Redox Trapping of Arsenic During Groundwater Discharge in Sediments from the Meghna Riverbank in Bangladesh

Redox trapping of arsenic during groundwater discharge in sediments from the Meghna riverbank in Bangladesh

S. 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 Environmental Sciences, 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 Holocene Results aquifers of Bangladesh. In the dry season, the shallow groundwa- Aquifers in the GBMD. The Ganges and Brahmaputra rivers coalesce ter discharges to major rivers. This process may influence the northwest of Dhaka and then join the Meghna River south of chemistry of the river and the hyporheic zone sediment. To assess Dhaka before flowing into the Bay of Bengal (see Fig. 1). Bang- the fate of As during discharge, surface (0–5 cm) and subsurface ladesh is less than 10 m above sea level, except for the uplifted (1–3 m) sediment samples were collected at 9 sites from the bank Pleistocene terraces, the Chittagong Hills, and the hilly area in the of the Meghna River along a transect from its northern source northeast. The sandy, unconsolidated Pleistocene to Holocene (25° N) to the Bay of Bengal (22.5° N). Bulk As concentrations of fluvial and deltaic sediments that underline much of Bangladesh -Subsurface form prolific aquifers in this highly energetic depositional environ .(9 ؍ surface sediment averaged 16 ؎ 7 mg/kg (n sediment contained higher mean concentrations of As of 4,000 ment. The shallow Holocene aquifers are typically elevated in As, .ranging from 1 to 23,000 mg/kg As, with >100 whereas the older Pleistocene aquifers appear to contain little As ,(14 ؍ mg/kg (n Ϸ mg/kg As measured at 8 sites. X-ray absorption near-edge struc- The shallow aquifer sediments have been deposited since 10,000 ture spectroscopy indicated that As was mainly arsenate and to 11,000 years B.P. (1). During the early Holocene (11,000–7,000 GEOLOGY arsenite, not As-bearing sulfides. We hypothesize that the elevated years B.P.), the river sediment flux was sufficient to infill 50-m incisions formed during glacial periods of low sea level. The deeper sediment As concentrations form as As-rich groundwater dis- Pleistocene aquifers are typically separated from the shallow Ho- charges to the river, and enters a more oxidizing environment. A locene aquifers by multiple layers of silt and clay. Aquifers are significant portion of dissolved As sorbs to iron-bearing minerals, recharged from precipitation directly or through flooding during which form a natural reactive barrier. Recycling of this sediment- the wet season (1). Shallow aquifers are discharged via flow into the bound As to the Ganges-Brahmaputra-Meghna Delta aquifer pro- rivers and other low-lying surface water bodies, as well as by vides a potential source of As to further contaminate groundwater. evapotranspiration. Furthermore, chemical fluxes from groundwater discharge from Sediment samples were collected on the Meghna riverbank at the Ganges-Brahmaputra-Meghna Delta may be less than previous 9 locations in January 2003 (Table 1) and at 2 locations in estimates because this barrier can immobilize many elements. January 2006 (Fig. S1). In 2006, continuous sediment cores from surface to up to 6-m depth were obtained. In 2003, surface (0–5 Ganges-Brahmaputra ͉ hyporheic zone ͉ natural reactive barrier ͉ cm) and subsurface (variable depth between 1 and 3 m) sediment submarine groundwater discharge ͉ XANES samples were obtained at each site. Four sites are located on the tributaries to the Meghna River at Ϸ25° N at an elevation of 3 n the Ganges-Brahmaputra-Meghna Delta (GBMD) of India to 10 m. Five sites are located on the main channel of the Meghna Iand Bangladesh, more than 30 million people have been River between 22.5° N and 24° N at 0 to 3 m elevation. Eight sites drinking groundwater with elevated levels of arsenic (As) for at are located on sandy deposits along the riverbank. RS-1 is located on a sandbar in the river. At RS-4, an additional least the last decade (1). Many studies have focused on the sediment core was obtained from the riverbed (see Table 1). biogeochemical processes that release As in the GBMD aquifers, Samples from the top 5 cm and bottom 5 cm were subjected to but little work has been done on the fate of As during ground- leaching and acid digestion (see SI Analytical and Spectroscopic water discharge. In Bangladesh, rivers are potential areas of Methods) before determination of As (see Table 1). groundwater discharge (2). Because of the change in redox conditions at the interface, As may precipitate or sorb onto iron As in Surface Sediment. Bulk As concentrations of surface sedi- (Fe)-bearing sediments in the riverbank or riverbed. ment samples subjected to total acid dissolution ranged from 7 It has been recently demonstrated that when anoxic ground- to 27 mg/kg, averaging 16 Ϯ 7 mg/kg (n ϭ 9) (see Table 1). These water discharges to the ocean or to a lake, Fe in groundwater is values were similar to As concentrations in soils from around the oxidatively precipitated onto the sands (3), immobilizing many world (8) and in Bangladesh (9). elements, including As and phosphorus (4). Growing evidence supports the important role that processes in the hyporheic zone As in Subsurface Sediment. High concentrations of As (Ϸ100 to have in regulating the composition of groundwater discharging Ϸ20,000 mg/kg) were extracted from the sediment using 1.2 N to rivers (5–7). In this study, we examine accumulations of As bound to riverbank sediment and implications these enrichments have on As cycling in Bangladesh. We determined the As 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., concentrations of shallow sediments along the entire length of B.M., H.-B.J., and Y.Z. analyzed data; and S.D., B.M., and Y.Z. wrote the paper. the Meghna River. This system was chosen because most of the The authors declare no conflict of interest. shallow groundwater As east of the Meghna River between 22 1To whom correspondence may be addressed. E-mail: [email protected] or and 24° N (Fig. 1) contains Ͼ50 ␮g/L As (1). Our results have [email protected]. important implications for cycling of As in deltaic environments This article contains supporting information online at www.pnas.org/cgi/content/full/ and groundwater discharge as a source of elements to the ocean. 0908168106/DCSupplemental. www.pnas.org͞cgi͞doi͞10.1073͞pnas.0908168106 PNAS Early Edition ͉ 1of6 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 circles represent HCl-leachable As concentrations in subsurface sediments at a depth between 1 and 3 m, with values of Ͻ10 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 previous study (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/kg and by light gray squares for those with values between 100 and 758 mg/kg. Groundwater As concentrations for wells with depth Ͻ25m(n ϭ 743) are plotted as blue, red, and dark red circles to represent Ͻ50, 50 to Ͻ100, and 100 to 1,090 ␮g/L (1). hydrochloric acid (HCl) at 8 of the 9 sample sites (see Table 1 is most likely caused by leaving Ϸ1 ml of phosphate solution to and Fig. 2). The concentrations of As released by 1-M phosphate react with the sediments for about a year before the residual leaches were elevated (Ϸ200 to Ϸ1,000 mg/kg) at 7 sites (see sediment was separated for total acid dissolution. We suspect Table 1). Six samples with 3 to 1,000 mg/kg of phosphate- that As continued to be leached by this discarded aliquot of extractable As displayed comparable HCl-extractable As con- phosphate solution. Nevertheless, the results are consistent with centrations (R2 ϭ 0.99) (see Table 1). Solid residues from the greater concentrations of As bound in the phases that are phosphate extractions of these samples had low concentrations leached either by phosphate or HCl in subsurface sediments than of As ranging from 3 to 30 mg/kg (see Table 1). Collectively, is bound in residual more refractory phases. these results indicate that the majority of the As in these 6 subsurface sediment samples are easily extracted. Depth Profiles of As and Fe(II)/Fe. Enrichment of sediment As was For 5 samples with very high HCl-extractable As (Ϸ4,000 to found at shallow depths, corresponding to a redox transition Ϸ20,000 mg/kg), As concentrations were also elevated in the zone, as indicated by the increasing sediment Fe(II)/Fe ratio with residues of the phosphate extracts, ranging from 500 to Ϸ3,000 depth (Fig. 3). Samples were analyzed for HCl-leachable mg/kg (see Table 1). However, the sum of the phosphate- Fe(II)/Fe within 12 h to characterize sediment redox condition. extractable As and the residual As after phosphate-extraction, At RS-30, the depth interval from 0.75 to 0.90 m displayed should be higher than, or at least equal to the HCl-extractable increasing bulk sediment As concentration from Ϸ30 mg/kg to As. This was not the case for these 5 samples. This discrepancy Ϸ200 mg/kg. The sediment from this depth interval was gray and

2of6 ͉ www.pnas.org͞cgi͞doi͞10.1073͞pnas.0908168106 Datta et al. Table 1. Composition of the Meghna riverbank sediment Subsurface sediment (1–3 m) Surface sed (0–5 cm) DPCSV P-ext HR ICP-MS HCl HCl HCl HCl Latitude (N) Bulk As Position As(III) P-ext As Residual As Sum As ext. As ext-Fe ext-Mn ext-S Site Longitude (E) (mg/kg) in core (mg/kg)a (mg/kg) (mg/kg)b (mg/kg)c (mg/kg) (%) (mg/kg) (mg/kg)

25° 00.327Ј Top 431 457 759 1,217 4,077 2.17 215 267 RS-5 92° 15.773Ј 27 24° 53.105Ј Top 3,321 3.49 486 233 RS-4 92° 12.66Ј Bottom 134 917 10.1 927 806 3.25 423 291 21 Riverbedd 461 264 825 1,090 6,415 3.75 560 274 24° 52.87Ј Top 582 367 10.7 378 352 2.34 322 223 RS-2 91° 52.81Ј 24 Bottom 266 241 8.4 249 281 2.12 327 240 24° 41.885Ј Top 96 238 30.3 268 249 3.81 1,338 292 RS-3 91° 55.866Ј 26 Bottom 5 2.6 3.0 6 3 3.06 1,162 159 24° 02.992Ј Top 0.3 173 1.27 168 31 RS-1 90° 00.832Ј 7 Bottom 0.2 1 1.59 183 34 23° 36.622Ј Top 1,168 1321 1,963 3,284 5,109 2.75 399 284 RS-11 90° 37.486Ј 11 23° 31.739Ј Top 996 843 2,632 3,475 23,744 5.04 973 254 RS-10 90° 42.453Ј 12 23° 13.553Ј Top 2,121 1,293 552 1,845 10,893 2.99 468 310 RS-9 90° 38.338Ј 11 22° 58.779Ј Top 1.0 3 2.69 550 53 RS-8 90° 41.259Ј 12 Average Ϯ SD 16 Ϯ 7 649 Ϯ 646 457 Ϯ 481 679 Ϯ 927 3,919 Ϯ 6,556 2.88 Ϯ 1.00 566 Ϯ 365 210 Ϯ 100 GEOLOGY Abbreviations: As, arsenic; DPCSV, differential pulse cathodic stripping voltammetry; Fe, iron; HCL, hydrochloric acid; HR ICP MS, high-resolution inductively coupled plasma mass spectrometry; Mn, manganese; P, phosphate; S, sulfur. aOne sigma error for this measurement is Ϸ 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 Ϸ 1 year before HR-ICP MS analysis cThe 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 than HCl-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 characterized by a high Fe(II)/Fe ratio of Ϸ0.85, although the 2.9 Ϯ 1.0% (see Table 1). HCl-extractable manganese concen- sediment immediately above at 0.3 m had a low Fe(II)/Fe ratio trations were 570 Ϯ 370 mg/kg (see Table 1). of 0.25. At RS-33, depth intervals of 0.24 m to 0.30 m, and 1.45 to 1.5 m displayed sediment As enrichment up to Ϸ100 mg/kg. Mineralogy of As and Fe in Subsurface Sediment. The X-ray absorp- The shallower interval had sediment Fe(II)/Fe of 0.48, whereas tion near-edge structure (XANES) spectra of 6 subsurface the deeper interval had sediment Fe(II)/Fe of 0.24. Sediment sediment samples from 5 sites indicated that As is present as Fe(II)/Fe reached 0.9 at 3 m, suggesting that this location has a arsenate and arsenite. These 2 species accounted for 79 to 98% thick redox transition zone. One sample displayed higher HCl- of As in the sample (Table 2). Only in RS-9, which is located in leachable As concentration than bulk sediment As content (see the downstream area of the Meghna River where tidal influences Fig. 3). The difference likely reflects the focused analysis of the are stronger and which can provide sulfate in seawater (see Fig. extraction on a 0.5-g sample compared to the concentration 1), was where 56% of As found to be associated with sulfides. averaged over a 5-cm depth interval. That As in the subsurface sediment is of mixed arsenate and arsenite form is consistent with the differential pulse cathodic Acid Leached Fe and Manganese in Sediment. In contrast to the 5 stripping voltammetry (DPCSV) analysis of phosphate-extract so- orders of magnitude variations of HCl-extractable As concen- lutions (see Table 1). Compared to the total As determined by trations (see Fig. 2), HCl-extractable Fe varied little, averaging high-resolution inductively coupled plasma (HR ICP)-MS on the phosphate extract, the DPSCV analyses indicate that most samples have only a small fraction of phosphate-extractable As as arsenate (see Table 1). Results of the DPSCV and XANES analyses indicate similar arsenic speciation. In RS-4, 85% of phosphate-extractable As is arsenate, whereas XANES identified 79%. In RS-11, 12% of phosphate-extractable As is arsenate, whereas XANES identified 0%. RS-9 displayed much higher phosphate-extractable As(III) than total As quantified by HR ICP-MS beyond the 1 sigma error (Ϸ25%) of this measurement, possibly because of a very large dilution factor (250 times) used in the analyses. The extended X-ray absorption fine structures (EXAFS), near edge (XANES), and X-ray diffraction spectra of 11 subsurface sediment samples from 8 sites show several Fe-bearing minerals, including biotite, hornblende, goethite, magnetite, vivianite, but Fig. 2. Comparison of concentrations of As in surface sediment obtained by not siderite (see SI Analytical and Spectroscopic Methods). Fe- acid digestion and in subsurface sediment obtained by HCl-leach. bearing clays were present at all sites and represented the sum

Datta et al. PNAS Early Edition ͉ 3of6 to 5.5 mg/kg (10, 11). Surface sediment samples from all 9 sites also show bulk sediment concentrations Ͻ20 mg/kg, suggesting the source of enrichment is in the subsurface. Therefore, enrichment of As in subsurface sediment implies that groundwater is the source of As. The enrichment occurs in a redox transition zone that is saturated during the wet season but unsaturated during the dry season. Hydrological and geochemical comparisons between Waquiot Bay, Massachusetts (4) and the Meghna River in this study (Table 3) support this interpretation. Sediment samples were obtained at sandy locations along the Meghna River, similar to the setting at Waquiot Bay, where the shallow aquifer consists of permeable sand. Because of the high permeability, a short residence time of Ͻ10 years was observed for the upper 10 m of the aquifer at Waquiot Bay and in Bangladesh. Charette and Sholkovitz (3) reported that oxidative precipitation of groundwater Fe at a few m depths below the surface at Waquiot Bay (4). This ‘‘iron curtain’’ acted as a barrier, preventing a large plume of groundwater-derived phosphorus and several other elements from entering the surface water of the bay. We postulate that a similar natural reactive barrier consists of secondary Fe minerals also formed along the Meghna River. The redox transition zone may extend to different depths, depending on variation of the seasonality, with a particularly dry year corresponding to a very deep penetrating unsaturated zone, and thus may explain the multiple layers of enrichment observed in RS-33. Secondary Fe minerals formed in situ, goethite, and vivianite were present in our samples (Table S1). Magnetite and Fe-bearing clays were also present, but they could be secondary or primary. Green rust may also be part of a consortium of secondary Fe minerals in this barrier. Can the natural reactive barrier in the Meghna riverbank Fig. 3. Depth proﬁles of bulk (solid square) and HCl-leachable (open square) sediment accommodate the high As/Fe ratios observed? In the sediment As concentrations, and sediment HCl-leachable Fe(II)/Fe for RS-30 upstream area, where 4 of our sites are located and the ground- (0–2 m) and RS-33 (0–6.5 m), located close to RS-11 (see Fig. S1). water As data are few (see Fig. 1), a survey by UNICEF found that 20 to 80% of the wells contain Ͼ50 ␮g/L As (12). An average sediment HCl-extractable As/Fe ratio of 89 Ϯ 114 mmole/mole Ϯ of chlorite, illite, and hydrated smectite. Goethite was identified was determined for Meghna River sediments, compared to 0.7 1.1 mmole/mole for the Waquiot Bay (see Table 3). In other at sites along the tributaries of the Meghna River but not along Ϸ the main channels. Goethite was used to represent all of the Fe words, 100 times more As was trapped by the reactive barrier along the Meghna riverbank on a per mole basis of Fe. This high (III) hydroxides, as the spectra are similar for these minerals. As/Fe ratio, however, is still within the sorption capacity for Further work is needed to confirm that reduced Fe minerals, arsenate and arsenite on a number of iron oxides, such as such as green rust or vivianite, are important components of ferrihydrite and goethite (13, 14). Further study of the Meghna reactive barrier along the main channel of the Meghna River, riverbank sediment is needed to determine the sorption capacity which is consistent with sediment Fe(II)/Fe data (see Fig. 3). for arsenate and arsenite in these natural sediments. Discussion Recycling of As. Enrichment of As up to 758 mg/kg associated with A Natural Reactive Barrier for Groundwater As. The enrichment of ferric oxides in shallow subsurface sediments (1–2 m) were also As, several orders of magnitude higher than the crustal abundance reported for 8 other sites located within Ϸ80 km of each other of As, observed in subsurface sediment along nearly the entire (see Fig. 1), mostly east of the Meghna River (15). The As- length of the Meghna River, cannot be explained by processes of enriched ferric oxides form by oxidation of Fe(II) and As(III) sediment deposition. Suspended particulate matter sampled from near the top of the capillary fringe, where exposure to oxidants dozens of locations from the Ganges, Brahmaputra, and Meghna enables bacterial oxidation and coprecipitation (15). We realize Rivers in Bangladesh displayed crustal values of As from 4 mg/kg that further study is needed to determine the extent of As

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 0.01 RS-4 Bottom 0.79 0.00 0.14 0.07 0.026 RS-1 Top 0.87 0.09 0.00 0.08 0.034 RS-11 Top 0.00 0.87 0.00 0.03 0.024 RS-10 Top 0.78 0.09 0.14 0.00 0.043 RS-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 derivative normalized by the respective maximum peak heights. The lower the value, the better the fit.

4of6 ͉ www.pnas.org͞cgi͞doi͞10.1073͞pnas.0908168106 Datta et al. 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 ϭ 44 n ϭ 743 As (␮g/L) 0.01 9.4 1.5 Ϯ 2.4 Ͻ0.5 1,109 114 Ϯ 168 Fe (mg/L) 0.01 5 0.5 Ϯ 0.9 Ͻ0.01 22.5 4.1 Ϯ 5.5 As/Fe (mmole/mole) 0.01 41 7.5 Ϯ 11.6 0.02 119 67 Ϯ 245 Sedimentb n ϭ 12 n ϭ 14 As (mg/kg) 0.36 7.5 2.2 Ϯ 2.1 1 23744 5319 Ϯ 7,770 Fe (%) 0.08 7.6 0.32 Ϯ 0.22 1.27 5.04 2.91 Ϯ 1.03 As/Fe (mmole/mole) 0.1 4.1 0.7 Ϯ 1.1 0.04 352 89 Ϯ 114

aWaquiot data are from piezometers in Bone et al. (4) and the Meghna River data are from BGS and DPHE (1) for 743 groundwater wells (Ͻ25 m) located south of 25° N. bBone et al. (4) used 1M hydroxylamine hydrochloride in 25% acetic acid to leach As and Fe. Meghna River sediments were leached by 1M HCl.

enrichment in near-surface sediment, but our results and pre- Is the amount of As trapped on the riverbank significant vious studies suggest a systematic feature driven by seasonally compared to the sediment As inventory in the shallow aquifer active hydrological and biogeochemical interactions between sediment? The shallow aquifer in Bangladesh is Ϸ30-m thick, discharging anoxic groundwater and more oxic river water or air. with an area of Ϸ40,000 km2 and a sediment As concentration Recently, a unique yet unproven explanation of As release was of Ϸ3 mg/kg (1, 22, 23). Therefore, the sediment As inventory is proposed, invoking mobilization during recharge in near-surface estimated to be 7 ϫ 109 kg, assuming a 25% porosity and 2.5 sediment and followed by transport to depth (16). If near-surface g/cm3 grain density. The Meghna River discharges 500 m3/s in the sediment As enrichment is found to be widespread, then it dry season (1), most of which probably represents groundwater GEOLOGY represents part of an As cycle that is also responsible for elevated discharge, and therefore an annual discharge rate is estimated to As in groundwater. In this scenario, reversal of flow from river be 250 m3/s, equivalent to 0.2 m per year. (This is lower than the to aquifer at the onset of the wet season between April and June recharge rate because groundwater also discharges via evapo- ␮ (17), or reversal of flow from riverbank to aquifer because of transpiration.) Assuming all of the groundwater As (114 g/L) ϫ 5 irrigation pumping at the peak of the dry season between (see Table 3) is immobilized, then 9 10 kg of As per year is January and March (18), has the potential to remobilize As from trapped in the riverbank sediment. Over 10,000 years, this is the riverbank sediment. equivalent to all of the shallow aquifer sediment As inventory. Alternatively, sediments enriched with labile As may eventu- ally act as an As source to the groundwater system. In one Implications for Submarine Groundwater Discharge. Submarine scenario, in the modern prograding GBMD, the enriched layers groundwater discharge has been shown to impact coastal (24) and perhaps global elemental budgets. A large flux of submarine are likely to be buried and result in a sedimentary As hot spot groundwater discharge, mostly as recirculated saline water, was in the aquifer. By placing the enriched zone into a strongly implied based on high fluxes of radium and barium during low reducing environment deeper in the subsurface, As is supplied river-discharge periods in the GBMD (25). Fluxes of strontium by to groundwater. In a slightly different scenario, sediment As may groundwater discharge was determined to be substantial based on be resuspended and redeposited, and will no longer be highly high groundwater strontium concentrations and high groundwater enriched after the homogenization but would remain in an recharge rates (26), although the recharge rates used may contain aquifer as dispersed grains of potentially labile As. The time substantial error because of irrigation-induced recharge. Neverthe- scale for sediment redistribution is likely to be determined by the less, the behavior of elements across the iron curtain during tectonic and sedimentary processes that result in major river groundwater discharge must be carefully evaluated before their avulsions (1–3 hundred thousand years) (19) or local river fluxes via submarine groundwater discharge to the ocean can be migrations (tens to hundreds of years) (20). In these scenarios, calculated. This is because even for relatively conservative element arsenic is not discharged directly from the delta following initial like strontium, nonconservative behavior was observed (27). release to groundwater. Aquifer flushing over time was thought to have lowered Materials and Methods sedimentary As concentrations of the Pleistocene deeper aquifer Sediment Collection and Preservation. The location of each sediment sampling that today contains low groundwater As (21, 22). Flushing should site was recorded with a handheld GPS. Surface sediment (0–5 cm) was usually also lower the sedimentary As inventory in the shallow Holocene dry and was collected using a spatula into polyethylene Ziploc bag. An auger aquifer. However, our results suggest that not all of the As is was advanced into the subsurface until water-saturated gray sediment was Ϸ leaving the aquifer system with the water. In other words, the intersected, which was usually at 1 m below surface but could be as deep as 3 m. One subsurface sediment core was then collected by hammering (AMS flux of As discharge cannot be calculated using discharge fluxes compact slide hammer) a soil probe with a seven-eighth inch outside diameter of water multiplied by groundwater As concentrations. A simple with an Ϸ30 cm long plastic core liner (AMS) into the hole. The exact depth for back-of-the-envelope calculation illustrates this point. For the each sample varied among sites, but all were within 1 to 3 m. Usually this last 1,000 years, assuming groundwater is discharging at a rate of method retrieved a 25 to 30 cm wet core that was placed into a Mylar bag with 1 m per year with an average As concentration of 114 ␮g/L, the O2 absorbers. All samples were stored at 4 °C upon returning to the labora- riverbank sediment As concentration can increase to 1,800 mg/kg tory. Riverbed sediment samples were collected similarly but from a boat. if all of the As is removed in a reactive zone of 0.1-m thickness On January 25, 2006, a similar coring method was used near RS-11 (see Fig. 3 S1) to recover 4 continuous 30-cm cores from the ground surface to 1.2 m at with 25% porosity and a grain density of 2.5 g/cm . Previously it RS-30, and 6 sections of 30-cm cores with bottom depths of 0.3 m, 0.6 m, 1.5 m, was thought that this As left the delta. Our results imply that this 3.0 m, 4.6 m, and 6.1 m at RS-33. As remains in the sediment and can be a potential source for As Methods for leaching and acid digestion of sediment samples for As in groundwater. analysis are described in the SI Analytical and Spectroscopic Methods,as

Datta et al. PNAS Early Edition ͉ 5of6 well as the spectroscopic methods for sediment As speciation analysis and of the United States National Institute of Environmental Health Sciences/ Fe mineralogy. 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 Columbia ACKNOWLEDGMENTS. We thank George Breit of the United States Geolog- Earth Institute Post-Doctoral Fellow; H.-B.J. was a City University of New York ical Survey and two anonymous reviewers, and W. Rahman and M. Rahman for Graduate Center Science Fellow. This is Lamont-Doherty Earth Observatory their assistance with ﬁeldwork. This work was supported by Grant P42ES10349 contribution 7298.

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