Redox trapping of arsenic during groundwaterdischarge in sediments from the Meghnariverbank in BangladeshS. Dattaa,b, B. Maillouxc, H.-B. Jungd, M. A. Hoquee, M. Stutea,c, K. M. Ahmede, and Y. Zhenga,d,1

aLamont-Doherty Earth Observatory of Columbia University, 61 Route 9W, Palisades, New York, NY 10964; bKansas State University, Department of Geology,Manhattan, KS 66506; cBarnard College, Department of Environmental Sciences, New York, NY 10027; dQueens College, School of Earth and EnvironmentalSciences, City University of New York, Flushing, New York, NY 11367; and eUniversity of Dhaka, Department of Geology, Dhaka, 1000 Bangladesh

Communicated by Charles H. Langmuir, Harvard University, Cambridge, MA, July 30, 2009 (received for review September 5, 2007)

Groundwater arsenic (As) is elevated in the shallow Holoceneaquifers of Bangladesh. In the dry season, the shallow groundwa-ter discharges to major rivers. This process may influence thechemistry of the river and the hyporheic zone sediment. To assessthe fate of As during discharge, surface (0–5 cm) and subsurface(1–3 m) sediment samples were collected at 9 sites from the bankof the Meghna River along a transect from its northern source(25° N) to the Bay of Bengal (22.5° N). Bulk As concentrations ofsurface sediment averaged 16 � 7 mg/kg (n � 9). Subsurfacesediment contained higher mean concentrations of As of 4,000mg/kg (n � 14), ranging from 1 to 23,000 mg/kg As, with >100mg/kg As measured at 8 sites. X-ray absorption near-edge struc-ture spectroscopy indicated that As was mainly arsenate andarsenite, not As-bearing sulfides. We hypothesize that the elevatedsediment As concentrations form as As-rich groundwater dis-charges to the river, and enters a more oxidizing environment. Asignificant portion of dissolved As sorbs to iron-bearing minerals,which form a natural reactive barrier. Recycling of this sediment-bound As to the Ganges-Brahmaputra-Meghna Delta aquifer pro-vides a potential source of As to further contaminate groundwater.Furthermore, chemical fluxes from groundwater discharge fromthe Ganges-Brahmaputra-Meghna Delta may be less than previousestimates because this barrier can immobilize many elements.

Ganges-Brahmaputra � hyporheic zone � natural reactive barrier �submarine groundwater discharge � XANES

In the Ganges-Brahmaputra-Meghna Delta (GBMD) of Indiaand Bangladesh, more than 30 million people have been

drinking groundwater with elevated levels of arsenic (As) for atleast the last decade (1). Many studies have focused on thebiogeochemical processes that release As in the GBMD aquifers,but little work has been done on the fate of As during ground-water discharge. In Bangladesh, rivers are potential areas ofgroundwater discharge (2). Because of the change in redoxconditions at the interface, As may precipitate or sorb onto iron(Fe)-bearing sediments in the riverbank or riverbed.

It has been recently demonstrated that when anoxic ground-water discharges to the ocean or to a lake, Fe in groundwater isoxidatively precipitated onto the sands (3), immobilizing manyelements, including As and phosphorus (4). Growing evidencesupports the important role that processes in the hyporheic zonehave in regulating the composition of groundwater dischargingto rivers (5–7). In this study, we examine accumulations of Asbound to riverbank sediment and implications these enrichmentshave on As cycling in Bangladesh. We determined the Asconcentrations of shallow sediments along the entire length ofthe Meghna River. This system was chosen because most of theshallow groundwater As east of the Meghna River between 22and 24° N (Fig. 1) contains �50 �g/L As (1). Our results haveimportant implications for cycling of As in deltaic environmentsand groundwater discharge as a source of elements to the ocean.

ResultsAquifers in the GBMD. The Ganges and Brahmaputra rivers coalescenorthwest of Dhaka and then join the Meghna River south ofDhaka before flowing into the Bay of Bengal (see Fig. 1). Bang-ladesh is less than 10 m above sea level, except for the upliftedPleistocene terraces, the Chittagong Hills, and the hilly area in thenortheast. The sandy, unconsolidated Pleistocene to Holocenefluvial and deltaic sediments that underline much of Bangladeshform prolific aquifers in this highly energetic depositional environ-ment. The shallow Holocene aquifers are typically elevated in As,whereas the older Pleistocene aquifers appear to contain little As.The shallow aquifer sediments have been deposited since �10,000to 11,000 years B.P. (1). During the early Holocene (11,000–7,000years B.P.), the river sediment flux was sufficient to infill 50-mincisions formed during glacial periods of low sea level. The deeperPleistocene aquifers are typically separated from the shallow Ho-locene aquifers by multiple layers of silt and clay. Aquifers arerecharged from precipitation directly or through flooding duringthe wet season (1). Shallow aquifers are discharged via flow into therivers and other low-lying surface water bodies, as well as byevapotranspiration.

Sediment samples were collected on the Meghna riverbank at9 locations in January 2003 (Table 1) and at 2 locations inJanuary 2006 (Fig. S1). In 2006, continuous sediment cores fromsurface to up to 6-m depth were obtained. In 2003, surface (0–5cm) and subsurface (variable depth between 1 and 3 m) sedimentsamples were obtained at each site. Four sites are located on thetributaries to the Meghna River at �25° N at an elevation of 3to 10 m. Five sites are located on the main channel of the MeghnaRiver between 22.5° N and 24° N at 0 to 3 m elevation. Eight sitesare located on sandy deposits along the riverbank. RS-1 islocated on a sandbar in the river. At RS-4, an additionalsediment core was obtained from the riverbed (see Table 1).Samples from the top 5 cm and bottom 5 cm were subjected toleaching and acid digestion (see SI Analytical and SpectroscopicMethods) before determination of As (see Table 1).

As in Surface Sediment. Bulk As concentrations of surface sedi-ment samples subjected to total acid dissolution ranged from 7to 27 mg/kg, averaging 16 � 7 mg/kg (n � 9) (see Table 1). Thesevalues were similar to As concentrations in soils from around theworld (8) and in Bangladesh (9).

As in Subsurface Sediment. High concentrations of As (�100 to�20,000 mg/kg) were extracted from the sediment using 1.2 N

Author contributions: Y.Z. designed research; S.D., H.-B.J., M.A.H., M.S., K.M.A., and Y.Z.performed research; S.D., B.M., and H.-B.J. contributed new reagents/analytic tools; S.D.,B.M., H.-B.J., and Y.Z. analyzed data; and S.D., B.M., and Y.Z. wrote the paper.

The authors declare no conflict of interest.

1To whom correspondence may be addressed. E-mail: yzheng@ldeo.columbia.edu oryan.zheng@qc.cuny.edu.

This article contains supporting information online at www.pnas.org/cgi/content/full/0908168106/DCSupplemental.

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hydrochloric acid (HCl) at 8 of the 9 sample sites (see Table 1and Fig. 2). The concentrations of As released by 1-M phosphateleaches were elevated (�200 to �1,000 mg/kg) at 7 sites (seeTable 1). Six samples with 3 to 1,000 mg/kg of phosphate-extractable As displayed comparable HCl-extractable As con-centrations (R2 � 0.99) (see Table 1). Solid residues from thephosphate extractions of these samples had low concentrationsof As ranging from 3 to 30 mg/kg (see Table 1). Collectively,these results indicate that the majority of the As in these 6subsurface sediment samples are easily extracted.

For 5 samples with very high HCl-extractable As (�4,000 to�20,000 mg/kg), As concentrations were also elevated in theresidues of the phosphate extracts, ranging from 500 to �3,000mg/kg (see Table 1). However, the sum of the phosphate-extractable As and the residual As after phosphate-extraction,should be higher than, or at least equal to the HCl-extractableAs. This was not the case for these 5 samples. This discrepancy

is most likely caused by leaving �1 ml of phosphate solution toreact with the sediments for about a year before the residualsediment was separated for total acid dissolution. We suspectthat As continued to be leached by this discarded aliquot ofphosphate solution. Nevertheless, the results are consistent withgreater concentrations of As bound in the phases that areleached either by phosphate or HCl in subsurface sediments thanis bound in residual more refractory phases.

Depth Profiles of As and Fe(II)/Fe. Enrichment of sediment As wasfound at shallow depths, corresponding to a redox transitionzone, as indicated by the increasing sediment Fe(II)/Fe ratio withdepth (Fig. 3). Samples were analyzed for HCl-leachableFe(II)/Fe within 12 h to characterize sediment redox condition.At RS-30, the depth interval from 0.75 to 0.90 m displayedincreasing bulk sediment As concentration from �30 mg/kg to�200 mg/kg. The sediment from this depth interval was gray and

Fig. 1. Locations of sediment sampling sites along the Meghna River are indicated by circles with dots in the center. White, light gray, and dark gray circlesrepresent HCl-leachable As concentrations in subsurface sediments at a depth between 1 and 3 m, with values of �10 mg/kg, between 100 and 1,000 mg/kg,and between 1,000 and 23,000 mg/kg, respectively. There are no samples with concentrations between 10 and 100 mg/kg. At 14 other sites reported in a previousstudy (15), maximum HCl-leachable As concentrations in shallow sediments (1–3 m) are indicated by open squares for those with values between 6 and 30 mg/kgand by light gray squares for those with values between 100 and 758 mg/kg. Groundwater As concentrations for wells with depth �25 m (n � 743) are plottedas blue, red, and dark red circles to represent �50, 50 to �100, and 100 to 1,090 �g/L (1).

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characterized by a high Fe(II)/Fe ratio of �0.85, although thesediment immediately above at 0.3 m had a low Fe(II)/Fe ratioof 0.25. At RS-33, depth intervals of 0.24 m to 0.30 m, and 1.45to 1.5 m displayed sediment As enrichment up to �100 mg/kg.The shallower interval had sediment Fe(II)/Fe of 0.48, whereasthe deeper interval had sediment Fe(II)/Fe of 0.24. SedimentFe(II)/Fe reached 0.9 at 3 m, suggesting that this location has athick redox transition zone. One sample displayed higher HCl-leachable As concentration than bulk sediment As content (seeFig. 3). The difference likely reflects the focused analysis of theextraction on a 0.5-g sample compared to the concentrationaveraged over a 5-cm depth interval.

Acid Leached Fe and Manganese in Sediment. In contrast to the 5orders of magnitude variations of HCl-extractable As concen-trations (see Fig. 2), HCl-extractable Fe varied little, averaging

2.9 � 1.0% (see Table 1). HCl-extractable manganese concen-trations were 570 � 370 mg/kg (see Table 1).

Mineralogy of As and Fe in Subsurface Sediment. The X-ray absorp-tion near-edge structure (XANES) spectra of 6 subsurfacesediment samples from 5 sites indicated that As is present asarsenate and arsenite. These 2 species accounted for 79 to 98%of As in the sample (Table 2). Only in RS-9, which is located inthe downstream area of the Meghna River where tidal influencesare stronger and which can provide sulfate in seawater (see Fig.1), was where 56% of As found to be associated with sulfides.

That As in the subsurface sediment is of mixed arsenate andarsenite form is consistent with the differential pulse cathodicstripping voltammetry (DPCSV) analysis of phosphate-extract so-lutions (see Table 1). Compared to the total As determined byhigh-resolution inductively coupled plasma (HR ICP)-MS on thephosphate extract, the DPSCV analyses indicate that most sampleshave only a small fraction of phosphate-extractable As as arsenate(see Table 1). Results of the DPSCV and XANES analyses indicatesimilar arsenic speciation. In RS-4, 85% of phosphate-extractableAs is arsenate, whereas XANES identified 79%. In RS-11, 12% ofphosphate-extractable As is arsenate, whereas XANES identified0%. RS-9 displayed much higher phosphate-extractable As(III)than total As quantified by HR ICP-MS beyond the 1 sigma error(�25%) of this measurement, possibly because of a very largedilution factor (250 times) used in the analyses.

The extended X-ray absorption fine structures (EXAFS), nearedge (XANES), and X-ray diffraction spectra of 11 subsurfacesediment samples from 8 sites show several Fe-bearing minerals,including biotite, hornblende, goethite, magnetite, vivianite, butnot siderite (see SI Analytical and Spectroscopic Methods). Fe-bearing clays were present at all sites and represented the sum

Table 1. Composition of the Meghna riverbank sediment

SiteLatitude (N)

Longitude (E)

Surface sed(0–5 cm)Bulk As(mg/kg)

Subsurface sediment (1–3 m)

Positionin core

DPCSV P-extAs(III)

(mg/kg)a

HR ICP-MSP-ext As(mg/kg)

Residual As(mg/kg)b

Sum As(mg/kg)c

HClext. As(mg/kg)

HClext-Fe

(%)

HClext-Mn(mg/kg)

HClext-S

(mg/kg)

25° 00.327� Top 431 457 759 1,217 4,077 2.17 215 267RS-5 92° 15.773� 27

24° 53.105� Top 3,321 3.49 486 233RS-4 92° 12.66� Bottom 134 917 10.1 927 806 3.25 423 291

21 Riverbedd 461 264 825 1,090 6,415 3.75 560 27424° 52.87� Top 582 367 10.7 378 352 2.34 322 223

RS-2 91° 52.81� 24 Bottom 266 241 8.4 249 281 2.12 327 24024° 41.885� Top 96 238 30.3 268 249 3.81 1,338 292

RS-3 91° 55.866� 26 Bottom 5 2.6 3.0 6 3 3.06 1,162 15924° 02.992� Top 0.3 173 1.27 168 31

RS-1 90° 00.832� 7 Bottom 0.2 1 1.59 183 3423° 36.622� Top 1,168 1321 1,963 3,284 5,109 2.75 399 284

RS-11 90° 37.486� 1123° 31.739� Top 996 843 2,632 3,475 23,744 5.04 973 254

RS-10 90° 42.453� 1223° 13.553� Top 2,121 1,293 552 1,845 10,893 2.99 468 310

RS-9 90° 38.338� 1122° 58.779� Top 1.0 3 2.69 550 53

RS-8 90° 41.259� 12Average � SD 16 � 7 649 � 646 457 � 481 679 � 927 3,919 � 6,556 2.88 � 1.00 566 � 365 210 � 100

Abbreviations: As, arsenic; DPCSV, differential pulse cathodic stripping voltammetry; Fe, iron; HCL, hydrochloric acid; HR ICP MS, high-resolution inductivelycoupled plasma mass spectrometry; Mn, manganese; P, phosphate; S, sulfur.aOne sigma error for this measurement is � 25% because of large dilution employed in the analysis.bTotal acid digestion was performed on residual sediment reacting with 1.2 M phosphate solution for � 1 year before HR-ICP MS analysiscThe sum of phosphate-extractable As and residual As should reflect the bulk As concentration of the sediment. In 5 cases (in italics), the sum of As is lower thanHCl-extracted As, most likely because of loss to the phosphate solution in over 1 year of reaction time.dThis sediment sample is collected on a boat in the river at the same location as RS-4

Fig. 2. Comparison of concentrations of As in surface sediment obtained byacid digestion and in subsurface sediment obtained by HCl-leach.

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of chlorite, illite, and hydrated smectite. Goethite was identifiedat sites along the tributaries of the Meghna River but not alongthe main channels. Goethite was used to represent all of the Fe(III) hydroxides, as the spectra are similar for these minerals.Further work is needed to confirm that reduced Fe minerals,such as green rust or vivianite, are important components ofreactive barrier along the main channel of the Meghna River,which is consistent with sediment Fe(II)/Fe data (see Fig. 3).

DiscussionA Natural Reactive Barrier for Groundwater As. The enrichment ofAs, several orders of magnitude higher than the crustal abundanceof As, observed in subsurface sediment along nearly the entirelength of the Meghna River, cannot be explained by processes ofsediment deposition. Suspended particulate matter sampled fromdozens of locations from the Ganges, Brahmaputra, and MeghnaRivers in Bangladesh displayed crustal values of As from 4 mg/kg

to 5.5 mg/kg (10, 11). Surface sediment samples from all 9 sites alsoshow bulk sediment concentrations �20 mg/kg, suggesting thesource of enrichment is in the subsurface.

Therefore, enrichment of As in subsurface sediment impliesthat groundwater is the source of As. The enrichment occurs ina redox transition zone that is saturated during the wet seasonbut unsaturated during the dry season. Hydrological and geo-chemical comparisons between Waquiot Bay, Massachusetts (4)and the Meghna River in this study (Table 3) support thisinterpretation. Sediment samples were obtained at sandy loca-tions along the Meghna River, similar to the setting at WaquiotBay, where the shallow aquifer consists of permeable sand.Because of the high permeability, a short residence time of �10years was observed for the upper 10 m of the aquifer at WaquiotBay and in Bangladesh. Charette and Sholkovitz (3) reportedthat oxidative precipitation of groundwater Fe at a few m depthsbelow the surface at Waquiot Bay (4). This ‘‘iron curtain’’ actedas a barrier, preventing a large plume of groundwater-derivedphosphorus and several other elements from entering the sur-face water of the bay. We postulate that a similar natural reactivebarrier consists of secondary Fe minerals also formed along theMeghna River. The redox transition zone may extend to differ-ent depths, depending on variation of the seasonality, with aparticularly dry year corresponding to a very deep penetratingunsaturated zone, and thus may explain the multiple layers ofenrichment observed in RS-33. Secondary Fe minerals formedin situ, goethite, and vivianite were present in our samples (TableS1). Magnetite and Fe-bearing clays were also present, but theycould be secondary or primary. Green rust may also be part ofa consortium of secondary Fe minerals in this barrier.

Can the natural reactive barrier in the Meghna riverbanksediment accommodate the high As/Fe ratios observed? In theupstream area, where 4 of our sites are located and the ground-water As data are few (see Fig. 1), a survey by UNICEF foundthat 20 to 80% of the wells contain �50 �g/L As (12). An averagesediment HCl-extractable As/Fe ratio of 89 � 114 mmole/molewas determined for Meghna River sediments, compared to 0.7 �1.1 mmole/mole for the Waquiot Bay (see Table 3). In otherwords, �100 times more As was trapped by the reactive barrieralong the Meghna riverbank on a per mole basis of Fe. This highAs/Fe ratio, however, is still within the sorption capacity forarsenate and arsenite on a number of iron oxides, such asferrihydrite and goethite (13, 14). Further study of the Meghnariverbank sediment is needed to determine the sorption capacityfor arsenate and arsenite in these natural sediments.

Recycling of As. Enrichment of As up to 758 mg/kg associated withferric oxides in shallow subsurface sediments (1–2 m) were alsoreported for 8 other sites located within �80 km of each other(see Fig. 1), mostly east of the Meghna River (15). The As-enriched ferric oxides form by oxidation of Fe(II) and As(III)near the top of the capillary fringe, where exposure to oxidantsenables bacterial oxidation and coprecipitation (15). We realizethat further study is needed to determine the extent of As

Fig. 3. Depth profiles of bulk (solid square) and HCl-leachable (open square)sediment As concentrations, and sediment HCl-leachable Fe(II)/Fe for RS-30(0–2 m) and RS-33 (0–6.5 m), located close to RS-11 (see Fig. S1).

Table 2. Arsenic species in subsurface sediment by XANES

Sample Arsenate Arsenite Arsenopyrite As-Sulfides Fita

RS-4 Top 0.90 0.08 0.00 0.00 0.01RS-4 Bottom 0.79 0.00 0.14 0.07 0.026RS-1 Top 0.87 0.09 0.00 0.08 0.034RS-11 Top 0.00 0.87 0.00 0.03 0.024RS-10 Top 0.78 0.09 0.14 0.00 0.043RS-9 Top 0.10 0.37 0.22 0.34 0.017

aThe linear combination fit parameter is the average error of the XANES spectra and the first derivativenormalized by the respective maximum peak heights. The lower the value, the better the fit.

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enrichment in near-surface sediment, but our results and pre-vious studies suggest a systematic feature driven by seasonallyactive hydrological and biogeochemical interactions betweendischarging anoxic groundwater and more oxic river water or air.Recently, a unique yet unproven explanation of As release wasproposed, invoking mobilization during recharge in near-surfacesediment and followed by transport to depth (16). If near-surfacesediment As enrichment is found to be widespread, then itrepresents part of an As cycle that is also responsible for elevatedAs in groundwater. In this scenario, reversal of f low from riverto aquifer at the onset of the wet season between April and June(17), or reversal of f low from riverbank to aquifer because ofirrigation pumping at the peak of the dry season betweenJanuary and March (18), has the potential to remobilize As fromthe riverbank sediment.

Alternatively, sediments enriched with labile As may eventu-ally act as an As source to the groundwater system. In onescenario, in the modern prograding GBMD, the enriched layersare likely to be buried and result in a sedimentary As hot spotin the aquifer. By placing the enriched zone into a stronglyreducing environment deeper in the subsurface, As is suppliedto groundwater. In a slightly different scenario, sediment As maybe resuspended and redeposited, and will no longer be highlyenriched after the homogenization but would remain in anaquifer as dispersed grains of potentially labile As. The timescale for sediment redistribution is likely to be determined by thetectonic and sedimentary processes that result in major riveravulsions (1–3 hundred thousand years) (19) or local rivermigrations (tens to hundreds of years) (20). In these scenarios,arsenic is not discharged directly from the delta following initialrelease to groundwater.

Aquifer flushing over time was thought to have loweredsedimentary As concentrations of the Pleistocene deeper aquiferthat today contains low groundwater As (21, 22). Flushing shouldalso lower the sedimentary As inventory in the shallow Holoceneaquifer. However, our results suggest that not all of the As isleaving the aquifer system with the water. In other words, theflux of As discharge cannot be calculated using discharge fluxesof water multiplied by groundwater As concentrations. A simpleback-of-the-envelope calculation illustrates this point. For thelast 1,000 years, assuming groundwater is discharging at a rate of1 m per year with an average As concentration of 114 �g/L, theriverbank sediment As concentration can increase to 1,800 mg/kgif all of the As is removed in a reactive zone of 0.1-m thicknesswith 25% porosity and a grain density of 2.5 g/cm3. Previously itwas thought that this As left the delta. Our results imply that thisAs remains in the sediment and can be a potential source for Asin groundwater.

Is the amount of As trapped on the riverbank significantcompared to the sediment As inventory in the shallow aquifersediment? The shallow aquifer in Bangladesh is �30-m thick,with an area of �40,000 km2 and a sediment As concentrationof �3 mg/kg (1, 22, 23). Therefore, the sediment As inventory isestimated to be 7 � 109 kg, assuming a 25% porosity and 2.5g/cm3 grain density. The Meghna River discharges 500 m3/s in thedry season (1), most of which probably represents groundwaterdischarge, and therefore an annual discharge rate is estimated tobe 250 m3/s, equivalent to 0.2 m per year. (This is lower than therecharge rate because groundwater also discharges via evapo-transpiration.) Assuming all of the groundwater As (114 �g/L)(see Table 3) is immobilized, then 9 � 105 kg of As per year istrapped in the riverbank sediment. Over 10,000 years, this isequivalent to all of the shallow aquifer sediment As inventory.

Implications for Submarine Groundwater Discharge. Submarinegroundwater discharge has been shown to impact coastal (24) andperhaps global elemental budgets. A large flux of submarinegroundwater discharge, mostly as recirculated saline water, wasimplied based on high fluxes of radium and barium during lowriver-discharge periods in the GBMD (25). Fluxes of strontium bygroundwater discharge was determined to be substantial based onhigh groundwater strontium concentrations and high groundwaterrecharge rates (26), although the recharge rates used may containsubstantial error because of irrigation-induced recharge. Neverthe-less, the behavior of elements across the iron curtain duringgroundwater discharge must be carefully evaluated before theirfluxes via submarine groundwater discharge to the ocean can becalculated. This is because even for relatively conservative elementlike strontium, nonconservative behavior was observed (27).

Materials and MethodsSediment Collection and Preservation. The location of each sediment samplingsite was recorded with a handheld GPS. Surface sediment (0–5 cm) was usuallydry and was collected using a spatula into polyethylene Ziploc bag. An augerwas advanced into the subsurface until water-saturated gray sediment wasintersected, which was usually at �1 m below surface but could be as deep as3 m. One subsurface sediment core was then collected by hammering (AMScompact slide hammer) a soil probe with a seven-eighth inch outside diameterwith an �30 cm long plastic core liner (AMS) into the hole. The exact depth foreach sample varied among sites, but all were within 1 to 3 m. Usually thismethod retrieved a 25 to 30 cm wet core that was placed into a Mylar bag withO2 absorbers. All samples were stored at 4 °C upon returning to the labora-tory. Riverbed sediment samples were collected similarly but from a boat.

On January 25, 2006, a similar coring method was used near RS-11 (see Fig.S1) to recover 4 continuous 30-cm cores from the ground surface to 1.2 m atRS-30, and 6 sections of 30-cm cores with bottom depths of 0.3 m, 0.6 m, 1.5 m,3.0 m, 4.6 m, and 6.1 m at RS-33.

Methods for leaching and acid digestion of sediment samples for Asanalysis are described in the SI Analytical and Spectroscopic Methods, as

Table 3. Comparison between Waquoit Bay and the Meghna River

Waquiot Bay, Massachusetts Meghna River, Bangladesh

Min Max Mean SD Min Max Mean SD

Groundwatera n � 44 n � 743As (�g/L) 0.01 9.4 1.5 � 2.4 �0.5 1,109 114 � 168Fe (mg/L) 0.01 5 0.5 � 0.9 �0.01 22.5 4.1 � 5.5As/Fe (mmole/mole) 0.01 41 7.5 � 11.6 0.02 119 67 � 245

Sedimentb n � 12 n � 14As (mg/kg) 0.36 7.5 2.2 � 2.1 1 23744 5319 � 7,770Fe (%) 0.08 7.6 0.32 � 0.22 1.27 5.04 2.91 � 1.03As/Fe (mmole/mole) 0.1 4.1 0.7 � 1.1 0.04 352 89 � 114

aWaquiot data are from piezometers in Bone et al. (4) and the Meghna River data are from BGS and DPHE (1) for743 groundwater wells (�25 m) located south of 25° N.bBone et al. (4) used 1M hydroxylamine hydrochloride in 25% acetic acid to leach As and Fe. Meghna Riversediments were leached by 1M HCl.

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well as the spectroscopic methods for sediment As speciation analysis andFe mineralogy.

ACKNOWLEDGMENTS. We thank George Breit of the United States Geolog-ical Survey and two anonymous reviewers, and W. Rahman and M. Rahman fortheir assistance with fieldwork. This work was supported by Grant P42ES10349

of the United States National Institute of Environmental Health Sciences/Superfund Basic Research Program (to Y.Z.). S.D. was a Columbia Earth Insti-tute and a Mellon Foundation Post-Doctoral Fellow; B.M. was a ColumbiaEarth Institute Post-Doctoral Fellow; H.-B.J. was a City University of New YorkGraduate Center Science Fellow. This is Lamont-Doherty Earth Observatorycontribution 7298.

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