ARTICLESPUBLISHED ONLINE: 13 DECEMBER 2009 | DOI: 10.1038/NGEO723

Arsenic release from paddy soils duringmonsoon floodingLinda C. Roberts1,2*, Stephan J. Hug1*, Jessica Dittmar2, Andreas Voegelin1,2, Ruben Kretzschmar2,Bernhard Wehrli1,2, Olaf A. Cirpka3, Ganesh C. Saha4, M. Ashraf Ali5 and A. Borhan M. Badruzzaman5

Bangladesh relies heavily on groundwater for the irrigation of dry-season rice. However, the groundwater used for irrigationoften contains high concentrations of arsenic, potentially jeopardizing the future of rice production in the country. In seasonallyflooded fields, topsoil arsenic concentrations decrease during the monsoon season, suggesting that flooding attenuates arsenicaccumulation in the soils. Here we examine the chemistry of soil porewater and floodwater during the monsoon season inrice paddies in Munshiganj, Bangladesh, to assess whether flooding releases significant quantities of arsenic from the soils.We estimate that between 51 and 250 mg m−2 of soil arsenic is released into floodwater during the monsoon season. Thiscorresponds to a loss of 13–62% of the arsenic added to soils through irrigation each year. The arsenic was distributedthroughout the entire floodwater column by vertical mixing and was laterally removed when the floodwater receded. Weconclude that monsoon floodwater removes a large amount of the arsenic added to paddy soils through irrigation, and suggestthat non-flooded soils are particularly at risk of arsenic accumulation.

Rice production in Bangladesh has more than doubled overthe past three decades1, mainly owing to the increasingcultivation of dry-season rice (boro), which requires intense

irrigation2. As groundwater from shallow tubewells, often high inAs, is predominantly used for irrigation1, an estimated 1,360 tonsof As are yearly added to arable soils in Bangladesh3. Accumulationof As in paddy soils may adversely affect rice yields, owingto As phytotoxicity, and rice quality, owing to increasing Aslevels in grains and straw. Both effects have been observed ingreenhouse studies with As-spiked soils4–6. Increased As levels insoils7–10 and rice grains8,11,12 have also been demonstrated in fieldstudies conducted in As-affected areas, and a recent study hasshown alarming evidence of As-related yield reduction under fieldconditions in Bangladesh13.

Long-term trends of As concentrations in irrigated paddysoils are difficult to assess owing to spatial heterogeneity andtemporal variability14,15. Irrigation with As-rich groundwater leadsto laterally heterogeneous As input to soil12–15, affecting the topfew centimetres most strongly14. In areas with monsoon flooding,topsoil As concentrations seasonally increase during irrigation anddecrease over the monsoon season14,16,17, suggesting attenuationof soil arsenic accumulation by flooding. As yet, direct evidenceof As release during monsoon flooding, and more importantly,of the pathways of As removal from paddy soil, is howeverlacking. Several pathways have been suggested, including Asdiffusion into the overlying floodwater and lateral removal withreceding floodwater13,14,18,19, As transport to deeper soil layers orthe underlying aquifer by floodwater infiltration18 and microbialmethylation of As to volatile methylarsines causing As loss tothe atmosphere16,18,19.

The objectives of this study were to obtain a first quantitativeestimate of As release from paddy soil into overlying floodwater

1Eawag, Swiss Federal Institute of Aquatic Science and Technology, Ueberlandstrasse 133, CH-8600 Dübendorf, Switzerland, 2Institute of Biogeochemistryand Pollutant Dynamics, Department of Environmental Sciences, ETH Zurich, CH-8092 Zurich, Switzerland, 3Center for Applied Geoscience, University ofTübingen, D-72076 Tübingen, Germany, 4Department of Civil Engineering, Dhaka University of Engineering and Technology, Gazipur-1700, Bangladesh,5Department of Civil Engineering, Bangladesh University of Engineering and Technology, Dhaka-1000, Bangladesh. *e-mail: linda.roberts@eawag.ch;hug@eawag.ch.

and to assess lateral removal of As with receding floodwater. Wetherefore measured As and other major elements and chemicalparameters in floodwater and soil porewater in paddy fieldsnear Sreenagar (Munshiganj) and in a channel connecting thefields to the local river. We investigated soil processes relevantfor As mobilization under flooded conditions and applied threeindependent approaches to estimate As flux from soil intofloodwater. The field site, exclusively used for boro rice productionin the past ∼18 years, is located 5 km north of the Ganges and1 km east of the Ichamati River, which discharges into the Ganges7 km south of the field site (Fig. 1). The area is subject tointense monsoon flooding between mid-June and late October.Floodwater levels are determined predominantly by Ganges waterlevels (Supplementary Fig. S1) and vary strongly over the courseof the season, reaching up to 4.5m above rice field level intypical years (Fig. 2)20. The hydrology of the area, the annual Asinput through irrigation with groundwater containing 400 µg l−1As and the spatial As distribution in the studied fields have beencharacterized previously14,15,20.

Arsenic release into floodwaterFloodwater was sampled at four locations in rice fields duringthe 2006 and 2007 monsoon seasons (Figs 1 and 2). Distinctconcentration gradients indicating limited vertical mixing weremeasured on various days in both years (Figs 3a, 4a–e). Wheneversuch gradients were present, floodwater As concentrations werehighest close to the soil surface (40–120 µg l−1) and decreasedsharply towards shallower water depths, indicating As releasefrom soil. Arsenic gradients were observed both in the initial andfinal stages of flooding, implying that As mobilization occurredthroughout the entire flooded period (Fig. 3a). Days of limitedvertical mixing alternated with periods of vertically well-mixed

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Eawag_06327

ARTICLES NATURE GEOSCIENCE DOI: 10.1038/NGEO723

Channel

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Figure 1 |Map and schematic vertical cross-section of the field area. a, IKONOS satellite image (2001; ref. 45) showing the sampling locations on tworice fields and at the mouth of a channel connecting the field area to the Ichamati River. Arrows (year-tagged for field I) point to the positions of theirrigation water inlets. Insets show the location of the field site within Bangladesh. b, Schematic representation of the field area topography. The roads, builton banked-up terrain, constitute a network of dams. Channel samples were taken at a road bridge, that is, a location where floodwater could enter and exitthe field area. The curved blue line with arrows in a shows this water exchange.

floodwater (Fig. 3b,c). The upper part of the water column (>80 cmabove the soil surface), termed bulk floodwater in the following,generally showed vertically uniform As concentrations that rangedbetween 5 and 20 µg l−1 (Figs 2 and 3). On a scale of hundredsof metres, bulk floodwater was also well mixed horizontally, asapparent from data obtained at different field locations 70–190mdistant from each other (Supplementary Fig. S2). Figure 2 shows Asconcentrations in bulk floodwater along with the floodwater level in2007. While floodwater receded in July 2007, As concentrations inbulk floodwater increased, reflecting the ongoing release of As intodecreasing water volumes. Mobilization of As into floodwater wasparalleled by the release of Fe,Mn and P (Fig. 4a–c).

Geochemistry controlling arsenic releaseSoil porewater profiles, obtained at 3m distance from the irrigationinlet of field I after 2.5 and 4.5 weeks of flooding in 2006 and induplicate after 4, 12 and 16 weeks of flooding in 2007 (Figs 1, 2and 4f–i), were compared with solid-phase profiles (total andoxalate-extractable As, P and organic carbon) obtained 1m fromthe inlet before flooding in May 2005 (ref. 14) and 2007 (Fig. 4j,Supplementary Fig. S3).

Dissolved Fe and Mn in porewater indicated Mn- and Fe-reducing conditions throughout the 50 cm depth range and thetime period sampled (Fig. 4g,h, Supplementary Fig. S4). Broadpeaks of Fe and Mn concentrations in the top 20 cm of soil,that is, the zone above the plough pan14, were probably due to

greater availability of reducible Fe- andMn-phases14 anddegradableorganic carbon within this depth range (Fig. 4j). Concentrations ofdissolved organic carbon likewise peaked in the top 20 cm (Fig. 4h).Porewater As and P (predominantly phosphate), developed profilessimilar to Fe (Fig. 4f–h), suggesting mobilization of As andphosphate by reductive dissolution of Fe(hydr)oxides. Solid-phasecontents of As and P above the plough pan were higher than below(Fig. 4j) and largely oxalate-extractable (Supplementary Fig. S3)14,supporting the suggested release from reactive Fe(hydr)oxides21.

With the exception of one profile obtained after 4 weeks offlooding, porewater profiles were depleted in S in the top 15 cmat all sampling times (Fig. 4g, Supplementary Fig. S4), suggestingthat sulphate reduction proceeded rapidly within this depth range.Below 15 cm, porewater S concentrations, dominated by sulphate(see Supplementary Information), were considerably higher anddid not show a temporal trend (Supplementary Fig. S4), whereassulphide was measurable at low concentrations after 16 weeks offlooding (Fig. 4g). Sulphate reduction in the subsoil thus seemed tooccur at slow rates and after prolonged flooding. In floodwater, Sconcentrations decreased towards the soil–water interface on daysof limited vertical mixing, whereas detectable sulphide showed theopposite trend (Fig. 4b). Sulphate reduction near the soil surfacewas therefore maintained by sulphate resupply from the floodwaterthroughout the flooded period.

Arsenic in soil porewater was mainly arsenite (AsIII) (Fig. 4f),indicating that As mobilization was also promoted by reduction

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NATURE GEOSCIENCE DOI: 10.1038/NGEO723 ARTICLES

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Figure 2 | Floodwater level, sampling times and As concentrations in bulk floodwater and channel water. a, Floodwater level, expressed as height abovethe paddy field surface, is indicated by filled symbols on days when floodwater was sampled. Dated arrows and ellipses identify the days and periods forwhich floodwater As profiles are shown in Fig. 3. Arsenic concentrations in bulk floodwater on paddy fields (>80 cm above the soil) are compared with Asconcentrations in channel water (data from 2007). b, Coloured bars show the periods of porewater sampler deployment. The approximate beginning offlooding is indicated by vertical black lines for 2006 and 2007.

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Figure 3 |Arsenic profiles in floodwater on field I. All data were obtained at 3 m distance from the irrigation inlet. a, Distinct concentration gradientsmeasured at different stages of flooding in 2006 and 2007. b,c, Profiles measured while floodwater underwent vertical mixing during decreasing (b) orincreasing (c) floodwater levels. The dotted lines and open symbols show initial gradients before mixing. The black lines indicate initial and final floodwaterlevels; the arrows show the direction of floodwater level change.

of arsenate (AsV) to the less competitively sorbing AsIII (ref. 22).Porewater As concentrations and As/Fe ratios increased over timein the top 15 cm,most strongly betweenweeks 12 and 16 of flooding.Porewater P concentrations (but not P/Fe ratios) showed only aslight increase in this depth range (Supplementary Figs S4 andS5). Whereas P/Fe ratios in porewater closely followed oxalate-extractable solid-phase P/Fe ratios14, As/Fe ratios in porewater

exceeded those in the oxalate-extractable solid phase at all samplingtimes and depths (Supplementary Fig. S5), supporting the suggestedcontribution of both reductive Fe dissolution and AsV reductionto As release. The prominent increase in topsoil porewater Asand As/Fe ratios after prolonged flooding could be attributable toincreasing As reduction23,24 and/or to increasingly limited surfacesites for As re-adsorption.

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ARTICLES NATURE GEOSCIENCE DOI: 10.1038/NGEO723

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Figure 4 |Geochemistry of arsenic release. a–e, Floodwater profiles obtained on 2 July 2007 showing distinct concentration gradients in the watercolumn. TOC: total organic carbon. f–h, Porewater profiles after 16 weeks of flooding (sampler PS-13, extracted on 8 October 2007). DOC: dissolvedorganic carbon. i, Changes in porewater As profiles over time of flooding in 2007. j, Soil profiles of total As, P (note the axis breaks) and organic carbon(May 2007). The plough pan is indicated by a grey bar. Sampling location: Field I, distance from irrigation inlet 3 m (floodwater and porewater) and 1 m(solid phase), respectively.

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Figure 5 |Arsenic flux estimates. Flux estimates obtained from porewatergradients and changes in floodwater As concentrations expressed as dailyfluxes scaled to average topsoil As concentrations in field I. Estimates ofthe total amount of As seasonally released were derived by integrating anexponential fit of the porewater-based fluxes obtained in 2007 (solid line)over time of flooding (140 days) and by multiplying the daily flux valuesobtained from floodwater data with the total duration of flooding (dashedlines). The brown dotted lines show the minimum and maximum As fluxesderived from soil data from 2005 (ref. 14).

Sorption of As to potentially forming mixed FeII/FeIII phasessuch as magnetite25–27 or green rust28,29, co-precipitation with FeIIand sulphide and/or sorption to Fe sulphides30–33 may counteractreductive As mobilization to some extent. Sulphide-related Assequestration may have initially attenuated As release in the top15 cm while providing a likely sink for AsIII at greater depthsafter prolonged flooding.

Estimation of arsenic fluxes from porewater gradientsWe estimated diffusive As flux across the soil–floodwater interfaceon the basis of Fick’s first law, that is, assuming steady-statediffusion of As through the topsoil of porosityφ and tortuosity θ :

J =−φ

θ 2D0

dCdz

(1)

where J is diffusive flux, D0 denotes the molecular diffusioncoefficient and dC/dz represents the vertical As gradientat the soil–water interface, determined in topsoil porewater(Supplementary Fig. S6). Soil porosity was calculated fromcores collected under non-flooded conditions, tortuosity wasestimated with a semi-empirical relationship (θ 2 = 1 − ln(φ2);ref. 34) and values for D0As(III) (ref. 35) and D0As(V) (ref. 36)were temperature-adjusted and combined to a weighted diffusioncoefficient reflecting the mean AsIII/AsIII+V-ratio at the soil–water interface (see details in the Supplementary Information).Arsenic fluxes calculated with equation (1) ranged from 0.18to 1.3mgm−2 d−1 and agreed to within 3–22% for porewatersamplers deployed in duplicate (Supplementary Table S1).To capture porewater As dynamics best, samplers had beendeployed near the irrigation inlet where soil As concentrationswere above average. On the basis of soil As contents14 andthe assumption that As release was proportional to topsoil Asconcentrations before monsoon flooding, we estimated average,that is, spatially-scaled As fluxes from field I into floodwater torange between 0.12 and 0.83mgm−2 d−1 (details see SupplementaryInformation). Fitting the spatially scaled flux data from 2007with an exponential function and integrating over total time offlooding (Fig. 5), yielded a seasonal As release of 51mgm−2 As,corresponding to an average diffusive flux of 0.37mgm−2 d−1.

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NATURE GEOSCIENCE DOI: 10.1038/NGEO723 ARTICLESAlternative fitting approaches yielded similar results (±4%,Supplementary Table S2).

Arsenic fluxes derived from porewater data increased overtime of flooding (Fig. 5), reflecting the build-up of porewater Asconcentrations to a prominent peak in the topsoil after prolongedflooding (Fig. 4i). The absence of As gradients in porewaterbelow 20 cm implies that downward diffusion from this peak wasrestricted to the soil above the plough pan after 12 and 16 weeksof flooding. Vertical As translocation within this soil volume,annually mixed by puddling, is likely to have a minor impact onthe long-term fate of As in paddy soil.

Estimation of arsenic fluxes from floodwater dataWe derived As flux estimates from concentration changes infloodwater in situations of negligible and complete vertical mixing,respectively. First, we used two two-day periods during whichvertical As gradients built up or became more pronounced in thebottom 80 cm of the floodwater column (Supplementary Fig. S7).We fitted and integrated the floodwater As profiles over this depthand from the difference between the two time points, obtainedan estimate of As release from the soil, assuming negligible lateralflow in the bottom floodwater and negligible mixing with the bulkfloodwater (details as Supplementary Information). For the twoperiods 30 June–2 July and 5–7 October 2007, we obtained Asreleases of 5.68 and 5.71mgm−2, respectively. Spatial scaling of theresulting fluxes (obtained at 3m distance from the inlet) yieldedaverage fluxes of 1.80 and 1.76mgm−2 As d−1 for both periods.By excluding vertical mixing with the bulk floodwater during thetwo-day periods, we potentially overestimated the time of build-upof the final As profile measured. As the soil As concentrationswere above average at the inlet-near location sampled14, lateralflow would probably have had a diluting effect, partially maskingAs release. Both assumptions underlying this estimate thereforecontributed to a potential underestimation of As flux.

Second, we derived a flux estimate from the increase in bulkfloodwater As concentrations characterizing the period of recedingfloodwater level in July 2007 (Fig. 2). Between 4 and 15 July2007, the floodwater column was in good approximation verticallyand horizontally mixed, as evidenced from floodwater profiles(Fig. 3b) and the good agreement between As concentrations inbulk floodwater at different field locations and in channel water(Fig. 2, Supplementary Fig. S2). Rainfall and evaporation werenegligible during this period, water level at the field site and theGanges receded (Supplementary Fig. S1) and flow direction inthe channel (Fig. 1) was towards the river. The floodwater leveldecrease at the field site was therefore exclusively attributable todischarge into the river system. Arsenic release from soil intofloodwater could thus be determined by adding the As masslaterally removed with receding floodwater to the increase in Asmass in the floodwater column per surface area over the 11-dayperiod. On the basis of linear regressions of floodwater level andbulk floodwater As concentrations (Supplementary Fig. S8), wecalculated As release to amount to 9.3mgm−2 As over the 11-dayperiod, corresponding to an average flux of 0.84mgm−2 d−1 As(details as Supplementary Information).

Magnitude of arsenic release into floodwaterOur estimates from floodwater data exceed the average As fluxderived from porewater data by factors of 2.3–4.8 (Fig. 5). Theporewater-based flux values can be consideredminimum estimates,because they reflect diffusive flux calculations with conservativeporosity and tortuosity values (Supplementary Information). Re-suspension of soil owing to floodwater currents and bioturbation bybenthic macro-invertebrates such as oligochaetes and chironomidscommonly found in flooded paddy fields37,38 may increase Asfluxes significantly. Flux estimates derived from benthic chambers

capturing both diffusive and advective solute transfer typicallyexceed estimates from porewater samplers by factors of 2–14(refs 39, 40). Our estimates from floodwater data include bothdiffusive and advective As release. The highest values were derivedfrom bottom floodwater data obtained over two-day periods usingassumptions more likely to under- than overestimate As fluxesfor the periods in question. These periods, during which verticalmixing was limited, may however not be representative of the entireduration of flooding (Supplementary Table S3).

On the basis of soil data from May and December 2005(ref. 14), we estimated that As content in the top 10 cm of ourstudy field decreased by 2.2mg kg−1 over the 2005 monsoonseason (see Supplementary Information). Neglecting potential Asvolatilization, the resulting As loss of 185mgm−2 can be attributedto As flux from the zone of peak Asmobilization within the 0–10 cmdepth segment (Fig. 4i) across the upper and lower segmentboundary. Soil As content and porosity increase towards the surface(Fig. 4j, Supplementary Table S1), so release into the floodwaterprobably exceeded flux to depth. The soil data therefore suggestseasonal As release into floodwater to have ranged between 93and 185mgm−2 As, which lies between the minimum estimatederived from porewater data (51mgm−2 As) and the highestvalues derived from bottom floodwater data (250mgm−2 As) in2007 (Fig. 5).

Compared with the annual As input through irrigation of400 ± 50mgm−2 a−1 (1 ± 0.1m of groundwater17,20 containing400±10 µg l−1 As; ref. 15), our floodwater and porewater estimatesthus translate into a release range of between 13±2 and 62±8%,with soil data suggesting an upper limit of 46±6%.

Fate of floodwater arsenicAlthough As was predominantly released as AsIII at the soil surface,bulk floodwater was dominated by AsV (Fig. 4a), indicating Asoxidation within the water column, possibly by co-oxidation withFeII (ref. 41) or photo-oxidation42. Whereas the fraction of colloidalFe and P increased towards shallower depths, As remained dissolvedthroughout the water column (Fig. 4a–c), suggesting predominantphosphate rather than As sorption to Fe(hydr)oxide colloidsforming by FeII oxidation. Phosphate probably outcompeted Asfor Fe(hydr)oxide sorption sites owing to the high P/As ratio(∼15) and the prevalence of AsIII over AsV in the bottom-nearfloodwater (Fig. 4a)22. Arsenic was therefore largely unaffected bycolloid aggregation and settling, suggesting conservative behaviourin the floodwater. Owing to frequent vertical mixing (Fig. 3b,c),releasedAswas distributed into the entire floodwater column.

To assess horizontal As transport, we compared bulk floodwaterAs concentrations above paddy fields with As profiles at a bridgemarking the start of a channel connecting the village ponds tothe Ichamati River (Fig. 1). The 750-m-long channel constitutedthe shortest link between the river and the field area. Channelwater showed vertically uniformAs concentrations (SupplementaryFig. S9), which agreed to within ±8% with bulk floodwater Asconcentrations on 9 out of 11 days of measurement (Fig. 2). Whenfloodwater receded in July 2007, channel flow was towards theIchamati River. The parallel increase of As concentrations in bulkfloodwater and channel water therefore exemplarily showed Asexport to the river (Fig. 2). The absence of significant changes inAs concentrations over the 350–520m distance between field pointsand channel location (Fig. 2, Supplementary Fig. S9) supports thesuggested conservative behaviour of As in the floodwater column.

As floodwater receded, the Ganges and Ichamati River levelsdropped more quickly than groundwater level at the field site20.Floodwater therefore discharged laterally into the river system untilponded by the field boundaries, raised ∼40 cm above field level.Most As-containing floodwater was thus exported to the riversand only residual amounts remained on the paddy fields. Arsenic

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ARTICLES NATURE GEOSCIENCE DOI: 10.1038/NGEO723

sorption to suspended river sediments and sediment depositionfurther downstream may lead to some As redistribution withinflooded areas of Bangladesh. However, considering the high flowvelocities and discharge rates characterizing the monsoon regimeof the main rivers draining Bangladesh43, a large portion of theAs transported by river water is likely to be exported to theBay of Bengal.

Implications for other areasThis study identifies As release into floodwater, followed by lateralremoval to rivers, as a major pathway attenuating As accumulationin paddy soils. Other pathways, such as volatilization and transportto deeper layers may also have a role, but remain to be identifiedand quantified. Our field site is likely to be representative ofmost areas in Munshiganj district, 57% of which is flooded moredeeply than 0.9m during typical monsoon seasons44, and of otherdistricts with similar conditions. About 21% of the total area ofBangladesh is annually affected by monsoon floodwaters exceeding0.9m depth44.With As pools increasing in paddy soils, annual inputand release in seasonally flooded fields may approach a steady stateover the next decades, but trends are difficult to predict and dependon local conditions. Although As volatilization or downward Asleachingmay lead to someAs release fromAs-affected areas withoutpronounced monsoon flooding, these areas seem at greater risk ofsoil As accumulation13 than seasonally flooded fields.

MethodsField work during monsoon flooding was conducted by boat between mid-Juneand mid-July in 2006 and from July to mid-October in 2007 (Fig. 2). Floodwaterprofiles were taken in two locations on each of two rice fields, at 3m distance fromthe irrigation inlet and from the opposite corner, respectively, and at the startingpoint of a channel connecting the field site to the Ichamati River (Fig. 1). As theirrigation inlet of field I was shifted at the beginning of 2007, the inlet-near samplinglocation was moved accordingly (Fig. 1). Floodwater was sampled on 26 days at theinlet location of field I (n= 6 (2006); n= 20 (2007)) and compared with profilesfrom other locations on selected days (field I far corner: n= 4 (2006); n= 3 (2007);field A inlet: n= 1 (2007); field A far corner: n= 2 (2007)). When floodwater levelwas 61.6m, samples were taken 5 cm below the water surface and 2.5, 5, 10, 20, 40and 80 cm above the soil surface. In September 2007 (water level> 2m), an extrasample was taken at 160 cm depth. Channel samples were taken at approximately1.5m from the channel shore on 11 days in 2007 when field I was also sampled.Sampling depths were 0.2m above the channel bed and 0.5, 1 and 2m below thechannel surface. When floodwater level exceeded 2.5m above paddy field level,an extra sample was taken at 3m depth below the channel surface. Samples taken2.5, 5, 10, 20 and 40 cm above the channel bed on an exemplary day did not revealconcentration gradients in the bottom channel water (Supplementary Fig. S9).Profiles of conductivity, pH, dissolved O2 and redox potential, Eh, were measuredat the inlet location of field I (n= 6 (2006); n= 17 (2007)) and the channel location(n= 3 (2007)). Details concerning equipment and procedures for floodwater andchannel sampling are given as Supplementary Information.

Porewater samplers, yielding profiles with a 1.5 cm depth resolution over atotal depth of 48–67.5 cm (see details in the Supplementary Information), wereinserted at a distance of ∼3m from the irrigation inlet of field I, maintainingdistances of 0.7–1m between samplers (Supplementary Fig. S10). Porewatersamplers were kept anoxic until insertion and were inserted vertically into theflooded soil. Samplers were recollected after 2.5 and 4.5 weeks of flooding in2006 and in duplicate after 4, 12 and 16 weeks of flooding (3×2 samplers) in2007. Equilibration periods in the soil were >2weeks for all samplers (Fig. 2). Oncollection, samplers were placed into N2-flushed polyethylene bags for sampling(see details in the Supplementary Information).

Floodwater and porewater samples were analysed for total (unfiltered) and dis-solved (0.2 µm filtered) As by hydride generation atomic fluorescence spectroscopy(PS Analytical Ltd) and Fe, Mn, P, Si, Ca, Mg, Na, K and S by inductively coupledplasma optical emission spectrometry (Spectro Ciros CCD) as described in ref. 15and the Supplementary Information. Sulphide (measured in the field), orthophos-phate and total and dissolved organic carbon were determined by instrumentationand methods given in the Supplementary Information. The pH of porewater sam-ples wasmeasured in the field with amicro pH-electrode (Metrohm).Measurementerrors of analyticalmethods are provided in the Supplementary Information.

Soil data were collected with a vertical resolution of 0–1, 1–5 cm, andsubsequent 5 cm steps close to the inlet of field I in May 2007 (4 cores of 40 cmdepth and 3.4 cm diameter) and close to the inlet of a field adjacent to fieldI in May 2005 (soil profile to 60 cm depth14). Sampling locations are shownin Supplementary Fig. S10. Soil samples were oven-dried at 60 ◦C, ground to

<0.2mm, homogenized and analysed for total As and P by X-ray fluorescencespectrometry (2005 and 2007) and for As and P associated with poorly crystallineFe(hydr)oxides by extraction with acid NH4-oxalate21 (2005) as detailed in ref. 14.Total soil organic carbon content was determined with a CHNS analyser asdescribed in ref. 14. Soil profiles of total As and P from 2005 and 2007 agreed well(Supplementary Fig. S3). We assumed this to hold for oxalate-extractable data tooand based our discussion of oxalate-extractable fractions on the data from May2005 (ref. 14). In addition, soil cores with a depth resolution of 2 cm taken in fieldI (distance from inlet ∼6m) before (May 2005, May 2006) and after monsoonflooding (December 2005) were used to determine soil porosity values for fluxestimations (see details in the Supplementary Information).

Received 11 September 2009; accepted 18 November 2009;published online 13 December 2009

References1. Handbook of Agricultural Statistics (Ministry of Agriculture, 2007); available at

<http://www.moa.gov.bd/statistics/statistics.htm>.2. Field, D. I. Land Evaluation for Crop Diversification (Crop Diversification

Programme, 1995).3. Ali, M. A. et al. in Fate of Arsenic in the Environment (ed. Ahmed, M. F.) 7–20

(ITN International Training Network, 2003).4. Abedin, M. J., Cotter-Howells, J. & Meharg, A. A. Arsenic uptake and

accumulation in rice (Oryza sativa L.) irrigated with contaminated water.Plant Soil 240, 311–319 (2002).

5. Abedin, M. J., Cresser, M. S., Meharg, A. A., Feldmann, J. & Cotter-Howells, J.Arsenic accumulation and metabolism in rice (Oryza sativa L.). Environ. Sci.Technol. 36, 962–968 (2002).

6. Xu, X. Y., McGrath, S. P., Meharg, A. A. & Zhao, F. J. Growing riceaerobically markedly decreases arsenic accumulation. Environ. Sci. Technol. 42,5574–5579 (2008).

7. Alam, M. B. & Sattar, M. A. Assessment of arsenic contamination in soils andwaters in some areas of Bangladesh.Water Sci. Technol. 42, 185–193 (2000).

8. Meharg, A. A. & Rahman, M. M. Arsenic contamination of Bangladeshpaddy field soils: Implications for rice contribution to arsenic consumption.Environ. Sci. Technol. 37, 229–234 (2003).

9. Norra, S. et al. Impact of irrigation with As rich groundwater on soil and crops:A geochemical case study in West Bengal Delta Plain, India. Appl. Geochem. 20,1890–1906 (2005).

10. van Geen, A. et al. Impact of irrigating rice paddies with groundwatercontaining arsenic in Bangladesh. Sci. Total Environ. 367, 769–777 (2006).

11. Duxbury, J. M., Mayer, A. B., Lauren, J. G. & Hassan, N. Food chain aspectsof arsenic contamination in Bangladesh: Effects on quality and productivity ofrice. J. Environ. Sci. Health A A38, 61–69 (2003).

12. Hossain, M. B. et al. Spatial variability of arsenic concentration in soilsand plants, and its relationship with iron, manganese and phosphorus.Environ. Pollut. 156, 739–744 (2008).

13. Panaullah, G. M. et al. Arsenic toxicity to rice (Oryza sativa L.) in Bangladesh.Plant Soil 317, 31–39 (2009).

14. Dittmar, J. et al. Spatial distribution and temporal variability of arsenic inirrigated rice fields in Bangladesh. 2. Paddy soil. Environ. Sci. Technol. 41,5967–5972 (2007).

15. Roberts, L. C. et al. Spatial distribution and temporal variability of arsenic inirrigated rice fields in Bangladesh. 1. Irrigation water. Environ. Sci. Technol. 41,5960–5966 (2007).

16. Ali, M. A. et al. in Fate of Arsenic in the Environment (ed. Ahmed, M. F.) 85–112(ITN International Training Network, 2003).

17. Saha, G. C. & Ali, M. A. Dynamics of arsenic in agricultural soils irrigatedwith arsenic contaminated groundwater in Bangladesh. Sci. Total Environ. 379,180–189 (2007).

18. Heikens, A., Panaullah, G. M. & Meharg, A. A. Arsenic behaviour fromgroundwater and soil to crops: Impacts on agriculture and food safety.Rev. Environ. Contam. Toxicol. 189, 43–87 (2007).

19. Islam, M. R. et al. Behaviour of Arsenic in Aquifers, Soils and Plants: Implicationsfor Management, January 16–18, Dhaka <http://arsenic.tamu.edu/pub/pubpres/DHAKA/dhaka7.pdf> (2005).

20. Harvey, C. F. et al. Groundwater dynamics and arsenic contamination inBangladesh. Chem. Geol. 228, 112–136 (2006).

21. Wenzel, W. W. et al. Arsenic fractionation in soils using an improved sequentialextraction procedure. Anal. Chim. Acta 436, 309–323 (2001).

22. Roberts, L. C. et al. Arsenic removal with iron(II) and iron(III) in waterswith high silicate and phosphate concentrations. Environ. Sci. Technol. 38,307–315 (2004).

23. Burnol, A. et al. Decoupling of arsenic and iron release from ferrihydritesuspension under reducing conditions: A biogeochemical model.Geochem. Trans. 8, 12 (2007).

24. Burton, E. D., Bush, R. T., Sullivan, L. A., Johnston, S. G. & Hocking, R. K.Mobility of arsenic and selected metals during re-flooding of iron- andorganic-rich acid-sulphate soil. Chem. Geol. 253, 64–73 (2008).

58 NATURE GEOSCIENCE | VOL 3 | JANUARY 2010 | www.nature.com/naturegeoscience

© 2010 Macmillan Publishers Limited. All rights reserved.

NATURE GEOSCIENCE DOI: 10.1038/NGEO723 ARTICLES25. Coker, V. S. et al. XAS and XMCD evidence for species-dependent

partitioning of arsenic during microbial reduction of ferrihydrite to magnetite.Environ. Sci. Technol. 40, 7745–7750 (2006).

26. Kocar, B. D., Herbel, M. J., Tufano, K. J. & Fendorf, S. Contrasting effectsof dissimilatory iron(III) and arsenic(V) reduction on arsenic retention andtransport. Environ. Sci. Technol. 40, 6715–6721 (2006).

27. Tufano, K. J. & Fendorf, S. Confounding impacts of iron reduction on arsenicretention. Environ. Sci. Technol. 42, 4777–4783 (2008).

28. Randall, S. R., Sherman, D. M. & Ragnarsdottir, K. V. Sorption of As(V) ongreen rust (Fe4(II)Fe2(III)(OH)12SO4·3H2O) and lepidocrocite (γ -FeOOH):Surface complexes from EXAFS spectroscopy. Geochim. Cosmochim. Acta 65,1015–1023 (2001).

29. Su, C. & Wilkin, R. T. Arsenate and arsenite sorption on and arseniteoxidation by iron(II, III) hydroxycarbonate green rust. ACS Symp. Ser. 915,25–40 (2006).

30. Bostick, B. C., Chen, C. & Fendorf, S. Arsenite retention mechanismswithin estuarine sediments of Pescadero, CA. Environ. Sci. Technol. 38,3299–3304 (2004).

31. Bostick, B. C. & Fendorf, S. Arsenite sorption on troilite (FeS) and pyrite (FeS2).Geochim. Cosmochim. Acta 67, 909–921 (2003).

32. Huerta-Diaz, M. A., Tessier, A. & Carignan, R. Geochemistry of trace metalsassociated with reduced sulfur in freshwater sediments. Appl. Geochem. 13,213–233 (1998).

33. O’Day, P. A., Vlassopoulos, D., Root, R. & Rivera, N. The influenceof sulfur and iron on dissolved arsenic concentrations in the shallowsubsurface under changing redox conditions. Proc. Natl Acad. Sci. USA 101,13703–13708 (2004).

34. Boudreau, B. P. The diffusive tortuosity of fine-grained unlithified sediments.Geochim. Cosmochim. Acta 60, 3139–3142 (1996).

35. Leaist, D. G. Mutual diffusion coefficients for binary aqueous solutionsof arsenous, arsenic, and malonic acids. J. Chem. Eng. Data 52,1319–1325 (2007).

36. Li, J. H. & Gregory, S. Diffusion of ions in sea water and in deep-sea sediments.Geochim. Cosmochim. Acta 88, 703–714 (1974).

37. Kirk, G. The Biogeochemistry of Submerged Soils (Wiley, 2004).38. Simpson, I. C., Roger, P. A., Oficial, R. & Grant, I. F. Density and

composition of aquatic oligochaete populations in different farmers’ricefields. Biol. Fertility Soils 16, 34–40 (1993).

39. Grenz, C., Moutin, T., Picot, B. & Massé, H. Comparison between peepersand benthic chamber methods for the measurement of nutrient fluxes at thewater–sediment interface. C. R. Acad. Sci. III 313, 239–244 (1991).

40. Urban, N. R., Dinkel, C. & Wehrli, B. Solute transfer across the sedimentsurface of a eutrophic lake: I. Porewater profiles from dialysis samplers.Aquat. Sci. 59, 1–25 (1997).

41. Hug, S. J. & Leupin, O. Iron-catalyzed oxidation of arsenic(III) by oxygen andby hydrogen peroxide: pH-dependent formation of oxidants in the Fentonreaction. Environ. Sci. Technol. 37, 2734–2742 (2003).

42. Buschmann, J., Canonica, S., Lindauer, U., Hug, S. J. & Sigg, L. Photoirradiationof dissolved humic acid induces arsenic(III) oxidation. Environ. Sci. Technol.39, 9541–9546 (2005).

43. Brammer, H. The Geography of the Soils of Bangladesh (Univ. Press, 1996).44. Databases and Statistics (Bangladesh Agricultural Research Council, 2008);

available at<http://www.barc.gov.bd/Data_Stat.htm>.45. Harvey, C. F. et al. Arsenic mobility and groundwater extraction in Bangladesh.

Science 298, 1602–1606 (2002).

AcknowledgementsWe thank T. Ruettimann, J. Traber, K. Barmettler and D. Gaertner for analytical support,T. Rosenberg, C. Farnsworth, S. Chowdhury, M. S. Alam and A. Rahman for help withfield work, R. Neumann for help with procuring meteorological data and C. Dinkel forproviding the field spectrometer. We are indebted to the people of Bashailbhog villageand to M. F. Ahmed from the Bangladesh University of Engineering and Technologyfor their long-standing support of this project. Financial support of this research by theSwiss National Science Foundation (Grant 200021-15612/1 and 200020-113654/1) isgratefully acknowledged.

Author contributionsL.C.R. planned and carried out the fieldwork and wrote the manuscript as part ofher PhD thesis. L.C.R. and S.J.H. designed the sampling equipment and procedures.J.D., A.V. and R.K. carried out soil sampling and analyses. B.W. suggested the use ofporewater samplers and O.A.C. provided support in modelling the floodwater data.G.C.S., M.A.A. and A.B.M.B. co-initiated the project and provided technical and logisticsupport during field work. A.V., S.J.H., R.K. and B.W. contributed to data interpretationand the writing of the manuscript. All authors contributed to the final version of themanuscript. This study was part of a larger research project initiated and supervisedby S.J.H., A.V. and R.K.

Additional informationThe authors declare no competing financial interests. Supplementary informationaccompanies this paper on www.nature.com/naturegeoscience. Reprints and permissionsinformation is available online at http://npg.nature.com/reprintsandpermissions.Correspondence and requests formaterials should be addressed to L.C.R. or S.J.H.

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