abstraction history (cf. Polyaet al., 2005).

There are a variety of possible mechanisms for releaseof As within these reducing aquifers. For example, the oxi-dation and breakdown of As bearing pyrites by the draw-down of oxygenated water (Mallick and Rajagopal, 1996;Mandal et al., 1996; Das et al., 1996; Chowdhury et al.,1999), though this process has been contested, as the prod-ucts of the breakdown of pyrite under the neutral condi-tions of the aquifer would act as a sink for As as opposedto releasing it to the groundwater (Nickson et al., 1998,2000). The association of As with Fe(III) oxides within Asrich aquifers of Bangladesh and West Bengal (Nicksonet al., 1998, 2000; Bhattacharya et al., 2001; Dowlinget al., 2002; Gault et al., 2003; Swartz et al., 2004; Akaiet al., 2004), coupled with the reducing conditions of theaquifers could also lead to As release. Microbial degrada-tion of the naturally occurring organic matter present,drives the aquifer to anoxia where under such conditions,Fe(III) oxides are unstable and dissolve, releasing the As,as well as Fe and HCO�3 into the groundwaters – such a pro-cess would be expected to produce correlations betweenthese three components (Nickson et al., 1998, 2000; DPHE,1999; Smedley and Kinniburgh, 2002). In addition, mi-crobes can play a more direct role in mediating As releaserather than just by altering the redox conditions. Micro-cosm based experiments on sediments from West Bengaland Bangladesh show that stimulation of the indigenousmicrobial community leads to increased As release andFe(III) reduction (Harvey et al., 2002; van Geen et al.,2004; Akai et al., 2004; Islam et al., 2004; Gault et al.,2005b). After the consumption of O2, microbes can utilisea variety of other compounds as electron acceptors, includ-ing those of Fe(III) and As(V) (Oremland and Stolz, 2003;

Lloyd and Oremland, 2006). However, the specifics of thesecomplex microbial interactions are yet to be fullyunderstood.

Arsenic held at the surface of Fe(III) oxides is also sus-ceptible to mobilisation via competitive sorption with avariety of anions (HPO2�

4 , H2PO�4 HCO�3 ), cations (Fe(II))and other compounds (H4SiO4, dissolved organic matter)that also have a great affinity for the same surface sites(Acharyya et al., 1999; Grafe et al., 2001; Appelo et al.,2002; Meng et al., 2002; Dixit and Herring et al., 2003;Anawar et al., 2004). The mobilisation of As due to redoxcycling in near-surface soils/sediments has also been sug-gested (Polizzotto et al., 2006).

Therefore the aims of this study were to (i) determinethe geochemistry of As-rich waters present within a de-fined area, south of Phnom Penh in Cambodia, known tocontain elevated As, (Polya et al., 2003a, 2005; Buschmannet al., 2007; Berg et al., 2007), (ii) determine the lithologyand related mineralogy of the aquifer sediments withinthe same area, (iii) compare both the aqueous and solidphase distribution of As and other related parameters tothe As affected regions of West Bengal and Bangladesh,to see the extent of similarity between the regions, and(iv) identify the most important As mobilisation processesin place within the aquifers of Cambodia.

2. Geological setting

Cambodia lies within the Indo-China peninsula and cov-ers an area of approximately 176,500 km2 (ESCAP, 1993).The NW portion of the country is dominated by the TonleSap Lake region, which is drained by the Tonle Sap riverthat joins the Mekong River at Phnom Penh in the southernpart of the country. South of Phomn Penh, the river is splitinto two, with the western (Bassac River) and eastern (Me-kong River) distributaries cutting through low lying mar-shy and agricultural land into Vietnam and onwardsbefore ultimately draining into the South China Sea.

Recent countrywide surveys have identified a regionsouth of Phnom Penh within Kandal province lyingbetween the Bassac and the Mekong rivers (Fig. 1), ascontaining elevated groundwater As concentrations(>50 lg L�1, Polya et al., 2005; <1–1340 lg L�1, Buschmannet al., 2007; <1–212 lg L�1, Berg et al., 2007). The aquifersediments containing the As rich groundwater in the areaare dominated by unconsolidated alluvial deposits of theQuaternary (Workman, 1972, 1977; Anderson, 1978;ESCAP, 1993), where sedimentation was controlled by gen-eral sea level change and the flexing of the land surface dueto large basaltic eruptions occurring in southern Vietnam,southern Laos and eastern Cambodia that occurred duringthis time (Workman, 1972, 1977; Anderson, 1978; ESCAP,1993). Surface sediments in this As rich area consist of greyclays, silts and sands, with additional organic remains, dat-ing from the Holocene (ESCAP, 1993) which were depos-ited as the Mekong river delta system transgressedsouthwards after the Holocene high stand (6–7 ka) whensea level was 2.5–4.5 m above the present sea level (Taet al., 2002). Other units of importance within the SEregion of Cambodia that do not outcrop within the area

PHNOM PENH

Kandal

Prey Veaeng

Takaev

Mekong

Tonle Sap

Bassac

Mekong

11o 30

105o 00 105o 30

Capital city

River

Province border

Area A — Borehole location

Area B — Borehole location

Area sampled 0 5 10 km

Area B Area A

Fig. 1. Map of Kandal province, Cambodia, showing locations of sampling area and boreholes, as detailed in text.

H.A.L. Rowland et al. / Applied Geochemistry 23 (2008) 3029–3046 3031

of study but are thought to underlie the younger Holocenesediments include, (i) Lower Quaternary ‘Older Alluvium’(ESCAP, 1993) consisting of laterite, sands, silts and clay-stones of both fluvial and marine origin thought to datefrom 1.5 to 2.0 Ma, commonly associated with Pliocene-Quaternary plateau basalts to the NE of Phnom Penh,which form terraces of 25–150 m above sea level in theplains of the Tonle Sap and Mekong river plains and, (ii)Middle–Upper sediments (ESCAP, 1993) found outcroppingalong the western and eastern borders of the central Me-kong and Tonle Sap river system, consisting of grits, peb-bles, sands and clays. The latter unit (ESCAP, 1993) iscalled the Mochoa formation and is found on the MekongPlain, SE of Phnom Penh, outcropping as terraces of 10–15 m height, with the Middle Quaternary unit (ESCAP,1993) outcropping as red sandy terraces above 15 m inthe terraces of both the Mekong and Tonle Sap riversystems.

3. Methodology

3.1. Sample location and collection

Groundwater samples were collected south of PhnomPenh, within the Kandal province, along the river channelsof the Bassac and the Mekong, in an area previously re-ported to contain elevated groundwater As concentrations(Fig. 1) (Polya et al., 2003a, 2004, 2005; Berg et al., 2007;Buschmann et al., 2007). This area was relatively large(approximately 600 km2), but sampling was limited tothe wells lying close to the river channels due to transportand access restrictions. Both well water samples (n = 63)and sediment samples from 2 boreholes were taken. Differ-ent personnel collected aqueous and solid samples over 5separate field trips, broadly following the same sample col-lection protocol. To clarify sample locations, any samples(well water, and borehole cuttings) along the Bassac riverchannel are identified as ‘Area A’, with those collectedalong the Mekong, south of Phnom Penh, as being from‘Area B’.

To make sure that the samples collected were as repre-sentative as practicable to the groundwaters from theunderlying aquifers, (i) only hand pumps were sampled

as water from shallow open dug wells would be oxidisedafter exposure to the atmosphere, as well as exposed topossible contamination from rain and surface run off, and(ii) hand wells were pumped for at least 5 min prior tosampling so as to remove any standing water from withinthe borehole. pH and Eh were measured on site using cal-ibrated hand held water meters (HANNA, Watertest). Sam-ples for cation analysis were filtered with a 0.45 lmcellulose nitrate filter into acid washed poly-propylenebottles, preserved by the addition of HCl (Aristar, BDH,UK; acidified to �0.02 M HCl) and stored in opaque bottles.Samples for anion and DOC anaylsis were filtered with a0.45 lm cellulose nitrate filter into acid washed poly-pro-pylene bottles. All groundwaters were then kept cool in afreezer box on the day of sampling and subsequentlystored in the dark. Samples were then kept at 4 �C, exceptduring transit by airfreight from Cambodia to Manchester(approximately 3–4 days).

Solid samples were obtained by rotary drilling usinggroundwater as the drilling fluid. Samples were triplebagged and stored at 4 �C in the dark and transported byairfreight back to Manchester where they were repackagedand stored under N2 (further details are given in Rowlandet al., 2005).

3.2. Solid phase analytical methods

Preparation of different size fractions for mineralogicalanalysis was undertaken by dry sieving on freeze dried sam-ples. Clay sized (0–4 lm) particles were then separated fromthe finest sieved fraction (<45 lm) using a settling tube(15 �C, settling time 405 min as determined by Stokes Law).

Identification of the mineral phases present withinfreeze dried sediment was determined by X-ray diffraction(XRD, Phillips 1730 diffractometer with a CuKa source).Solid phase concentrations of As and Fe were determinedby X-ray fluorescence on air-dried sediments (PhilipsPW1400 XRF spectrometer).

Distribution of As within different operationally definedfractions of the solid phases was determined by sequentialextraction. The non-specifically sorbed, specifically sorbed,and As bound with amorphous, poorly crystalline and wellcrystallised hydrous oxides of Fe and Al and within the


residual (silicates and sulphides) were partitioned usingthe method as detailed in Gault et al. (2003), with the frac-tion held in association with the organic phases deter-mined using the method as detailed in Tessier et al.(1979). A stream sediment reference material (GBW07311) was also subjected to the sequential extraction pro-cedure. The sum of the As concentrations determined fromeach extraction step agreed within experimental errorwith the certified As concentration (measured:157 ± 59 lg g�1; certified: 188 ± 6 ug g�1).

Percentage abundances of total C were determined induplicate by flash combustion on freeze dried, groundsamples using a Carlo Erba EA 1108, with amounts of inor-ganic C determined on a Coulomat 702 (Strohleim). Totalorganic C (TOC) contents were then determined by sub-tracting the amounts of C present as carbonate from the to-tal amounts of C present.

3.3. Aqueous phase analytical methods

Water samples were analysed within 2 weeks of collec-tion, with the majority of samples being analysed wellwithin this time frame. Aqueous major and trace elementconcentrations in the acidified groundwater were deter-mined by inductively coupled plasma-atomic emissionspectrometry (ICP-AES) (Horizon, Fisons) and inductivelycoupled plasma-mass spectrometry (ICP-MS) (Fison’s Plas-ma Quad PQ II), respectively. Standards for both ICP-MSand ICP-AES were prepared immediately prior to analysisfrom dilution of concentrated multielement mixes (John-son Matthey, UK). Arsenic speciation was determined byion chromatography-inductively coupled plasma-massspectroscopy (IC-ICP-MS), according to the method ofGault et al. (2005a).

Anions were determined using ion chromatography (IC)with a Dionex DX 600 using a Dionex AS9-HC column,(9 mM Na2CO3 isocratic eluant, flow rate of 1.4 mL min�1)and a Dionex AG9-HC guard column. Dissolved organic C(DOC) was measured by combustion with a ShimadzuT5000 TOC analyser. Alkalinity was determined by acidi-metric titration with 0.01 M HCl. The Gran function foreach titration point was then calculated to determine theequivalence point and consequently the alkalinity.

Ammonium concentrations were determined by mea-suring the sample absorbance at 420 nm after addition ofNessler’s Reagent (K tetraiodomercurate) using a M330UV–visible spectrophotometer (CamSpec). The addition ofa 10% ZnSO4 and 25% NaOH solution prior to analysiscaused precipitation of any excess dissolved Ca to preventturbidity problems during analysis.

3.4. Data quality

To prevent precipitation of Fe and adsorption onto thecontainer surface of trace metals, both of which processescould lower the measured values of the cationic compo-nents of the water, all aqueous samples for cations analysiswere filtered and acidified with HCl (for details see Section3.1). In addition, such preservation methods limit changesin As speciation by minimising microbial activity (Polyaet al., 2003b). This method has been shown to preserve

the speciation distribution of As in aqueous samples forup to 15 weeks (Polya et al., 2003a, b).

Certified reference materials were analysed as un-knowns at regular intervals throughout each analyticalrun to check both cationic (NWRI, Canada) and anionic(SPEX, US) components. A number of duplicate analysesof field samples showed good agreement (data not shown).In addition, the electroneutrality (EN) was also calculatedfor each sample (where possible) to check that the sumof charges of the major cations (Ca2+, Mg2+, K+, Na+) andmajor anions (Cl�, HCO�3 , SO2�

4 , NO3�3 ) balanced. EN values

were typically 5% or less, though a number of samplesdid have slightly higher values that would be expected,this being ascribed to other components such as Fe(II)and NHþ4 making significant contributions to the chargebalance of the waters.

Handheld probes used in the field to measure pH werecalibrated regularly using buffer solutions of pH 4 and pH 7(Fisher Scientific, UK) during each trip to maintain accu-racy. Eh probes were checked prior to each field trip bytesting in fresh ZoBell’s solution (Fisher Scientific, UK). Val-ues are reported with respect to the standard hydrogenelectrode (SHE) scale.

4. Results and discussion

4.1. Sedimentary characteristics

4.1.1. Environment of depositionThe sedimentary sequences of alternating unconsoli-

dated clays and sands from both Cambodian boreholes fol-lowed very similar patterns (Fig. 2). The top layer in bothboreholes consisted of a clay cap (Area A, 0–16 m; Area B,0–12 m). The clay was red in colour to approximately 5 mdepth, indicating the limit of O2 ingress from the surface.At greater depths, the remaining clay and following finesand unit were grey (elephant grey) in colour, which wasconfined by a further silt clay (Area B – 3 m) or clay (AreaA – 5 m) layer at approximately 25 m depth (Fig. 2). Belowthis finer layer, the sediments predominantly consisted offining upward sequences of coarse to fine sands, typicallyreddish or white red in colour, with the deeper sandsappearing to be buff brown. The fining upward sequence ofdeposits of both area A and area B are suggestive of deposi-tion in a fluvial environment; with the coarser fractionsdeposited in channel bases and fining upward sequencesoccurring during channel migration or channel fill (cf. Tuck-er, 1991). The grey nature of the upper layer suggests it is ofHolocene age with the red colour of the deeper sedimentsbeing more typical of Pleistocene deposits (ESCAP, 1993).A more detailed quantitative analysis of the age of the sedi-ments within the sequence is discussed in Van Dongen et al.(2008). However, the sedimentary characteristics suggest aQuaternary age, where sediments of this age in the regionof study are typically alluvium deposits (Workman, 1972,1977; ESCAP, 1993).

4.1.2. Aquifer sediment mineralogyXRD analysis of a selection of bulk samples showed the

sediments from both areas were dominated by quartz,

Fig. 2. Comparison between down borehole profiles of solid phase As (closed triangles), Fe2O3 (open squares) and TOC (crosses) from area A (Bassac) and B(Mekong) boreholes, Combodia. Mud; slit/fine sand; fine stand; medium sand. Arsenic shows similar maxima and minima to those of Fe andTOC. Note the log scale for %TOC.


with the other phases including muscovite, biotite, feld-spars (predominantly alkali feldspars, orthoclase, sanadine,albite and microcline, with occasional plagioclase feld-spars) and clay minerals (chlorite). Grains were typicallymoderately rounded to well rounded and, coupled withthe dominance of quartz over other minerals, suggeststhe sediments must have travelled some distance (to pro-duce rounding of the grains) and undergone extensive

chemical weathering (to leave sediments dominated bythe most stable mineral quartz). Such characteristicswould be expected from sediments deposited within alarge river in a warm humid environment, such as the Qua-ternary sediments deposited in this region of Cambodia(Workman, 1972, 1977; ESCAP, 1993). Separation of thesediments into different sized fractions revealed a morecomplex mineralogical distribution, with the finest clay


fraction (<4 lm) dominated by a variety of clays, such aschlorite, kaolinite, illite and smectites, presumably weath-ering products from chemical degradation of other miner-als, such as muscovite and feldspars, present within theaquifer (Deer et al., 1992), as well as the presence ofgoethite.

4.1.3. Arsenic distributionSolid phase As, Fe and TOC appear to have similar pat-

terns of distribution down both boreholes (Fig. 2), withsediments in the upper (<20 m) and deeper portions(>50 m) having slightly higher As and Fe concentrationsthan the sediments in the middle portions of both bore-holes. The higher concentration of both As and Fe at depthsshallower than 20 m correlates well with the younger grey(Holocene?) sediments from both area A and B. Correlationbetween As and Fe is further indicated by sequentialextraction of a selection of sediments from both area Aand B (Table 1) with As being found predominantly inthe specifically sorbed, amorphous and poorly crystallinehydrous oxides of Fe and Al, or the well crystallised hy-drous oxides of Fe and Al fractions. A detailed analysis ofthe types and sources of organic matter present withinthe borehole is discussed in Van Dongen et al. (2008).

4.2. General aqueous chemistry and aquifer redox conditions

Values for a variety of aqueous cationic componentsand anions measured for individual wells are shown inTable 2. All waters were circumneutral (mean average pH7.18, minimum 6.10, maximum 8.63), with low salinity,typically low SO2�

4 and anionically dominated by HCO�3(Table 2, Fig. 3). The groundwater tended to be dominatedeither by Ca2+ and Mg2+, or Na+ and K+ cations, with somesamples having chemistry along a mixing line betweenthese two extremes (Fig. 3). Such chemical signatures canoccur as water when being transported through an aquiferinteracts with the sediments and causes mineral dissolu-tion. This changes the chemical composition of the water,dependent on the reaction rates of differing minerals. Typ-ically waters with higher Ca2+ and Mg2+ are indicative ofwaters that have had a short interaction period with sedi-ments, allowing only minerals with high solubility, such ascarbonates, to dissolve (Appelo and Postma, 1993). Aswater travels through an aquifer, less soluble mineralssuch as feldspars and micas (both of which are presentwithin these sediments) break down and release Na+ and

Table 1Distribution of As (lg g�1) between sequentially extracted operationally defined p

Operationally defined phase D

Non-specifically sorbed (lg g�1) <Specifically sorbed (lg g�1)Amorphous and poorly crystalline hydrous oxides of Fe and Al (lg g�1)Well crystallised hydrous oxides of Fe and Al (lg g�1) <Organics (lg g�1) <Residual (silicates) (lg g�1) <Sediment type SDepth (m) 5Area B

K+ (Appelo and Postma, 1993). Cambodian well waters ap-pear to have a range of chemical composition from thosedominated by Ca2+ and Mg2+ (least interaction with sedi-ments), to those dominated with Na+ and K+ (most interac-tion with sediments), suggesting the waters have a rangeof residence times (Fig. 3), though the use of more accurategroundwater dating methods would have to be used toidentify more accurately the variation in age within theaquifer.

All samples contained typically low NO�3 (<3.2 mg L�1),with the exception of a single sample (49.8 mg L�1,FS1:C10, Table 2). This coupled with the presence of dis-solved Fe (0–26.5 mg L�1) and NHþ4 (0–23.7 mg L�1), andpredominantely low SO2�

4 (80% of wells sampled contain-ing <10 mg L�1) at all depths (Table 2, Fig. 4) suggests thatthe groundwaters are anoxic and that any available O2 andNO�3 have been utilised during microbial activity (Appeloand Postma, 1993). Such conditions allow microbiallymediated Fe(III) reduction (producing aqueous Fe(II) ionsfrom the reduction of solid phase Fe(III)), SO2�

4 reduction(utilising SO2�

4 , producing low dissolved SO2�4 concentra-

tions) and ammonification (oxidation of organic matter,producing NHþ4 ) (Appelo and Postma, 1993). Under suchconditions, groundwaters sampled in Cambodia would beexpected, at neutral pH, to have Eh values of between�500 and 0 mV (Appelo and Postma, 1993). However, theEh values measured are much higher, between +50 and+300 mV (Table 2, Fig. 4), more indicative of NO�3 reducingconditions (Appelo and Postma, 1993). This discrepancy isthought to reflect redox disequilibrium which is wide-spread in low temperature systems (Chapelle et al., 1996;Christensen et al., 2000; Nordstrom and Wilde, 2005).Furthermore, the redox potential was measured at thewell-head from standing water as opposed to a sealedflow-through cell, the latter of which is recommended forEh analysis as it limits the interaction of the groundwaterwith the atmosphere, preventing oxidation and redoxchange during sampling (Christensen et al., 2000; Nord-strom and Wilde, 2005). Therefore, the elevated Eh valuesare taken to suggest rapid oxidation of the groundwaterduring sampling and are probably not representative ofthe redox conditions present within the aquifer. Neverthe-less the redox values, although not useful as absolute mea-surements, can be used for comparative purposes if it ispresumed that similar oxidation occurs for all the samplescollected. The entirety of the geochemical data suggeststhat the groundwaters are typically sub-oxic to anoxic, as

hases in a variety of Cambodian aquifer sediments

S54 SR25 DS23 DS27

0.15 <0.15 <0.15 <0.150.33 ± 0.24 1.13 ± 0.31 <0.25 0.38 ± 0.260.38 ± 0.15 1.09 ± 0.20 2.45 ± 0.26 1.43 ± 0.210.25 3.06 ± 0.45 <0.25 0.85 ± 0.350.12 <0.12 <0.12 <0.120.6 <0.6 <0.6 <0.6ilt/fine sand Silt/fine sand Mud Mud4 25 23 27

A B B

Table 2Geochemistry of Cambodian groundwaters

Samplea UTM(E)

UTM (N) Depth(m)

Wellage

pH Ehb

mVAs(III)(lg L�1)

Err As(V)(lg L�1)

Err RAs(lg L�1)

Err TOC(%)

HCO3

(mg L�1)Fe(mg L�1)

Err S(mg L�1)

Err Si(mg L�1)

Err Mn(mg L�1)

Err P(mg L�1)

Err NO�3c

(mg L�1)SO2�

4c

(mg L�1)NHþ4

c

(mg L�1)

FS1:c10 (a) – – 23 2002 6.1 301 – – – – 0.5 – – 683 0.1 – 25.0 – 21.0 – – – – – 49.8 58.2 –FS1:c2 (a) – – 40 1996 7.3 335 0.0 0.1 0.8 0.2 0.8 0.2 – 495 0.0 – 1.0 – 15.0 – – – – – ND 1.9 –FS1:c4 (a) – – 60 1997 6.2 201 79.0 3.0 46.0 3.0 125 4.2 – 202 9.0 – 4.0 – 24.0 – – – – – ND 8.0 –FS1:c5 (a) – – 27 2002 6.4 171 151.0 5.0 26.0 2.0 177 5.4 – 255 14.0 – 4.0 – 21.0 – – – – – ND 9.2 –FS1:c8 (a) – – 60 – 6.4 128 219.0 7.0 47.0 3.0 266 7.6 – 241 9.0 – 0.8 – 16.0 – – – – – 2.1 1.6 –FS2:JJ01 (b) 505565 1271713 49 1998 6.7 118 173.0 – 30.0 – 203 – – 316 3.0 1.0 0.1 0.7 14.0 2.0 0.3 0.02 0.40 0.20 0.1 ND 6.0FS2:JJ02 (b) 504185 1272598 65 1998 7.1 134 122.0 – 14.0 – 136 – – 383 0.1 0.7 0.1 1.0 11.0 2.0 0.2 0.02 0.00 0.20 0.3 ND 6.0FS2:JJ 03 (b) 504185 1272598 65 1998 7.1 134 117.0 – 18.0 – 135 – – 400 0.4 0.7 0.1 0.5 11.0 2.0 0.2 0.02 0.02 0.20 0.2 ND 8.0FS2:JJ 04 (a) 498005 1266695 45 2001 7.1 248 1.0 – 0.7 – 1.7 – – 85 0.1 0.7 0.4 2.0 14.0 2.0 0.2 0.02 0.00 0.20 ND 1.2 0.7FS2:JJ 05 (a) 503584 1266159 43 1995 6.6 118 50.0 – 9.0 – 59.0 – – 334 6.7 0.7 1.0 2.0 16.0 2.0 0.5 0.02 0.07 0.20 ND 4.3 2.0FS2:JJ 06 (a) 504674 1262213 48 1998 7.1 216 8.0 – 6.0 – 14.0 – – 252 0.1 0.7 0.2 0.5 9.0 2.0 0.5 0.02 0.03 0.20 0.1 0.5 2.0FS2:JJ 07 (a) 504674 1262213 48 1998 7.1 216 9.0 – 3.0 – 12.0 – – 245 0.0 0.7 0.2 2.0 11.0 2.0 0.5 0.02 0.05 0.20 0.1 0.5 8.0FS2:JJ 08 (a) 539097 1255625 54 2000 6.9 188 13.0 – 3.0 – 16.0 – – 543 1.5 0.8 153 4.0 21.0 2.0 0.8 0.02 0.20 0.20 ND 408 8.0FS2:JJ 09 (a) 497161 1254701 31 2003 6.7 228 0.0 – 0.0 – 0.0 – – 150 0.2 0.7 20.0 20.0 28.0 2.0 4.8 0.06 0.10 0.20 ND 61.0 4.0FS2:JJ 10 (a) 500591 1256004 52 1999 7.4 221 0.0 – 0.0 – 0.0 – – 401 0.0 0.7 30.0 10.0 21.0 2.0 0.1 0.02 0.01 0.20 ND 57.0 0.5FS2:JJ 11 (a) 500591 1256004 52 1999 7.4 221 0.0 – 0.0 – 0.0 – – 395 0.0 0.7 30.0 20.0 21.0 2.0 0.1 0.02 0.03 0.20 ND 56.0 0.5FS3:1.1 (a) 498003 1266698 43 2001 7.4 909 0.0 0.1 0.0 0.4 0.0 0.4 – – 0.1 0.2 0.5 0.1 12.9 0.3 – – 0.00 0.20 – – –FS3:1.2 (a) 503572 1266148 42 1995 7.0 376 54.3 1.4 9.3 2.5 63.6 2.9 – 351 5.1 0.2 1.6 0.1 14.8 0.3 – – 0.00 0.30 – – –FS3:1.3 (a) 504670 1262219 48 1998 7.7 233 7.2 0.5 2.6 0.9 9.8 1.1 – 195 0.1 0.2 0.3 0.1 10.2 0.3 – – 0.00 0.30 – – –FS3:1.5 (a) 500125 1257007 47 2002 6.9 196 317 4.1 41.4 6.5 358 7.7 – – 9.1 0.2 0.1 0.1 29.7 0.4 – – 1.20 0.30 – – –FS3:2.1 (b) 508757 1271191 57 2000 7.4 94 699 8.6 60.6 6.9 759 11.0 – – 3.1 0.3 0.1 0.1 11.2 0.3 – – 0.03 0.30 – – –FS3:2.2 (b) 515054 1268366 42 2001 6.9 225 8.2 0.7 12.4 4.6 20.6 4.7 – 161 0.4 0.2 0.9 0.1 18.7 0.3 – – 0.10 0.30 – – –FS3:2.3 (b) 520004 1264742 38 2002 7.6 204 1.5 0.2 –0.3 0.3 1.2 0.4 – 165 0.2 0.2 0.0 0.1 14.1 0.3 – – 0.00 0.30 – – –FS3:2.4 (b) 519937 1236745 36 2002 7.4 117 103 1.8 12.9 5.9 116 6.2 – – 1.9 0.2 0.1 0.1 13.2 0.3 – – 0.00 0.30 – – –FS3:2.5 (b) 492432 1266143 30 1990 6.5 174 41.2 1.3 46.7 6.6 87.9 6.7 – 41 9.0 0.3 2.8 0.3 26.3 0.4 – – 0.10 0.30 – – –FS3:4.3 (b) 526736 1254167 55 2001 7.4 101 417 5.2 24.3 6.3 441 8.2 – 240 2.0 1.0 0.2 0.1 16.5 0.3 – – 1.20 0.50 – – –FS3:5.1 (a) 498825 1272710 45 2001 7.2 104 195 2.7 20.4 6.2 216 6.8 – 199 3.4 0.3 0.1 0.1 20.4 0.3 – – 1.10 0.50 – – –FS3:5.2 (a) 498849 1272715 34 1988 6.9 266 0.2 0.1 0.3 0.7 0.5 0.7 – 517 0.1 0.3 40.0 2.0 16.5 0.3 – – 0.20 0.50 – – –FS3:5.3 (a) 498771 1272750 35 1997 7.1 85 210 2.8 22.3 6.2 232 6.8 – 185 6.4 0.3 0.1 0.1 18.5 0.3 – – 1.30 0.50 – – –FS3:5.5 (a) 499660 1272739 35 1998 7.0 116 196 2.7 29.1 6.3 225 6.9 – 189 6.3 0.3 0.1 0.1 19.6 0.3 – – 1.40 0.60 – – –FS3:6.2 (a) 483630 1250443 43 1989 7.6 346 0.1 0.1 0.0 0.6 0.1 0.6 – 833 0.2 0.3 8.0 1.0 15.0 0.3 – – 0.20 0.60 – – –FS3:6.3 (a) 498825 1272673 34 1988 7.1 136 204 2.8 20.7 6.2 225 6.8 – 198 4.6 0.3 0.1 0.1 21.0 0.3 – – 1.70 0.80 – – –FS4:1.1 (b) 503575 1266162 42 1995 7.1 88 73.3 1.6 7.3 1.3 80.6 2.1 – 418 6.5 0.4 2.0 0.2 14.3 1.3 0.4 0.01 0.03 0.03 <0.1 5.7 1.7FS4:1.2 (b) 500130 1257685 48 2001 7.0 60 345 5.7 47.6 2.9 392 6.4 – 282 8.4 0.5 0.1 0.0 25.1 1.9 0.1 0.01 1.04 0.11 <0.1 <0.1 2.7FS4:1.3 (b) 506171 1236966 34 1999 7.5 248 1.3 0.2 233.5 7.4 235 7.4 – 541 0.0 0.0 133 6.3 12.5 1.3 0.2 0.01 0.00 0.03 0.5 398 0.4FS4:1.4 (b) 503717 1249746 – 1996 6.8 161 0.2 0.1 5.6 1.5 5.8 1.5 – 282 0.2 0.0 0.7 0.1 18.8 1.6 0.7 0.02 0.15 0.04 <0.1 1.7 0.3FS4:2.1 (b) 506957 1270341 45 2001 7.1 63 29.1 1.2 1.6 0.7 30.7 1.4 – 388 2.8 0.3 0.1 0.1 17.9 1.5 0.2 0.01 0.03 0.03 <0.1 <0.1 1.7FS4:3.2 (b) 506977 1270363 35 2003 7.4 – 95.5 1.9 8.1 2.5 104 3.1 – 385 0.0 0.0 0.2 0.1 14.3 1.3 0.1 0.01 0.00 0.03 <0.1 <0.1 2.7FS4:4.1 (a) 498825 1272710 45 2001 7.0 58 247 4.2 18.0 2.6 265 4.9 – 242 3.9 0.4 0.1 0.0 16.1 1.4 0.2 0.01 1.48 0.36 <0.1 0.1 0.0FS4:4.2 (b) 505318 1272390 70 1995 7.4 58 269 4.5 21.1 2.6 290 5.2 – 421 4.0 0.4 0.1 0.1 21.5 1.7 0.5 0.02 1.00 0.21 <0.1 <0.1 9.2FS4:4.3 (b) 505305 1272373 16 2003 7.1 48 302 14.0 30.5 8.0 333 16.1 – 507 26.5 0.7 0.3 0.2 18.8 1.6 0.2 0.01 1.68 0.48 0.2 0.3 33.0FS4:5.1 (a) 492426 1266146 23 1990 6.4 131 137 2.5 21.2 2.6 158 3.6 – 211 12.7 0.5 9.7 0.9 26.9 2.0 3.4 0.15 0.28 0.08 <0.1 0.1 0.8FS4:7.1 (b) 506972 1270461 24 2003 8.2 226 50.3 7.1 1.8 0.6 52.1 7.1 – – 0.0 0.0 3.2 0.8 12.6 1.3 0.6 0.06 0.00 0.05 – – –FS4:7.2 (b) 506972 1270461 70 2003 8.6 – – – – – – – – 382 0.2 0.1 12.4 1.0 16.1 1.4 0.1 0.02 0.00 0.04 <0.1 33.0 0.5FS4:8.1 (a) 498865 1266315 49 1996 7.0 180 – – – – – – – 117 0.1 0.0 0.5 0.3 20.6 1.6 0.1 0.01 0.04 0.03 <0.1 1.1 0.2FS4:8.2 (a) 498872 1266286 54 2003 8.3 322 – – – – – – – 380 0.0 0.0 17.4 1.2 19.7 1.6 0.0 0.01 0.00 0.03 <0.1 46.0 0.3FS4:9.1 (a) 498713 1272780 40 2003 – – 127 11.0 9.8 2.3 137 11.2 – 268 7.4 0.4 0.2 0.1 15.3 1.4 0.5 0.06 0.45 0.25 <0.1 0.1 1.8FS4:KS1 (a) 506972 1270461 20 2003 6.8 – 257 13.0 20.4 7.9 277 15.2 – 269 14.5 0.5 0.2 0.2 17.1 1.5 0.3 0.06 0.08 0.07 <0.1 <0.1 12.2FS5:CF01 (a) 506171 1235966 34 1999 7.3 – 0.0 0.8 0.7 0.2 0.8 0.8 – 427 0.0 0.3 102 2.8 18.5 1.2 0.9 0.35 0.05 0.59 – – –FS5:CF02 (a) 503695 1249740 28 1996 7.2 190 3.3 1.0 3.4 0.5 6.7 1.1 – 274 0.2 0.3 1.3 0.2 23.1 1.6 0.7 0.30 0.30 0.59 – – –FS5:CF03 (a) 500130 1257685 48 – 7.3 107 370 28.0 30.0 11.0 400 30.1 – 290 13.3 1.9 0.0 0.1 33.5 2.6 0.1 0.15 2.11 0.59 – – –FS5:CF04 (a) 503572 1266148 38 1995 7.2 116 29.4 9.8 0.2 0.6 29.6 9.8 – 396 2.3 0.6 2.2 0.2 16.2 1.4 0.4 0.24 0.05 0.60 – – –FS5:CF05 (a) 504670 1262241 46 2000 7.7 201 98.5 7.1 23.6 3.9 122 8.1 – 335 0.2 0.3 0.1 0.1 13.9 1.3 0.4 0.24 0.42 0.60 – – –FS5:CF06 (b) 498002 1266697 30 – 7.4 215 0.1 0.7 0.2 0.2 0.3 0.7 2.4 119 0.1 0.3 0.3 0.1 12.8 1.3 0.4 0.30 0.05 0.60 ND 1.2 0.5FS5:CF07 (b) 526773 1254185 55 2001 7.4 94 455 29.0 33.0 11.0 488 31.0 6.6 313 2.7 0.7 0.1 0.1 15.1 1.7 0.2 0.19 1.39 0.60 ND 0.6 8.7FS5:CF08 (b) 519979 1264737 50 – 4.2 220 3.3 0.9 0.2 0.2 3.5 0.9 5.6 250 0.1 0.3 0.0 0.1 14.0 1.7 0.9 0.66 0.05 0.61 ND ND 4.9FS5:CF09 (b) 519908 1264818 32 2002 7.5 102 108 26.0 7.0 3.2 115 26.2 5 237 2.6 0.8 0.0 0.1 16.4 2.7 0.7 0.73 0.35 0.62 ND ND 2.3FS5:CF10 (b) 519908 1264818 32 2002 7.5 99 – – – – – – 5.9 237 1.0 0.6 0.1 0.1 16.4 2.7 0.7 0.71 0.04 0.63 ND ND 2.2FS5:CF27 (b) 515052 1268381 32 2001 7.1 209 0.0 0.7 0.0 0.2 0.1 0.7 5.2 221 0.4 0.4 0.7 0.2 20.2 2.7 2.7 1.80 0.36 0.73 ND 2.0 1.5FS5:CF28 (b) 508768 1271204 53 1995 7.4 73 720 130 135 55.0 855 141 16.7 531 5.1 3.4 0.1 0.2 11.6 2.7 0.1 0.28 0.35 0.74 0.3 ND 26.3FS5:CF29 (b) 526755 1254169 67 1998 7.5 97 521 30.0 41.0 11.0 562 32.0 8.1 321 2.8 2.2 0.7 0.3 15.6 2.7 0.2 0.59 1.81 0.78 ND 2.0 10.3FS5:CF30 (b) 508773 1271185 57 1998 7.6 70 525 130 28.5 16.5 554 131 18.3 534 2.7 2.1 0.1 0.2 12.1 2.7 0.0 0.15 0.41 0.79 ND ND 33.7FS5:CF32 (a) 495833 1268379 45 2001 – – 0.0 1.4 1.5 1.0 1.6 1.7 2.7 121 0.0 0.4 0.4 0.2 1.0 0.5 0.0 0.14 0.05 0.80 3.2 2.0 0.1

– Denotes measurements not taken.a Samples name. Letter in parentheses determines location, A and B refer to Areas A (Bassac) and B (Mekong), respectively.b Eh normalised to SHE.c ND, Non detectable, <0.5 mg L�1.

H.A

.L.Row

landet

al./Applied

Geochem

istry23

(2008)3029–

30463035

Mg2+ SO42-

Ca+ Na+ + K+ HCO3- Cl-

(Ca+, Mg2+)(HCO3-) (Na+,K+)(SO4

2-,Cl-2)

(Ca+, Mg2+) (SO42-,Cl-2)

0 — 50 µg L-1 ΣAs

50 — 300 µg L-1 ΣAs

300 + µg L-1 ΣAs

Freshestwater

Increased interaction with aquifer minerals

Fig. 3. Piper plot of Cambodian well waters. Samples with high RAs (>300 lg L�1) appear to have tightly clustered chemistry, identified by HCO�3 , Ca2+ andMg2+ rich waters with low to negligible SO2�

4 and Cl�. Samples with lower concentrations of RAs have a broader chemistry, ranging from high Mg2+ and Ca2+

waters to those dominated by Na+ and K+. Only samples with low RAs (<50 lg L�1) have notable SO2�4 .

0 20 40

As Fe Eh NH4+ DOC

(ug L-1) (mg L-1) (mV) (mg L-1) (mg L-1)

0

10

20

30

40

50

60

70

80

0 200 400 600 800 1000

Dep

th (m

)

300 10 20 0 200 400 0 10 20

Area A

Area B

Σ

Fig. 4. Depth variation in aqueous RAs, Fe, Eh, NHþ4 and TOC in areas A (Bassac) and B (Mekong) well waters, Cambodia. The highest RAs concentrationsappear to occur below 48 m (dashed line), predominantly within area B (Mekong) and correlate with higher NHþ4 and DOC.



previously shown (Polya et al., 2003a; Buschmann et al.,2007; Berg et al., 2007).

4.3. Arsenic speciation and distribution

Analysis of Cambodian groundwater samples yielded arange of As concentrations (0–855 lg L�1 As) (Fig. 4, Table2). Arsenic was mainly in the reduced As(III) form, typicallyconstituting 90% of the total present, with As(V) making upsome 10% of the total. The predominance of As(III) is inagreement with the geochemical evidence of the watersbeing sub-oxic to anoxic. Concentrations of As subse-quently reported are the sum of As(III) and As(V) (identi-fied as RAs).

Despite wells from both Areas A and B producing gener-ally similar, circumneutral, sub-oxic to anoxic waters, theregions yielded highly variable RAs concentrations (Table2, Fig. 4). Over 40% of the wells in Area A were below theWHO provisional drinking water guide value of 10 lg L�1

(WHO, 2004) with 53% of wells being below the Cambo-dian legal limit of 50 lg L�1 (Smith et al., 2000) (Fig. 4).By contrast, less than 20% in Area B fell below the 10 lg L�1

threshold, with only 26% of all wells being below the high-er Cambodian standard. Wells in Area A produced RAs in arange of concentrations between 0 and 400 lg L�1 withArea B containing a much broader range from 0 to855 lg L�1. Although values are high, they were to be ex-pected due to the sampling bias of selecting wells from aknown As hotspot, with the broad range in RAs reflectingthe heterogeneous nature of As distribution within thisarea (Polya et al., 2005; Buschmann et al., 2007; Berget al., 2007). RAs concentration in wells in Area A appearedto be independent of depth (Fig. 4), whereas higher valuesof RAs values in Area B were found exclusively in wells be-low 48 m, with wells above this depth having a range anddistribution more typical of Area A (0–400 lg L�1) (Fig. 4).

There appeared to be no relationship between RAs andwell age (Fig. 5), however, high RAs (>300 lg L�1) wellwa-ters had tightly clustered (bar one sample) compositionsdominated by Ca2+ and Mg2+ (Fig. 3). Such chemistry isindicative of fresher waters that have not undergoneextensive water/rock interaction (see Section 4.2). Thissuggests that the risk of groundwater containing excessive

0

200

400

600

800

1000

1200

1985 1990 1995 2000

ΣAs(

µg L

-1)

Year of well construction

Fig. 5. Comparison of aqueous RAs with well age in Cambodian well waters. Ther

As (>300 lg L�1 RAs) decreases with residence time ofgroundwater within the aquifer. Although groundwaterswith lower RAs concentrations also plotted within thiscluster, these waters had a much wider range of chemicalcompositions.

4.4. Comparison of Cambodian groundwater and aquifersystem with West Bengal and Bangladesh

Sediments containing waters with elevated As withinWest Bengal and Bangladesh are Quaternary in age andbroadly divisible into two main deposition events, (i)early-mid Pleistocene sediments consisting of cyclicstacked river channels of interbedded gravels, sands andmuds, subsequently Fe stained (red/brown in colour) dueto exposure during the last glacial maximum, and (ii)younger, finer grey Holocene deposits consisting of finersands and silts with abundant mica (Alam, 1989; Umitsu,1993; Acharyya et al., 2000; Goodbred and Kuehl, 2000;Ravenscroft et al., 2001). Solid phase As within the aquifersis typically correlated to Fe, and thought to be mainly asso-ciated with Fe(III) oxides (Nickson et al., 1998, 2000; Bhat-tacharya et al., 2001; Anawar et al., 2003; Akai et al., 2004;Swartz et al., 2004). Arsenic has also been found associatedwith sulphides in Bangladesh (Polizzotto et al., 2006).Groundwaters present within these Quaternary aquifersare typically characterised by high Fe and HCO�3 , low Cl�

and SO2�4 , circumneutral pH and reducing conditions, with

As distribution being highly heterogeneous in nature, andpredominantly in the reduced As(III) form (Nickson et al.,1998, 2000; DPHE, 1999; McArthur et al., 2001, 2004;Smedley and Kinniburgh, 2002; Harvey et al., 2002; Dow-ling et al., 2002; van Geen et al., 2003).

The aquifer sediments present within Cambodia showmany similar characteristics to those of West Bengal andBangladesh. The presence of an upper layer of finer greysands and muds, thought to be Holocene (ESCAP, 1993)in Cambodia ties in well with similar grey coloured sedi-ments known to have been deposited within the Holocenein West Bengal and Bangladesh. The underlying red sedi-ments dominated by fluvially deposited fining upward se-quences of coarse to very fine sand found below the grey(Holocene?) sediment in Cambodia also ties in with that

2005

Area A Area B Combined Area A and B average ΣAs

e appears to be little correlation between RAs concentration and well age.


found within West Bengal and Bangladesh. The down bore-hole solid phase correlation of As and Fe (Fig. 2) and theassociation between As and Fe(III) oxides (Table 1) is alsovery similar to that observed in West Bengal and Bangla-desh. Groundwater characteristics within Southern Cam-bodia, as with the sediments, show many similarities tothe As-rich waters of West Bengal and Bangladesh includ-ing high but heterogeneous As distribution, high Fe, lowSO2�

4 (in the majority of samples), moderate to highHCO�3 , circumneutral pH and reducing conditions (as iden-tified from geochemical characteristics) (Table 2).

If the characteristics of the groundwater and sedimentsin West Bengal, Bangladesh and Cambodia are so similar, itwould then follow that the processes of As mobilisationinto the groundwater could also be the same. Therefore,various processes of As mobilisation previously proposedto be of importance in West Bengal and Bangladesh aretested in comparison to Cambodia, to identify a likelymode of As release.

4.5. Arsenic mobilisation processes

4.5.1. Arsenic associated with sulphidesDissolution of As bearing pyrites has previously been

postulated as a possible route of As release into groundwa-

0

200

400

600

800

1000

0 50 100 150 200

Area A

ΣAs

(µg

L-1)

Fe (m

g L-1

)

S (mg L-1)

S (mg L-1)

0

200

400

600

800

1000

0 5

0

5

10

15

20

25

30

0 50 100 150 2000

5

10

15

20

25

30

0 5

Fig. 6. Comparison of aqueous RAs, S and Fe in Cambodian well waters. Sulphurboth Area A and B.

ters of West Bengal and Bangladesh (Mallick and Rajag-opal, 1996; Mandal et al., 1996; Das et al., 1996;Chowdhury et al., 1999). However, this has been subse-quently disputed, as any As released would be resorbedby the products produced from the breakdown of pyrite(Nickson et al., 2000; McArthur et al., 2001; Ravenscroftet al., 2001).

Although SO2�4 was only measured in just over half of

the well samples in this study (n = 41), comparison withthe total S indicated that the majority of the S in the wellsis SO2�

4 (Table 2). Waters analysed from Cambodia have asimilar negative covariance between S (presumed to bein the form of SO2�

4 , produced as a byproduct of the break-down of pyrite), Fe and RAs (Fig. 6) as found in aquifers ofWest Bengal and Bangladesh (DPHE, 1999; Ravenscroftet al., 2001; McArthur et al., 2001, 2004; Harvey et al.,2002; Anawar et al., 2003; Swartz et al., 2004). Indeed, ifthese three components are plotted as a function of rela-tive redox conditions (Fig. 7) or general chemistry(Fig. 3), it is clear that As is found almost exclusively inwaters that contain negligible S. The few samples that docontain higher concentrations of S (presumed to be in theform of SO2�

4 ), are found only in less reducing waters (bycomparison to other wells) with waters containing Feand RAs found almost exclusively in more reducing waters

Area A All wells

Area B Shallow wells (>48 m) Deep wells (<48 m)

Area B

0 100 150 200

0 100 150 200

(presumed in the form SO2�4 ) has a negative correlation with RAs and Fe in

Area A

As Fe S Area B

As (<48m depth) As (>48m depth) Fe S

S (m

g L-1

)

200

150

100

50

00

50

100

150

200

250

300

350

400

450

500

As (µ

g L- 1

)

0

5

10

15

20

Fe (m

g L-1

)

Area - A

More Less reducing reducing Redox conditions

160

140

120

100

80

60

40

20

0

S (m

g L-1

)

0

200

400

600

800

1000

As (µ

g L- 1

)

Fe (m

g L-1

)

0

5

10

15

20

25

30 Area - B

More Less reducing reducing Redox conditions

Fig. 7. Comparison between aqueous RAs, Fe and S (presumed in the form SO2�4 ) and relative redox conditions in areas A (Bassac) and B (Mekong) well

waters, Cambodia. Wells with moderate to high RAs and Fe appear to be more reducing (open checked area), with the highest RAs values (deep wells >48 mof area B) found in the most reducing waters (close checked area). Wells with moderate S contents appear to be less reducing and contain negligible Fe andRAs (open area).


(Fig. 7). Therefore, pyrite oxidation appears not to be animportant process of As mobilisation in relation to Cambo-dian groundwaters.

No evidence was found for As bound to sulphides inthis study, with As being either sorbed, or associatedwith amorphous, poorly crystalline or well crystallisedhydrous Fe and Al oxides (Table 1). Previous X-rayabsorption near edge structure (XANES) spectroscopicanalysis, used to determine the chemical speciation ofAs within a range of aquifer sediments from Cambodia,found that only a minor fraction was associated with S(Rowland et al., 2005), again implying, for this area atleast, that As is associated predominantly with Fe-oxi-

des. This is in contrast to the study by Polizzotto et al.(2006) where As was found to be predominantly associ-ated with sulphides (>60%), with these workers citingmobilisation of As as being due to redox cycling at thesurface causing As release which is then flushed intothe underlying aquifer, and, due to a lack of Fe(III) oxi-des, either remains in solution or is weakly sorbed andso easily desorbed due to chemical changes. Lowerset al. (2007) also found that pyrite harboured a signifi-cant proportion of the sediment-bound As in their studysites in Bangladesh, but also found a large fraction ofsedimentary As contained in Fe(III) oxyhydroxides andsecondary Fe minerals.


4.5.2. Arsenic and reductive dissolution of Fe(III) oxidesArsenic is commonly found associated with Fe(III) oxi-

des in sediments of West Bengal and Bangladesh (Nicksonet al., 1998, 2000; Bhattacharya et al., 2001; Anawar et al.,2002, 2003; Akai et al., 2004; Swartz et al., 2004). Microbi-ally mediated oxidation of organic matter can drive aqui-fers to anoxia causing destabilisation and dissolution ofAs bearing Fe(III) oxides producing positive correlationsbetween Fe, As and HCO�3 , and is a theory commonly citedfor As mobilisation in groundwaters of Bangladesh andWest Bengal (Nickson et al., 1998, 2000; Bhattacharyaet al., 2001; Ravenscroft et al., 2001; Dowling et al.,2002; Akai et al., 2004; Zheng et al., 2004; McArthuret al., 2004).

Cambodian groundwaters that contain high RAs, invari-ably contain Fe and occur within more reducing waters,with the very high (>300 mg L�1) RAs concentrations typ-ically occurring in the most reducing waters (Fig. 7). Inaddition, all wells sampled show a positive correlation be-tween RAs and Fe (Fig. 8), as would be expected to resultfrom this process. Also, RAs shows good correlation withboth NHþ4 and DOC (Fig. 9), which has also been noted inwell samples from West Bengal and Bangladesh, and is ta-ken to indicate higher levels of microbial activity and or-ganic matter degradation (McArthur et al., 2001, 2004;

Area A

0

200

400

600

800

1000

0 10 20 30

Fe (mg L-1) 0 1

ΣAs

(μg

L-1)

Fig. 8. Comparison between aqueous RAs and Fe in areas A (Bassac) and B (Mekshow a good positive correlation between RAs and Fe. The RAs in the deepest (>4greater RAs compared to Fe concentrations with respect to the other well grou

NH4+ (mg L-1)

0

As (µ

g L-1

)

0

200

400

600

800

1000

0 10 20 30 40

Fig. 9. Comparison between aqueous RAs, NHþ4 and DOC from Area B well waterhave a positive correlation with both NHþ4 and TOC in all samples regardless of

Dowling et al., 2002; Harvey et al., 2002; Smedley andKinniburgh, 2002; Anawar et al., 2003; Swartz et al.,2004). Greater rates of microbial activity would drive thesystem to greater anoxia, lead to an increase in the reduc-tive dissolution of Fe(III)-oxides and greater As release, andwould explain why the most reduced wells sampled in thisstudy contain the highest RAs and Fe concentrations(Fig. 7).

The relationship between RAs and HCO�3 is howevervariable, (Fig. 10). Area B shows a good positive correlationbetween RAs and HCO�3 , with samples showing a distribu-tion of HCO�3 and RAs similar to As rich well water samplesfrom West Bengal and Bangladesh (Nickson et al., 1998,2000; Smedley and Kinniburgh, 2002; Swartz et al.,2004). The Cambodian wells have an intercept of approxi-mately �150 mg L�1 HCO�3 , presumably representing thebackground alkalinity due to mineralogical dissolutionwithin the aquifer. However, Area A appears to have an al-most unimodal distribution, with high RAs only appearingin waters within a range of HCO�3 concentration, from 150to 400 mg L�1 (Fig. 10). The waters that contain the highestHCO�3 concentrations in Area A are almost exclusively oneswith high S (>5 mg L�1, presumed to be in the form ofSO2�

4 ), (Fig. 10), previously shown to have a negativecovariance with RAs (Fig. 6), and which by nature appear

Area B

0 20 30

Area A All wells Area B

Shallow wells (<48 m) Deep wells (>48 m)

ong) well waters, Cambodia. Both area A and the shallow wells in area B8 m) wells of area B also correlate with Fe, but samples appear to have far

ps (hashed area).

DOC (mg L-1) 10 20 30

Area B Shallow wells (<48 m) Deep wells (>48 m)

samples, Cambodia. Both graphs share the same RAs scale. RAs appears todepth.

HCO3- (mg L-1)

ΣAs

(µg

L-1)

0 500 10000

200

400

600

800

1000

0

200

400

600

0 500 1000

Area A Low S (<5 mg L-1) High S (>5 mg L-1)


Area A Area B

Fig. 10. Comparison between aqueous RAs and HCO�3 , in areas A (Bassac) and B (Mekong) well waters, Cambodia. Area B shows a good positive correlationbetween RAs and HCO�3 . Area A shows a more unimodal distribution with samples containing RAs predominantly falling within a narrow range of HCO�3contents (hashed area). Samples with high HCO�3 in area A appear to correspond to waters containing high S (presumed to be predominantly in the formSO2�

4 ).


to be less reducing (Fig. 7). These waters also contain littleFe, which, when coupled to the excess HCO�3 present im-plies Fe is removed from solution to form siderite thatcould scavenge As from solution (Islam et al., 2005). Inaddition, the lack of Fe in waters containing higher S couldbe due to the formation of diagenetic pyrite (Appelo andPostma, 1993) which would also scavenge RAs from thewaters (Lowers et al., 2007). Therefore re-precipitation ofFe minerals could explain the lack of correlation betweenRAs and HCO�3 in these specific high HCO�3 , high S, lowFe waters within area A.

Iron and HCO�3 , if both are released via reductive disso-lution into the groundwater, should be produced at a molarratio of 1:2 (Nickson et al., 2000). When Fe and HCO�3 areplotted together for the Cambodian groundwater samples(Fig. 11), all plot well to the right of this line (adjusted torepresent the background alkalinity of �150 mg L�1), sug-gesting that if reductive dissolution of Fe(III) oxides is ulti-mately controlling their behaviour, then Fe is not behavingconservatively within the system and is being lost from theaqueous phase. Calculation of the SI of the samples withPHREEQC found that around half of the samples were over-saturated with respect to siderite, implying the formation

0

0.1

0.2

0.3

0.4

0.5

0 5 10 15

ΣFe

(mM

)

0

Area A

HCO3- mM

Fig. 11. Comparison between aqueous Fe and HCO�3 , in areas A (Bassac) and B (Mline represents the Fe(II)/HCO�3 molar ratio (1:2) produced by reductive dissolutiline indicating that Fe is not behaving conservatively and is being lost from solu

of this phase is a likely candidate for shifting the dissolvedFe concentrations from the line of equal relation withHCO3. Another possible explanation could be the sorptionof Fe(II) onto Fe(III) oxide surfaces not already reduced(Appelo et al., 2002). This effect could explain why the veryhigh (>300 lg L�1) RAs groundwaters present at depth(>48 m) in Area B have much lower Fe contents than wouldbe expected by comparison to other samples (Fig. 8) andhas previously been cited as a possible explanation of highRAs, low Fe waters in Bangladesh (van Geen et al., 2004;Horneman et al., 2004).

4.5.3. Arsenic and microbial activityThe activity of microbes within the aquifers of West

Bengal and Bangladesh have also been shown to have amore direct impact on the mobilisation of As. Microbescan directly utilise Fe(III) to provide energy for growth,and follow the same chemical reaction as reductive disso-lution, to produce HCO�3 and Fe(II) (Lovley, 1991). This pro-cess could therefore explain the relationships found in theCambodian wells with lower RAs, Fe and HCO�3 concentra-tions (Area A, Area B, shallow wells). Indeed, microbiallymediated reduction of Fe(III) oxides, with subsequent

5 10 15

Area A Low S (<5 mg L-1) High S (>5 mg L-1)


Area B

ekong) well waters, Cambodia. Both graphs share the same Fe scale. Theon of Fe(III) oxides to Fe(II) and HCO�3 . All samples plot to the right of thistion. This effect appears to be greatest in wells from area B.


mobilisation of As(III) has been shown to occur withinaquifer sediments from this ‘As hotspot’ in Cambodia(Rowland et al., 2007; Pederick et al., 2007).

It has been shown that this process does not necessarilyrelease all of the available Fe(II) into solution but can retainit in the solid phase by the creation of Fe(II) or mixed Fe(II/III) minerals (Konhauser, 1997; Fredrickson et al., 1998;Roh et al., 2003). This would keep Fe in the solid phasebut release As, therefore producing waters with exces-sively high As, but not necessarily high Fe, as seen in thevery high RAs (>300 lg L�1) samples in Area B. This hasbeen cited as the reason behind high As, low Fe watersstudied in Bangladesh (Horneman et al., 2004; van Geenet al., 2004). It is of course possible that the new Fe second-ary phases could re-sorb As mobilised via Fe(III) reduction(Islam et al., 2005), but if the new minerals have a lowercapacity for sorption than the parent materials, then thesorption capacity of the sediments would be reduced andAs would remain in solution, this effect possibly beingdependent on the As oxidation state (cf. Coker et al.,2006). However, why this effect would be so marked in justthis one area is not clear.

Microbes can also cause mobilisation of As via directutilisation of As(V) as an electron acceptor (Oremlandand Stolz, 2003; Lloyd and Oremland, 2006). This processwould mobilise As(III), but not Fe, and could explain howthe deep samples within Area B have very high RAs butwithout comparably high Fe (Fig. 8). Most As(V) respiringbacteria are opportunistic and capable of utilising a varietyof terminal electron acceptors, including Fe(III) (Lloyd andOremland, 2006). In an environment containing mixedelectron acceptors, these opportunistic microbes, whichmight be out competed for more common electron accep-tors (such as Fe(III) or SO2�

4 ) will instead utilise other elec-tron acceptors (such as As(V)), leading to the release ofAs(III) into the groundwater. Indeed, microbes capable ofutilising As(V) as an electron acceptor have been found inCambodian aquifer sediments (Lear et al., 2007).

4.5.4. Competitive sorption and desorptionArsenic held on Fe(III) oxides can also be mobilised by

competition for sorption sites with other chemical compo-nents such as H4SiO4, PO4, Fe(II), HCO�3 and dissolved or-ganic C (Grafe et al., 2001; Smedley and Kinniburgh,2002; Appelo et al., 2002; Meng et al., 2002; Anawaret al., 2004). Competitive sorption is an extremely complexmechanism, dependent not only on absolute concentra-tions, but also on pH, redox conditions, the order in whichdifferent components are sorbed/released and interactionbetween the competing components. Even so, if it was animportant process within these sediments, especially as aroute to producing waters with very high As, but not nec-essarily high Fe, then some clear relationships should beobserved. All of these components have been found withinCambodian well waters (Table 2) and so could have an ef-fect on promoting the mobilisation of As beyond thatoccurring by microbial activity (as previously discussed).

Comparison of RAs with total Si and total P shows lim-ited correlation within the sediments (Fig. 12). The correla-tion between RAs and P in all samples (apart from the veryhigh RAs, >600 lg L�1 observed in Area B) is intriguing and

could suggest a causal link. However, PO3�4 is chemically

analogous to As(V) (Abedin et al., 2002), so it is entirelypossible that this relationship is due to the similar behav-ioural patterns of As and PO3�

4 , and not due to the compet-itive effect of PO3�

4 with As. Comparison of RAs with HCO�3(Fig. 10) again suggests competitive sorption may beimportant with Area B having a positive correlation be-tween RAs and HCO�3 in both the shallow and the deepwells. However, it is not possible to separate the relativecontributions to the correlation between RAs and HCO�3of microbially mediated breakdown of As bearing Fe(III)oxides and desorption of As due to competitive adsorptionwith HCO�3 . The most likely evidence for competitive sorp-tion playing any role in exacerbating RAs mobilisation inCambodian waters is the correlation between RAs andDOC seen within Area B (Fig. 9). Organic compounds areknown to compete with RAs for surface sites (Grafeet al., 2001), so the very high RAs could be from competi-tion with DOC for surface sites on Fe(III) oxides. However,this is only seen in a small selection of groundwaters asDOC was not analysed in all of the wells waters sampled.Therefore, although the importance of competitive sorp-tion in promoting RAs mobilisation is not disregarded,the interactions between competing ions and As withinthis complex natural system cannot be reliably delineatedfrom other processes.

4.5.5. Arsenic and anthropogenic activityThe impact of groundwater extraction on the mobilisa-

tion of As is an area of great debate. The introduction of la-bile organic C into the aquifer has been shown to promoteAs mobilisation both in situ (Harvey et al., 2002), and dur-ing incubation experiments using aquifer sediments (Islamet al., 2004; Akai et al., 2004; van Geen et al., 2004; Row-land et al., 2007). In both Pakistan (Nickson et al., 2005)and Thailand (Lawrence et al., 2000), anthropogenic activ-ity has lead to the introduction of labile organic C from thesurface into underlying aquifers, increasing microbialactivity and causing As release. However, in other areas,typically West Bengal and Bangladesh, the impact ofincreasing groundwater extraction on dissolved As concen-trations, especially over recent times, is still unclear (Char-let and Polya, 2006).

Well age has been used as a proxy for the amount ofwater extracted over time, with a rise in aqueous As con-centrations in conjunction with older wells cited as evi-dence for human activity exacerbating As levels ingroundwaters (Burgess et al., 2002a, b). No clear correla-tion could be found between RAs and well age in the pres-ent work (Fig. 5). This of course may reflect the lowsampling frequency, nevertheless, the well age is not agood record of the level of human impact on the aquifersas each well would have different extraction volume histo-ries, and well ages given at source are not always reliable(McArthur et al., 2004). If As concentrations within anaquifer were sensitive to extraction (by whichever Asmobilisation process), areas with a short history of ground-water extraction might be expected to have waters withsubstantially lower As concentrations than those exposedto longer-term human abstraction of groundwater. Cambo-dia has had a shorter history of groundwater extraction

0

200

400

600

800

1000

0 1 2 3 0 1 2 3

0

200

400

600

800

1000

0 10 20 30 40 0 10 20 30 40

Si (mg L-1)

ΣAs

(µg-1

) ΣA

s (µ

g-1)

P (mg L-1)

Area A Area B

Area A Area B

Area A All wells


Fig. 12. Comparison between aqueous RAs, P and Si in area A (Bassac) and B (Mekong) well waters, Cambodia. All graphs share the same RAs scale. Area Ashows a positive correlation between RAs, P and Si. Area B shows no correlation with Si, and a slight correlation with P, though samples with very high RAs(>600 lg L�1) do not share this correlation.


than West Bengal and Bangladesh (Polya et al., 2005), so itwould be expected to have significantly lower As concen-trations if anthropogenic activity was exacerbating As con-centrations. However, this is not the case, with Cambodiangroundwaters having As concentrations ranging from 0 to855 lg L�1 (this study) and other workers reporting maxi-mum levels of up to 1700 lg L�1 (Polya et al., 2005; Busch-mann et al., 2007; Berg et al., 2007). Such concentrationsare only slightly lower than reported values from bothWest Bengal and Bangladesh. This suggests that elevatedAs concentrations in these aquifers pre-dates massivegroundwater exploitation.

However, this is not to say that human activity does nothave the potential to exacerbate As concentration withinthe groundwaters. Comparison between the variability ofthe major cations within the aquifer (Na+, K+, Mg2+ andCa2+) indicate the presence of very high (>300 lg L�1)RAs occurs predominantly in waters that appear to havethe lowest residence time within the aquifer (Section 4.2,Fig. 3). A possible theory for this relationship is that‘fresher’ waters have less contact with the microbial com-munity in the aquifer and so will retain higher levels ofDOC and nutrients, whereas older groundwater would bemore depleted in DOC and nutrients and so would not becapable of supporting as much microbial activity. Why this‘fresher’ water is found predominantly at depth (>48 m)

could be due to heterogeneity within the aquifer, withchannels of coarser sand that would be expected to occurin fluvial deposits providing a fast route for water throughthe aquifer system, with movement of water in the upperportions of the aquifer being restricted by finer sediments.If human activity and extensive groundwater abstractioncould cause an increase in drawdown of this fresh water,it may have the potential to increase As concentrationswithin the aquifers.

5. Conclusions

Analysis of groundwaters in Kandal province, Cambo-dia, an area previously reported to contain elevated As(Polya et al., 2003a, 2004, 2005; Buschmann et al., 2007;Berg et al., 2007), found high but heterogeneous As distri-butions with As predominantly in the reduced As(III) form.The circumneutral waters were characterised by their highconcentrations of Fe and HCO�3 , low SO2�

4 concentrations(in the majority of wells) and reducing conditions (as iden-tified from geochemical components present). Sequentialextraction analysis of the aquifer sediments indicated thatAs was largely bound with Fe(III) oxides. Such characteris-tics are typical of those found in similar high As aquifers inboth West Bengal and Bangladesh (Ravenscroft et al., 2001;Smedley and Kinniburgh, 2002; Nordstrom, 2002; Charlet


and Polya, 2006) and imply that the types of processes ofAs release in Cambodia would be similar to those foundin these regions.

The lack of relationship between S, and RAs and Fe indi-cated that pyrite oxidation is not an important process inmediating As release into Cambodian waters. However,evidence for a variety of other possible mechanisms mobil-ising As could be found. Good positive correlations in theaqueous phase between RAs and Fe, NHþ4 and DOC andmoderate correlation with HCO�3 suggests that microbiallymediated processes leading to the dissolution of Fe(III) oxi-des under reducing conditions could be centrally involvedin the release of As, as suggested for similar aquifer watersin both Bangladesh and West Bengal (Nickson et al., 2000;Bhattacharya et al., 2001; Ravenscroft et al., 2001; Smedleyand Kinniburgh, 2002 McArthur et al., 2004; Zheng et al.,2004). These findings follow other studies that also con-clude that microbially mediated dissolution of As bearingFe(III) oxides is a dominant mechanism for the release ofAs into Cambodian groundwaters (Polya et al., 2003a; Bus-chmann et al., 2007; Berg et al., 2007; Rowland et al., 2007;Pederick et al., 2007).

The presence of high RAs and low Fe concentrations inthe deeper wells of area B implies that microbes could havea more direct role in mediating As release either throughthe reductive dissolution of As bearing Fe(III) oxides, asFe expected to be released into the aqueous phase throughthis process is often retained in the solid phase and notmobilised (Konhauser, 1997; Roh et al., 2003), or by the di-rect utilisation of As(V) as an electron acceptor whichcould release As without mobilising Fe (Oremland andStolz, 2003; Lloyd and Oremland, 2006). Very high concen-trations of RAs (>300 lg L�1) were found only in ground-waters with chemical signatures typical of short aquiferresidence times, possibly due to younger waters sustaininggreater microbial activity due to higher loads of DOC andnutrients. If such a situation exists, then this highlightsthe vulnerability of such aquifers to the drawdown of freshsurface waters, rich in organic matter, possibly via in-creased extraction for irrigation that would promotemicrobially mediated As release in the aquifer.

Despite the evidence for a variety of possible As mobili-sation processes within the aquifers studied here fromCambodia, there is insufficient data to determine whichare wholly responsible and this reveals limitations in thevalue of a study on this scale. However, it is apparent thatmicrobially mediated processes do play a major role in Asrelease and provides a direction for further study. Determi-nation of the specific role that microbes play, as well asidentification of the possible electron donors they use tomediate As release, would likely improve understandingof the As mobilisation processes involved, not only fromCambodian sediments but also in shallow, subsurface envi-ronments elsewhere in the world.

Acknowledgements

We gratefully acknowledge receipt of a NERC/CASE(with NHM) PhD studentship to HALR, an EPSRC researchgrant (GR/S30207/01) to DAP, Jon Lloyd, David Vaughanand Roy Wogelius; and a NERC/CASE (with CETAC Technol-

ogies) PhD studentship to AGG. We thank Vibol Long, Jes-sica Jones, David Fredericks and Mickey Sampson forassistance and advice during one or more field seasons inCambodia, and Alastair Bewsher, Tim Jenson and Cath Da-vies from the Manchester Analytical Geochemistry Unit forlaboratory support.

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Geochemistry of aquifer sediments and arsenic-rich groundwaters from Kandal Province, Cambodia

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