DSpace at VNU: Surface complexation modeling of groundwater arsenic mobility: Results of a forced gradient experiment in a Red River flood plain aquifer, Vietnam

Available online at www.sciencedirect.com Geochimica et Cosmochimica Acta 98 (2012) 186–201 www.elsevier.com/locate/gca Surface complexation modeling of groundwater arsenic mobility: Results of a forced gradient experiment in a Red River flood plain aquifer, Vietnam Søren Jessen a,⇑, Dieke Postma b, Flemming Larsen b, Pham Quy Nhan c, Le Quynh Hoa d, Pham Thi Kim Trang e, Tran Vu Long c, Pham Hung Viet e, Rasmus Jakobsen f a Department of Geography and Geology, University of Copenhagen, 1350 Copenhagen, Denmark Department of Geochemistry, Geological Survey of Denmark and Greenland (GEUS), 1350 Copenhagen, Denmark c Department of Hydrogeology, Hanoi University of Mining and Geology (HUMG), Hanoi, Vietnam d Department of Applied Physics, Graduate School of Engineering, Osaka University, Osaka, Japan e Research Centre for Environmental Technology and Sustainable Development (CETASD), Hanoi University of Science (VNU), Hanoi, Vietnam f Department of Environmental Engineering, Technical University of Denmark, 2800 Kgs Lyngby, Denmark b Received 26 August 2011; accepted in revised form 14 July 2012; available online 22 July 2012 Abstract Three surface complexation models (SCMs) developed for, respectively, ferrihydrite, goethite and sorption data for a Pleistocene oxidized aquifer sediment from Bangladesh were used to explore the effect of multicomponent adsorption processes on As mobility in a reduced Holocene floodplain aquifer along the Red River, Vietnam The SCMs for ferrihydrite and goethite yielded very different results The ferrihydrite SCM favors As(III) over As(V) and has carbonate and silica species as the main competitors for surface sites In contrast, the goethite SCM has a greater affinity for As(V) over As(III) while PO43À and Fe(II) form the predominant surface species The SCM for Pleistocene aquifer sediment resembles most the goethite SCM but shows more Si sorption Compiled As(III) adsorption data for Holocene sediment was also well described by the SCM determined for Pleistocene aquifer sediment, suggesting a comparable As(III) affinity of Holocene and Pleistocene aquifer sediments A forced gradient field experiment was conducted in a bank aquifer adjacent to a tributary channel to the Red River, and the passage in the aquifer of mixed groundwater containing up to 74% channel water was observed The concentrations of As (3 m water column The top end sections were sealed by rubber caps Sampling could then be conducted from a small boat Water levels in the channel and observation wells N1 and N4 were recorded by data loggers with an estimated uncertainty of 3–5 cm 2.1.2 Water sampling and field analysis Sampling was conducted with a submersible Grundfos MP1 pump and Ø10 mm polyethylene (PE) tubing Before sampling, three well volumes of water were removed with the pump positioned in the upper part of the borehole The pumping rate was then decreased, and the pump was lowered to a position at the top of the screen Temperature, O2, pH, and electrical conductivity (EC) were measured by probes mounted in a flow cell connected directly to the sampling tube The measurements were carried out with a WTW Multi197i instrument using a WTW Tetracon 96 EC probe, a WTW SenTix 41 pH electrode and for dissolved O2 a WTW EO 196-1.5 electrode Samples for CH4 were injected directly from the sampling tube through a butyl rubber stopper into a preweighed evacuated glass vial (Labco 819W), which was immediately frozen in an upside down position thereby trapping the gas phase in the head space above the sampled water Samples for all other parameters were collected in 50 mL polypropylene (PP) syringes and filtered through 0.2 lm cellulose acetate filters into 20 mL PE vials Aqueous As(V) and As(III) were separated by passing freshly collected sample through first a 0.2 lm membrane filter and then a disposable anion exchange cartridge at a rate of approximately mL/min using a syringe The cartridges contained 0.8 g aluminosilicate adsorbent that selectively adsorbs As(V) but not As(III) (Meng and Wang, 1998) The combined syringe, filter and cartridge were flushed five times with N2 before processing a sample Samples were acidified by 0.5 vol.% HNO3 suprapur As(V) was calculated as the difference between As(III) and As-total Fig (a) Location of the field site on the banks of the Red River 30 km upstream of Hanoi The dotted lines indicate channels that are fully connected to the main river course during the rainy season but may become disconnected during the dry season The location marked N indicates were the forced gradient experiment was carried out (b) Situation sketch of the field site for the forced gradient experiment, at location N S Jessen et al / Geochimica et Cosmochimica Acta 98 (2012) 186–201 Fe(II), phosphate and dissolved sulfide were measured spectrophotometrically using a Hach DR/2010 instrument and the ferrozine (Stookey, 1970), molybdate blue and methylene blue methods (Cline, 1967) with detection limits of 0.2, 1.1, and 0.5 lM, respectively Alkalinity was determined by Gran-titration (Stumm and Morgan, 1981) Samples for Si, Na+, Ca2+, Mg2+, and K+ were preserved by adding 0.5 vol.% suprapur HNO3 and refrigerated Samples for NH4+, NO3À, ClÀ and SO42À were frozen immediately after sampling Channel water samples were collected 2– m away from the shoreline and the electrodes were immersed directly into the water at a depth of 10–20 cm 2.1.3 Laboratory analysis Cations were analyzed by flame AAS on a Shimadzu AAS 6800 Arsenic was determined on the same instrument using a HVG hydride generator and a graphite furnace Anions were analyzed by ion chromatography on a Shimadzu LC20AD/HIC-20ASuper NH4+ and Si were determined spectrophotometrically using respectively the nitroprusside and the ammonium molybdate methods Head space CH4 was determined on a Shimadzu GC-14A with a m packed column (3% SP1500, Carbopack B) and a FID detector The aqueous CH4 concentration was calculated using Henry’s law The detection limits were: As 0.013 lM, Mn2+ 0.91 lM, Ca2+ 0.50 lM, NH4+ 5.6 lM, Si 0.2 lM, NO3À 0.8 lM, SO42À 2.1 lM, and CH4 1.9 lM 2.2 Elution of arsenic from the sediment The sediment consisting of grey to dark grey fine sand was collected by Postma et al (2007), using a sediment corer, at the nearby H-transect (Fig 1) in the anoxic part of the Holocene aquifer at 15.3 m depth This and other sediments from the H-transect have been characterized by Postma et al (2007, 2010) In a N2 filled glove box, 12 g wet sediment (8.6 g dry weight) was transferred from the core into a 100 mL septum bottle Added to the bottle was an aliquot of a carefully deoxygenated 10 mM NaHCO3 solution, prepared from boiled water, and equili- 189 brated with a 5% CO2/95% N2 gas mixture, corresponding to the 10 meq/L alkalinity and pH observed in the field at the coring site (Postma et al., 2007) Possible trace levels of oxygen in the CO2/N2 gas were trapped by passing the gas through an acetate buffered FeSO4 solution The bottle was capped by a 10 mm thick rubber stopper, removed from the glove box, and stripped for glove box-H2 by bubbling with the deoxygenated CO2/N2 gas The bottle was shaken sideto-side for two hours, and then left in an upright position for settling of the suspended particles for one to days Samples of the supernatant were withdrawn using a needle and a PP syringe, passed through a 0.2 lm syringe filter and stored refrigerated in PE vials After removal of most of the supernatant, the bottle received a new aliquot of 40–90 mL of the 10 mM NaHCO3 solution The mass of samples and amendments was determined gravimetrically This cycle was repeated five times Arsenic was measured in each batch of elutant with the As concentration decreasing for each subsequent step A sub-sample of the sediment was heated to 110 °C for 24 h to determine the water content 2.3 Modeling Surface complexation modeling was carried out using PHREEQC (version 2.16; Parkhurst and Appelo, 1999) For ferrihydrite, we used the two layer SCM by Dzombak and Morel (1990) extended with surface species for carbonate (Appelo and De Vet, 2003), silicate (Swedlund and Webster, 1999) and Fe(II) (Liger et al., 1999; Appelo et al., 2002) For goethite, the charge distribution-multisite complexation (CD MUSIC) model (Hiemstra and Van Riemsdijk, 1996) was used with the surface complexes compiled for PHREEQC by David Kinniburgh in the code PhreePlot (www.phreeplot.org) The surface complexation reactions, their affinity constants and source are listed in Table EA-1 in the Electronic Annex The capacitances for the 0–1 and 1–2 planes are C1 = 0.85 F/m2 and C2 = 0.75 F/m2, respectively (Stachowicz et al., 2008) The performance of the CD MUSIC model in PHREEQC was verified by successfully reproducing the model lines Fig Cross section of the field site for the forced gradient experiment (Fig 1), showing fine grained overbank deposits (hatched) on top of the sandy aquifer (white area) The position of three $5 m deep hand drillings in the flat channel bottom is indicated; the rightmost intersected presumed lenses of sand are indicated by small dots Well screens are indicated by horizontal dashes The well screens intersect a layer of coarse sand with pebbles indicated by the dotted interval at À7 to À9 masl The annual maximum (wet season) and minimum (dry season) groundwater table and channel water level is indicated 190 S Jessen et al / Geochimica et Cosmochimica Acta 98 (2012) 186–201 for single-sorbate As(III) adsorption (Fig in Stachowicz et al., 2006) and H3AsO3–PO43À–Ca2+ triple-sorbate coadsorption (Fig in Stachowicz et al., 2008) The model for adsorption on Pleistocene aquifer sediment from Bangladesh, by Stollenwerk et al (2007), uses experimentally determined adsorption constants for As(III), As(V), PO43À, and HCO3À, in combination with the Dzombak and Morel (1990) model for ferrihydrite for other components and for electrostatic effects The new reactions were formulated for the Hfo_w sites (termed Sites in Stollenwerk et al (2007)) and added to the PHREEQC input file overriding the standard Dzombak and Morel (1990) database The three models will be referred to in the text as the D&M model, the CD MUSIC model and the Stollenwerk et al (2007) model The standard Dzombak and Morel (1990) database extended with surface species for carbonate, silicate, and Fe(II) (see above) will be referred to as the D&M database Aqueous speciations were for all three models conducted using the wateq4f.dat database, although for the D&M and CD MUSIC models this database was modified with the aqueous As speciation constants from Langmuir et al (2006) 2.4 Hydrogeology The forced gradient experiment was conducted at the bank of a side channel of the Red River (Fig 1) Here, the Holocene aquifer consists of fine sand to pebbles underneath a surface layer of silty–clayey material and fine sand (Fig 2) Drillings in the channel bottom (Fig 2) reached a sand layer after penetrating the silty–clayey unit, suggesting that the sandy aquifer extends beneath the channel bottom In the drilling furthest away from the wells, a sandy sequence with only few thin clay layers was observed The surface water-groundwater interaction at the field site has been described by Larsen et al (2008) During the dry season the river stage decreases, the channel becomes disconnected from the Red River (Figs 1a and 2) and al- Fig The water level in the channel and in observation wells N1and N4 (Figs and 2) from 25 days before to 55 days after the start of pumping on July 2007 During the first 34 days of pumping the water level in the channel is above that of the wells and channel water intrudes into the aquifer After day 34, the channel water level drops quickly to below the groundwater level and the flow direction is reversed in spite of pumping most dries out near the end of the dry season (Fig 2) The groundwater flow direction is NE towards the Red River During the rainy season, the Red River stage rapidly increases and the channel and the Red River become connected (Fig 2) Fig displays the channel stage and the hydraulic head in observation wells N1 and N4 (Figs and 2) from 25 days before to 55 days after the start of pumping on July 2007 In the period from day À25 to day 34, the hydraulic head in the channel is higher than in wells N1 and N4, but on day 34 the channel stage decreases to below the hydraulic head in the wells From day zero to around day 35, N1 has a higher hydraulic head than N4 (Fig 3) indicating flow from the channel towards the pumping wells After 30 days of pumping, the channel stage drops causing the hydraulic gradient to reverse and the groundwater to flow in the opposite direction The natural hydraulic gradient between the aquifer and the channel now overrides the effect of the pumping RESULTS Table displays the composition of the pristine groundwater and the composition of the channel water after it has become flooded by the rising river The groundwater is anoxic with 2.5 lM As(III) and 0.6 lM As(V), the EC is 784 lS/cm with Ca2+ and Mg2+ as the main cations that are charge-balanced by alkalinity The channel water is oxic, contains no As and has a low EC of 168 lS/cm (average for first days of pumping) In terms of aqueous species, other than As, that may adsorb, one may note the much higher concentration of Fe(II), HCO3À, PO43À and Si in the pristine groundwater as compared to the channel water Fig displays the changes in concentration during the pumping experiment in observation wells N1 and N4 (Figs and 2) Nine days after the start of pumping, the EC begins to decrease, indicating the first appearance of Table The composition of pristine groundwater (boring N1 (Fig 1b) on day after initiation of pumping) and of channel water (average for first days) Parameter Groundwater (N1, day 9) Channel water (Avg day to 9) Unit EC Temp pH O2 Alkalinity Fe2+ PO43À CH4 As(III) As(V) Na+ K+ Ca2+ Mg2+ Mn2+ NH4+ Si SO42À 784 29 6.98 8.80 0.19 8.0 0.14 2.5 0.6 0.17 0.08 2.8 1.3 7.1 28 0.39 168 32 7.69 0.20 1.53 0.3 0 0.10 0.05 0.6 0.2 1.3 0.16 0.09 lS/cm °C – mM meq/L mM lM mM lM lM mM mM mM mM lM lM mM mM S Jessen et al / Geochimica et Cosmochimica Acta 98 (2012) 186–201 water derived from the channel The curves for EC are shifted by approximately days between the well closest to the channel (N1) and the well (N4) closest to the pumping well (Fig 2) The hydraulic gradient reverses on day 34 (Fig 3) where a minimum EC of 340 lS/cm is observed in both N1 and N4 (Fig 4) Thereafter, the EC increases again, but this time first in N4 and thereafter in N1 The EC in the two observation wells has returned to the initial values on day 54 The EC of the channel water increases after day 34, and approaches that of the groundwater This reflects the discharge of groundwater to the channel, which becomes of increasing importance subsequent to day 34, when the hydraulic head of the aquifer started to exceed that of the channel stage (Fig 3) During the flooding season in the year after the observations in Figs and were made, with no pumping, the EC in N1 did not shown any decrease, indicating that the observed changes are due to the pumping and not a natural phenomenon For most major ions, the changes in concentration follow those in EC (Fig and EA-1) Even though the channel water is oxic, the groundwater in N1 and N4 remained anoxic, and methanic, during the whole experiment As(III) follows the general trend with the lowest concentrations occurring near day 34 but for As(V) it is hard to identify a clear trend in the scatter of the data While the pH in the channel water was close to 7.7, the pH in N1 and N4 remained very close to 7.0 ± 0.1, as in the pristine groundwater 191 To separate the effects of mixing and chemical reactions on the water chemistry, conservative mixing calculations were carried out using channel- and groundwater in Table as endmembers The fraction of channel water mixed into the groundwater, fchannel, was calculated using alkalinity, which at neutral pH is close to the HCO3À anion concentration, as inert component (Appelo and Postma, 2005): fchannel ¼ ðmalk;sample À malk;groundwater Þ ; ðmalk;channel À malk;groundwater Þ ð1Þ where m denote concentration Alternatively EC could be used instead of alkalinity in Eq (1); in practice the resulting fchannel are barely distinguishable from those calculated using alkalinity Using fchannel the concentration expected for conservative mixing for each solute, i, is calculated from: mi;mix ẳ fchannel mi;channel ỵ ð1 À fchannel Þ Á mi;groundwater ð2Þ The results of the mixing calculations are included as the lines in Fig The maximum fraction of channel water in the waters sampled in N1 and N4 is 0.74 Fig shows that for most components the variation is very well described by the mixing lines The observation that conservative mixing between the channel water and groundwater may explain most of the variation indicates that the system has a high dispersivity Rather than a simple displacement of groundwater by the intruding channel water, it appears that chan- Fig Changes in water chemistry in observation boreholes N1 (filled symbols) and N4 (open symbols) and in the channel (crosses) For location see Fig The water composition calculated for conservative mixing between river- and groundwater is indicated for N1 by the solid line and N4 by the broken line 192 S Jessen et al / Geochimica et Cosmochimica Acta 98 (2012) 186–201 Fig Desorption of As from aquifer sediment from the nearby H-transect (Fig 1) by repeated equilibration with 10 mM NaHCO3 solutions The As concentration in each subsequent batch of extractant is shown on the X-axis The cumulative amount of eluted As is shown on the Y-axis Only As(III) was found to desorb The extrapolated content of desorbable As is 7.7 nmol/g nel water enters through highly permeable layers and becomes mixed with the groundwater residing in the adjacent layers However, Fig also shows that for As(III), As(V), PO43À and Si there are distinct differences between concentrations calculated for conservative mixing and the measured values, although the measured values for As(V) appear quite scattered The calculated values for Fe(II) are also somewhat different from the measured values These differences must be due to chemical reactions taking place in the aquifer Mobile As, present in the anoxic aquifer sediment, was determined by repeated elution with a 10 mM NaHCO3 solution Only As(III) was found to be present in the elutant Fig displays the As concentration in the elutant versus the cumulative amount of eluted As It shows a nearlinear relation which extrapolates to a concentration of mobile As(III) of 7.7 nmol/g sediment The continuous decrease in the As concentration in the repeated elution steps does not indicate the dissolution of a phase that controls the aqueous As concentration by a mineral equilibrium It is more consistent with As being desorbed from the sediment surface iment sampling (Table in Postma et al (2007)) Under in situ conditions, the mobile amount of As(III) of 7.7 nmol As/g sed (Fig 5) corresponds to 7.7 nmol As/g  6183 g/ L = 47.6 lmol adsorbed As(III) per liter of contacting groundwater, assuming a porosity of 0.3 and grain density of 2.65 g/cm3 By varying the amount of sorbent in the SCMs (grams of ferrihydrite, goethite or Stollenwerk et al (2007)’s Pleistocene sediment), the number of sites reactive towards As(III) in the CD MUSIC, D&M and Stollenwerk et al (2007) models was normalized, so that each SCM produced the same in situ adsorbed As(III) concentration of 47.6 lmol/L groundwater when at equilibrium with the water composition at the point of sediment sampling The resulting concentration of surface sites for which a surface reaction with As(III) is defined is for the CD MUSIC model 0.17, for the D&M model 0.28, and for the Stollenwerk et al (2007) model 0.51 lmol sites/g sediment; these respective site concentrations were applied in all model simulations presented in Figs 7–13 (Further details are available in the Electronic Annex.) Accordingly, the range of the concentration of surface sites that potentially may adsorb As(III) in the three models is quite small For comparison, the model predictions made by BGS and DPHE (2001) used 0.74 lmol sites/g For the sediment eluted by Polizzotto et al (2006), Swartz et al (2004) fitted a sorption capacity of 0.11 lmol sites/g (calculated using a DISCUSSION 4.1 The modeled speciation of adsorbed arsenic 4.1.1 Site density normalization When applying a surface complexation model to a sediment, assigning a plausible surface site density is an important, though non-trivial, step For example, surface site densities in previous studies have been calculated from the amount of M HCl-extracted Fe (Swartz et al., 2004) or fitted to water chemistry data (Postma et al., 2007) In the present study we elute As from the sediment (Fig 5), and thereby obtain the amount of As adsorbed in equilibrium with the groundwater composition at the point of sed- Fig Adsorption of As on Bangladesh aquifer sediments in the absence of competing anions, except for the Stollenwerk et al (2007) data points for Pleistocene sediment which were measured in a solution containing 70 mg/L Ca2+, 24 mg/L Mg2+ and 194 mg/L ClÀ at pH 6.8 The solid lines are predicted by the SCM by Stollenwerk et al (2007), derived from these and additional experiments Also included are adsorption data from Itai et al (2010) for As(III) on three different Holocene sediments measured at pH 7.3 in a 10 mM MOPS (3-morpholinopropanesulfonic acid) buffer solution The data points measured by Nath et al (2009) are for sorption of As(V) in mM NaNO3 at pH 7.5 or 7.7 on Holocene sediment S Jessen et al / Geochimica et Cosmochimica Acta 98 (2012) 186–201 porosity of 0.3 and a grain density of 2.65 g/cm3) Much lower surface site concentrations of 0.010 and 0.016 lmol sites/g were applied by Appelo et al (2002) and Postma et al (2007), respectively Because of competitive sorption with other solute ions, only the high surface site concentrations allow for the presence of the several nanomoles of adsorbed As per gram of sediment (Fig 5) 4.1.2 SCM for aquifer sediment In principle, models developed according to the general composite approach are site specific We therefore need to assess whether it is reasonable to apply the Stollenwerk et al (2007) model for Pleistocene aquifer sediment to data from our reduced Holocene aquifer setting Fig shows the adsorption data for As(V) and As(III) of Stollenwerk et al (2007), measured on Pleistocene aquifer sediment from Bangladesh in the absence of competing anions The results show much stronger adsorption for As(V) than for As(III) The solid lines shown in Fig are predicted by the model by Stollenwerk et al (2007) Itai et al (2010) measured the adsorption of As(III) on three Holocene aquifer sediment samples from Bangladesh and Nath et al (2009) measured the adsorption of As(V) on Holocene aquifer sediment from West Bengal, India Data from these studies are included in Fig Interestingly, the adsorption data for As(III) on Holocene sediment by Itai et al (2010) plot close to the As(III) adsorption data for Pleistocene sediment by Stollenwerk et al (2007) This suggests that the affinity of Holocene and Pleistocene sediments for As(III) is comparable A similar conclusion was reached by Itai et al (2010) based on discrete K0 d measurements on both Holocene and Pleistocene sediments For As sorption on Holocene sediments, Itai et al (2010) found significantly higher K0 d values for As(V) than for As(III), in agreement with the results of Stollenwerk et al (2007) for Pleistocene sediment The results for As(V) sorption on Holocene sediments by Nath et al (2009) (Fig 6) indicate less sorption 193 than found for the Pleistocene sediments Nath et al (2009), however, did not determine the adsorption of As(III) to their sediment, so a direct comparison of As(III) and As(V) sorption is not available It is also obvious that sorption properties must differ among sediments, depending on mineralogy, surface area, organic matter content, etc But in this case the results in Fig suggest that these differences are not very big and therefore it is reasonable to test the Stollenwerk et al (2007) model for Pleistocene aquifer sediments on our Holocene aquifer system in order to compare the behavior of a natural sediment with that of synthetic Fe-oxides predicted by the D&M and CD MUSIC models 4.1.3 Comparison of modeled surface speciation The three SCMs, normalized in the previous section, were used to calculate the composition of the sediment surface in equilibrium with the pristine groundwater (Table 1) The results of these calculations are shown in Fig 7, for the surface sites that are able to adsorb As(III), and are surprisingly different The groundwater contains about five times as much As(III) compared to As(V) (Table 1) The CD MUSIC model for goethite, however, shows a higher surface concentration for As(V) as compared to As(III) indicating a much stronger sorption of As(V) than of As(III) In contrast, the D&M model for ferrihydrite shows stronger sorption for As(III) than for As(V) Also in terms of ions competing for sites with As on the surfaces of the synthetic Fe-oxides, ferrihydrite and goethite, the results are very different For ferrihydrite, the D&M model calculates as the main surface complexes, apart from protonated and deprotonated sites, bicarbonate (52%) and silica (27%) while phosphate surface complexes constitute only 5% For goethite, however, the CD MUSIC model calculates that 50% of the sites are occupied by phosphate, with Fe(II) surface complexes as the second most important species (30%) while both silica and bicarbonate cover less than 1% It Fig The surface speciation at equilibrium with pristine groundwater (Table 1), calculated for the sites that are able to adsorb arsenic, with the D&M model for ferrihydrite, the CD MUSIC model for goethite and the model for Pleistocene aquifer sediment by Stollenwerk et al (2007), using the normalized total site concentration of, respectively, Hfo_w = 0.28 (D&M model), Goe_uni = 0.17 (CD MUSIC model), and Site = 0.51 (Stollenwerk et al (2007) model) lmol sites/g of sediment The field for each element in the pie diagram may cover several surface species The field denoted “H” indicates the sum of all protonated or deprotonated surface sites 194 S Jessen et al / Geochimica et Cosmochimica Acta 98 (2012) 186–201 should further be noted that in the CD MUSIC model, As(III) binds almost exclusively ($99%) to the surface as a ternary As(III)-Fe(II) complex, Goe_uniOAs(OH)3Fe+0.5 (Hiemstra and van Riemsdijk, 2007), which covers 2% of the surface sites The surface speciation calculated with the Stollenwerk et al (2007) model (Fig 7) resembles most that of the goethite SCM Similar to goethite, the surface affinity for As(V) is much stronger than for As(III) Phosphate is again the most important ion competing for surface sites with a 41% coverage, while bicarbonate complexes are insignificant On the other hand, silica complexes cover 18% of the surface sites and in this respect it resembles more the ferrihydrite surface It is furthermore conspicuous that protonated and deprotonated surface sites are more important in the Stollenwerk et al (2007) model as compared to the two other SCMs In the SCM by Stollenwerk et al (2007) the dissociation constant for surface hydroxyls, pKa2, is nearly two orders of magnitude larger than in the D&M model, and consequently deprotonated surface sites are important (8% coverage) in the former model, while negligible in the latter 4.2 Sensitivity analysis The previous section has shown that the different SCMs produce very different results in terms of the amount of As(III) and As(V) adsorbed and the predominant ions competing for surface sites Because the relations between the aqueous solutions and the surface complexes are complex and non-linear, it is not easy to perceive how the different models would react towards changes in aqueous composition Therefore, a number of sensitivity tests were carried out with the different models in order to test their behavior with the range of groundwater compositions that are observed in S.E Asia In the first test, the distribution coefficients were calculated for As(III) and As(V) as a function of the concentration with the three models (Fig 8), using the pristine groundwater composition (Table 1) and the normalized surface site concentrations given in the previous section, but varying the As(V) or As(III) concentration For ferrihydrite, the D&M model predicts that As(III) sorbs about twice as strongly as As(V) However, for goethite the CD MUSIC model predicts that As(V) adsorption is more than three times stronger than for As(III) In the Stollenwerk et al (2007) model for aquifer sediment, the affinity of the surface for As(V) is even higher The As(III) and As(V) concentrations in the pristine groundwater (Table 1) are indicated on the isotherms, in Fig 8, except for As(V) in the Stollenwerk et al (2007) model, which is completely out of range The results obtained with the different models have major implications for the amount of mobile As, aqueous plus adsorbed, that is predicted to be present in the system This is particularly apparent for As(V) where the low groundwater concentration of 0.6 lM relates to a low mobile pool of As(V) in the D&M model, while the CD MUSIC model and particularly the Stollenwerk et al (2007) model suggest that a lot of surface bound, potentially mobilizable As(V) is present in the system, even when the aqueous concentration is low Finally, the large differences in the calculated distribution coefficients have a major bearing on the retardation and thereby on the mobility of As(III) and As(V) Retardation is defined as R = + Kd where Kd = [sorbate concentration in mol/L]/[solute concentration in mol/L] and therefore dimensionless (Appelo and Postma, 2005) All three models show a retardation for As(III) in the narrow range R = 9–20 (Kd = 8–19; Fig 8), while for As(V) the D&M model predict it to be very mobile (R = 5–10; Kd = 4–9) while the two other SCMs predict As(V) to have a decisively low mobility (R = 28–204; Kd = 27–203)! Fig displays the adsorption of As(III) and As(V) as a function of pH, using the same aqueous composition and surface properties as before but calculated for varying pH For ferrihydrite, the D&M model indicates that As sorption as a function of pH is complex At a pH higher than 8.5, sorption of both As(III) and As(V) strongly decreases For As(III), Hfo_wH2AsO3 is the only surface complexed species and this species becomes less stable as pH decreases For As(V) the surface species Hfo_wH2AsO4, Hfo_wHAsO4À and Hfo_wOHAsO4À3 are sequen- Fig Adsorption isotherms calculated for the groundwater in Table 1, while varying the As(III) or As(V) concentration, with the D&M model for ferrihydrite, the CD MUSIC model for goethite and the model for Pleistocene aquifer sediment by Stollenwerk et al (2007) The squares indicate the measured As(III) and As(V) concentrations (Table 1) As(V) in the Stollenwerk et al (2007) model runs out of range For normalized site densities refer to the caption of Fig S Jessen et al / Geochimica et Cosmochimica Acta 98 (2012) 186–201 195 Fig The effect of pH on the adsorption of As(III) and As(V) calculated for the groundwater in Table (while varying the pH) with the D&M model for ferrihydrite, the CD MUSIC model for goethite and the model for Pleistocene aquifer sediment by Stollenwerk et al (2007) For normalized site densities refer to the caption of Fig tially formed from low to high pH Because the Hfo_wHAsO4À species has a comparatively lower formation constant there is a depression in the As(V) sorption curve near pH For goethite, the CD MUSIC model predicts that in the range pH 6.5–10 the ternary As(III)–Fe(II) surface complex Goe_uniOAs(OH)3Fe+0.5 mediates a strong As(III) adsorption Outside this pH range, As(III) sorption becomes weak with the species Goe_uniOAs(OH)2À0.5 as most important at lower pH and (Goe_uniO)2AsOHÀ at higher pH For As(V), close to 100% remains adsorbed up to pH 8, while adsorption rapidly decreases towards higher pH In the Stollenwerk et al (2007) model, adsorbed As(III) is close to 100% above pH with Hfo_wAsO3À2 as the only important surface species, which becomes unstable below pH The strong affinity of the surface for As(V) in this model, is also displayed in the sorption behavior as a function of pH since close to 100% of As(V) stays adsorbed over the entire pH range The pristine groundwater (Table 1) has a pH of 6.98 and similarly most waters in As-bearing aquifers in S.E Asia have near neutral pH However, as Fig illustrates, in particular the CD MUSIC and the Stollenwerk et al (2007) models predict that even small decreases in pH may significantly lower As(III) adsorption In Fig 7, phosphate was found to adsorb very strongly according to both the CD MUSIC model for goethite and the Stollenwerk et al (2007) model for aquifer sediment, while the D&M model for ferrihydrite predicts much less PO43À adsorption In groundwater from our field site the PO43À concentration is only lM, but for Bangladesh groundwater Swartz et al (2004) reported a PO43À concentration in the range 26–68 lM Accordingly, it can be anticipated that variations in the phosphate concentration of the groundwater may have a strong influence on the adsorption or desorption of As Fig 10 shows the results of model runs, where PO43À (as H3PO4) was either added or removed from the system, while keeping pH fixed at 6.98 Zero on the X-axis corresponds to the measured groundwater concentrations for As(V), As(III) and PO43À (Table 1) The varying PO43À and derived As concentrations are plotted on the Y-axis The amount of PO43À added was adjusted to reach an aqueous PO43À concentration between 60 and 70 lM, corresponding to the Bangladesh situation reported by Swartz et al (2004) In the case of ferrihydrite, the D&M model calculates that 300 lM PO43À must be added to increase the aqueous PO43À concentration from to 68 lM The difference (300–68 = 232 lM) is the PO43À that is adsorbed on the surface Because of PO43À adsorption, the Fig 10 The effect of the aqueous PO43À concentration on the adsorption of As(III) and As(V) calculated for the groundwater in Table by adding or removing PO43À from the system The amount of PO43À added or removed in the model calculations with the D&M model for ferrihydrite, the CD MUSIC model for goethite and the model for Pleistocene aquifer sediment by Stollenwerk et al (2007) is shown towards right or left on the X-axis At zero on the X-axis the pristine groundwater concentrations of As(III), As(V) and PO43À can be read from the Y-axis For normalized site densities refer to the caption of Fig 196 S Jessen et al / Geochimica et Cosmochimica Acta 98 (2012) 186–201 aqueous As(V) concentration increases from 0.6 to 2.5 lM For As(III) the increase is only from 2.5 to 3.4 lM Going in the other direction, about 100 lM PO43À can be removed before the system becomes totally phosphate depleted The result is that aqueous As(V) decreases from 0.6 to 0.3 lM, while As(III) only decreases from 2.5 to 2.4 lM For goethite, the CD MUSIC model predicts that only half as much PO43À (150 lM) has to be added to increase the aqueous PO43À concentration to above 60 lM In comparison to ferrihydrite this probably reflects that so much PO43À already was adsorbed on the goethite surface (Fig 7) The effect of PO43À on the As(V) aqueous concentration is strong, the addition of 150 lM PO43À causes As(V) to increase from 0.6 to 4.5 lM But the effect on the As(III) concentration is as for the D&M model quite small, increasing from only 2.5 to 2.9 lM Removing PO43À from the system has the reverse effect All aqueous As(V) will adsorb to the goethite surface while the effect on As(III) is small The affinity of the aquifer sediment for PO43À is according to the Stollenwerk et al (2007) model much larger than for the pure Fe-oxides A total of 1200 lM phosphate needs to be added to increase the aqueous PO43À concentration above 60 lM Even though 40% of the sediment surface sites already hold adsorbed PO43À (Fig 7) the surface is, according to the model, able to adsorb much more PO43À (Fig 10) As a result, As(V) increases from 0.6 to 5.9 lM and As(III) from 2.5 to 14.3 lM, i.e., very strong displacements effects indeed The observation that As(III) adsorbs on the goethite surface as a combined Fe(II)–As(III) species in the CD MUSIC model (Hiemstra and van Riemsdijk, 2007) suggests that As(III) sorption becomes a function of the aqueous Fe(II) concentration Similarly as for phosphate, Fig 11 shows the response of the goethite system towards addition or removal of Fe(II) Addition of 0.4 mM Fe(II) causes aqueous Fe(II) to increase from 0.19 to 0.55 mM while aqueous As(III) decreases from 2.5 to 1.4 lM due to co-adsorption with Fe(II) As(V) increases slightly due to displacement from the surface It is also apparent in Fig 11 that the removal of Fe(II) from the system causes a very strong desorption of As(III) from the goethite surface In aquifers the Fe(II) concentration is often constrained by, e.g., siderite precipitation; on the other hand, an implication of the model result is that a decrease in dissolved Fe(II) could lead to a mobilization of adsorbed As(III) Finally, the displacement of As(V) from the surface of Fe-oxides by bicarbonate has been debated (Appelo et al., 2002; Radu et al., 2005; Stachowicz et al., 2007) The arguments of Appelo et al (2002) were based on calculations with the D&M model The groundwater at our field site has an alkalinity of 8.8 meq/L (Table 1) which at the neutral pH of the water is close to the TIC Addition or removal of about mM HCO3À from the system, while keeping the pH constant, appears to be reasonable range for the expected variation in floodplain aquifers of S.E Asia The results are shown in Fig 12 Adding mM HCO3À increases aqueous As(III) from 2.5 to 3.2 lM and As(V) from 0.6 to 0.9 lM and the reverse is noted when HCO3À is being removed Therefore, for ferrihydrite in the D&M model, the competition between As species and dissolved carbonate has some importance However, in the SCMs for goethite in the CD MUSIC model, and the aquifer sediment of Stollenwerk et al (2007), very little carbonate is adsorbed (Fig 7) and the effect on As is therefore insignificant and not shown Fig 11 The effect of the aqueous Fe(II) concentration on the adsorption of As(V) and As(III) as calculated for the groundwater in Table by adding or removing Fe(II) from the system Towards right or left on the X-axis, the amount of Fe(II) added or removed in the model calculations with the CD MUSIC model for goethite and the normalized total site concentration Goe_uni = 0.17 lmol site/g of sediment Fig 12 The effect of the aqueous HCO3À concentration on the adsorption of As(V) and As(III) as calculated for the groundwater in Table by adding or removing HCO3À from the system Towards right or left on the X-axis, the amount of HCO3À added or removed in the model calculations with the D&M model for ferrihydrite using the normalized total site concentration of Hfo_w = 0.28 lmol site/g of sediment 4.3 Modeling of the forced gradient experiment The conservative mixing calculations, shown in Fig 4, demonstrated that while the variations for most components can be described satisfactorily by conservative mix- S Jessen et al / Geochimica et Cosmochimica Acta 98 (2012) 186–201 Fig 13 Observed concentrations of desorbing elements (closed boxes) during the forced gradient field experiment in obs well N1 as compared with concentrations calculated by conservative mixing of groundwater and channel water (full line), and conservative mixing followed by equilibration with the model surface in equilibrium with the pristine groundwater using the D&M model for ferrihydrite, the CD MUSIC model for goethite and the model for Pleistocene aquifer sediment by Stollenwerk et al (2007) (broken lines as indicated) For normalized site densities refer to the caption of Fig The composition of groundwater and channel water is given in Table ing, distinct desorption from the sediment is observed for As(III), PO43À and Si Inspection of the break-through 197 curves for the major ions (Fig 4) indicates that during the forced gradient experiment, the aquifer between the channel and the observation wells was flushed less than one time by channel water In N1, the observation well most affected by flushing, half of the reduction in EC is completed around day 20–22 leaving less than 20 days of continued pumping before the flow reversal on day 34 Given the high retardations (Fig 8), the retarded fronts for As and other strongly retarded elements cannot have reached observation well N1 during the duration of the forced gradient experiment The observed desorption is similar to what is called the salinity effect in ion exchange (Appelo and Postma, 2005) When channel water with an ionic strength different from the pristine groundwater travels into the system, the water chemistry will change prior to the arrival of the retarded fronts, due to a reequilibration with the sediment surface at the new ionic strength Desorption or adsorption may occur as the intruding water passes through the aquifer For ions with a low retardation (i.e taking up few surface sites), the effects will be hard to distinguish from conservative mixing For ions with a high retardation the aqueous concentration will become effectively buffered by the surface (Itai et al., 2010) With respect to the forced gradient experiment, the pumping did not flush As from the aquifer sediment to any significant amount, but rather caused a water chemistry perturbation to which the pristine surface responded The three SCMs were tested on their ability to predict the desorption of As(III), PO43À and Si from the sediment as seen in Fig The procedure was to equilibrate the pristine groundwater (Table 1) with a SCM surface, i.e., resulting in the surface compositions shown in Fig Thereafter the surface was equilibrated with the mixed water, i.e the groundwater composition calculated by conservative mixing of pristine groundwater and channel water (Table 1) using Eq (1) and (2) Although the channel water is oxic, the overall redox state of the groundwater did not change during the forced gradient experiment Accordingly, the redox potential pe calculated from the aqueous speciation in PHREEQC for the CO2/CH4 and SO4/H2S redox couples ranged from À4.1 to À3.7 and À3.9 to À3.5, respectively, and showed no systematic variation over the experiment (Fig EA-2 in the Electronic Annex) Therefore, in the reactive mixing model we have omitted redox reactions with the main consequence for the modeled parameters that the As(V)/As(III) ratio remains unchanged The model results are shown in Fig 13 with the conservative mixing lines from Fig included for comparison The D&M model does predict some desorption of As(III) but no desorption of PO43À and Si, for which the calculated aqueous concentrations are lower than for conservative mixing, suggesting an enhanced PO43À and Si adsorption In contrast, the CD MUSIC model overpredicts the extent of As(III) desorption, while PO43À desorption is predicted correctly, but the release of Si is too low and close to conservative mixing, corresponding to the low amount of adsorbed Si predicted by the CD MUSIC model (Fig 7) The performance of the Stollenwerk et al (2007) model is superior compared to the two other models It predicts both As(III) and PO43À correctly and is closer to the field data for Si than the two other models 198 S Jessen et al / Geochimica et Cosmochimica Acta 98 (2012) 186–201 4.4 Evaluation of the SCMs It is remarkable that ferrihydrite and goethite, according to respectively the D&M and the CD MUSIC models, have such significantly different adsorption properties With regard to major adsorbing components the much stronger adsorption of PO43À on goethite than on ferrihydrite is particularly notable Reversely, ferrihydrite strongly adsorbs carbonate and Si which are not much adsorbed by goethite Finally goethite strongly adsorbs Fe(II) which only to a minor extent adsorbs onto ferrihydrite Particularly the difference in PO43À sorption must have important implications for phosphate cycling in natural environments in a broader sense because goethite rich environments will retain phosphate much stronger than ferrihydrite dominated environments With respect to As in a groundwater environment, the affinity in sorption of As(III) and As(V) makes a major difference While for ferrihydrite, As(III) adsorbs stronger than As(V), the reverse is the case for goethite Moreover, the adsorption of As(III) on goethite strongly depends on the Fe(II) concentration due to the ternary surface complex with Fe(II), which does not form according to the ferrihydrite model In general it appears that goethite is much more common in recent environments than ferrihydrite (van der Zee et al., 2003) and the SCM for goethite therefore appears most relevant For As-contaminated aquifer sediments, the information on the mineralogy of Fe-oxides is sparse Postma et al (2010) used Moăssbauer spectroscopy and identified only goethite and hematite as Fe-oxides in the Holocene sediments at our field locality Using TEM, Akai et al (2004) identified goethite and hematite, and in addition also ferrihydrite, in Holocene aquifer sediments from Bangladesh Rowland et al (2008), using XRD, identified goethite in sediments from the Kandal Province, Cambodia In general, however, the common occurrence of methane in As-contaminated aquifers suggests that ferrihydrite is not present because methanogenesis, thermodynamically, should not occur concomitantly with reduction of ferrihydrite since the energy gain of the latter process is much larger In the case of goethite the two processes may energetically proceed concurrently (Postma et al., 2007) Based on these considerations, goethite appears to be the better representative for the Fe-oxides present in As-contaminated aquifers of S.E Asia The large differences in the adsorption predicted for As, Si, CO32À and PO43À for ferrihydrite and goethite by the two SCMs (Fig 7) obviously calls for some sort of external validation, especially because of the surface structural similarities between ferrihydrite and goethite (Spadini et al., 2003) In principle the differences in predicted adsorption may either be the result of differences in the adsorption properties of the two minerals or they could be due to inconsistencies in the models and/or databases used Based on available information we cannot decide which of the two options is correct However, the following considerations could be taken into account The Dzombak and Morel (1990) SCM for ferrihydrite has a database based on a compilation of single-sorbate data on various preparations of hydrous ferric oxide from the literature Whether the model correctly predicts multi-component adsorption has to our knowledge never been tested rigorously Adsorption of carbonate was not present in the original database The only measurements of carbonate adsorption on ferrihydrite are by Zachara et al (1987) but were conducted at micromolar CO32À concentrations The CO32À affinity constants now present in the D&M database are based on sorption data for goethite (van Geen et al., 1994), reoptimized to the D&M model for ferrihydrite (Appelo and De Vet, 2003) by assuming a comparable adsorption to goethite and ferrihydrite on a per site basis Only the affinity constants for Si are derived from dual-sorbate experiments with Si, As(V) and As(III) (Swedlund and Webster, 1999) The CD MUSIC database for goethite is, with the exception of Si (Hiemstra et al., 2007), derived from multicomponent adsorption data sets, using only very few preparations of goethite (Stachowicz et al., 2006, 2008) This certainly increases confidence in the consistency of the CD MUSIC model and database Nonetheless, the exception of Si may be important While the surface speciation for goethite in Fig shows a negligible amount of adsorbed Si, both Waltham and Eick (2002) and Luxton et al (2006) have observed a significant competition between Si and As(III) on goethite in dual-sorbate experiments This, however, does not necessarily invalidate the result in Fig 7, as other competing ions may outcompete Si without doing so for As(III) The Stollenwerk et al (2007) experimental data and the derived SCM for Bangladesh Pleistocene sediments show a greater affinity of the surface for As(V) than for As(III) Itai et al (2010) measured distribution coefficients on Holocene Bangladesh sediments and also found much stronger adsorption for As(V) as compared to As(III) Both observations are in good agreement with the CD MUSIC model for goethite (Stachowicz et al., 2007, 2008) Also the strong adsorption of phosphate and the minor significance of adsorption of dissolved carbonate are found in both the Stollenwerk et al (2007) and the CD MUSIC (Stachowicz et al., 2007, 2008) model in contrast to the D&M model for ferrihydrite An important difference between the Stollenwerk et al (2007) and the CD MUSIC models is that the Stollenwerk et al (2007) model is extremely sensitive towards the PO43À concentration while the CD MUSIC model is very sensitive towards the Fe(II) concentration Another difference is that the Stollenwerk et al (2007) model, in agreement with our data as shown in Fig 13, predicts a significant Si adsorption to aquifer sediment which is not predicted by the CD MUSIC model for goethite This discussion naturally poses the question of what is the best approach for modeling the effect of multi-component adsorption on the mobility of As in As-contaminated aquifers As discussed above, the SCMs for ferrihydrite and goethite produce highly different results which can reduce the confidence in the predictions made In addition one could consider whether it is reasonable at all to model the complex sorbent(s) in a sediment, containing impure Feoxides, clays and organics, as if it was a pure synthetic Fe-oxide Conversely, one could intuitively expect that a sediment based SCM would be strongly site specific And looking at S Jessen et al / Geochimica et Cosmochimica Acta 98 (2012) 186–201 the sorption properties of trace metals on sediments from a variety of environments, Wang et al (1997) indeed found major differences However, the comparison of As sorption on aquifer sediments from S.E Asia presented in Fig 6, and the good behavior of the Stollenwerk et al (2007) model in predicting our forced gradient experiment (Fig 13) indicate quite similar sorption properties The reason for this could be that most As-contaminated aquifers in S.E Asia are situated on the floodplains of the major rivers that originate in the Himalayas and therefore may carry and deposit rather similar sediments On this background further exploration of sediment based SCMs could perhaps be the most promising approach towards quantifying the effect of multi-component adsorption on the mobility of arsenic in As-contaminated aquifers CONCLUSIONS The surface speciation of an As-contaminated Holocene aquifer sediment was calculated using SCMs for ferrihydrite by Dzombak and Morel (1990) and for goethite by CD MUSIC (Hiemstra and van Riemsdijk, 1996) The results were surprisingly different For ferrihydrite, As(III) adsorbs stronger than As(V) while HCO3À and Si are the main species that compete with As for surface sites In contrast, on goethite As(V) adsorbs much stronger than As(III) and in this case PO43À and Fe(II) form the main competing species The results demonstrate that predictions concerning the As mobility in As-contaminated aquifer that are based on SCMs for synthetic Fe-oxides will be strongly dependent on the model chosen An alternative approach is to use a SCM calibrated on sediment data to predict As mobility A compilation of As sorption data on sediment data from Bangladesh showed rather similar sorption behavior of Holocene and Pleistocene sediments The SCM of Stollenwerk et al (2007), which is based on sorption measurements of As, PO43À, Si and HCO3À onto Pleistocene oxidized aquifer sediment from Bangladesh, produced results rather similar to that of goethite, although it has a stronger affinity for As, particularly As(V), as well as for Si The three SCMs were tested for their ability to model the results of a forced gradient experiment where low As water intrudes into a As-contaminated aquifer and As(III), PO43À and Si desorb from the sediment The SCM by Stollenwerk et al (2007) behaved quite superior as compared to the models for synthetic Fe-oxides, despite being developed for sediments from an oxidized aquifer and applied to our reduced aquifer setting This result indicates that sediment calibrated SCMs may be a promising way to evaluate the effect of multi-component adsorption on the mobility of arsenic in As-contaminated aquifers ACKNOWLEDGEMENTS This paper is dedicated to the memory of Nguyen Van Hoan, our good friend and a promising student who tragically died from injuries caused by a gas explosion accident We thank Bui Hong Nhat, Dao Manh Phu and Tran Thi Luu, and especially Truong Quang Duc, Trinh Xuan Dai and Nguyen Thi Minh Hue, for assistance in water sampling and analysis, 199 and Hoang Van Hoan, Nguyen Bach Thao and Trieu Duc Huy for conducting water table measurements and assisting during drilling and pumping installations A data compilation for the CDMUSIC modeling and comments to it by David Kinniburgh are greatly acknowledged Dieke Postma thanks Yoshio Takahashi for the invitation to teach the application of surface complexation models on the arsenic problem at the University of Hiroshima This inspired the detailed comparison of the properties of the models presented in this paper We thank the three anonymous reviewers for their very constructive comments and criticism This research was supported by a grant from the Danish Research Council for Development Research under DANIDA APPENDIX A SUPPLEMENTARY DATA Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.gca.2012.07.014 REFERENCES Akai J., Izumi K., Fukuhara H., Masuda H., Nakano S., Yoshimura T., Ohfuji H., Anawar H M and Akai K (2004) Mineralogical and geomicrobiological investigations on groundwater arsenic enrichment in Bangladesh Appl Geochem 19, 215–230 Appelo C A J., van der Weiden M J J., Tournassat C and Charlet L (2002) Surface complexation of ferrous iron and carbonate on ferrihydrite and the mobilization of arsenic Environ Sci Technol 36, 3096–3103 Appelo C A J and De W W J M (2003) Modeling in situ iron removal from groundwater with trace elements such as As In Arsenic in Groundwater (eds A H Welch and K G Stollenwerk) Kluwer Academic, New York, pp 381–401 Appelo C A J and Postma D (2005) Geochemistry, Groundwater and Pollution, second ed A.A Balkema Publ, Amsterdam BGS and DPHE (2001) Arsenic contamination of groundwater in Bangladesh (eds D.G Kinniburgh and P.L Smedley) BGS Technical, Report WC/00/19 Chakraborty S., Wolthers M., Chatterjee D and Charlet L (2007) Adsorption of arsenite and arsenate onto muscovite and biotite mica J Colloid Interf Sci 309, 392–401 Charlet L., Morin G., Rose J., Wang Y., Auffan M., Burnol A and Fernandez-Martinez A (2011) Reactivity at (nano)particlewater interfaces, redox processes, and arsenic transport in the environment C R Geosci 343, 123–139 Cline J D (1967) Spectrophotometric determination of hydrogen sulfide in natural waters Limnol Oceanogr 14, 454–458 Davis J A., Coston J A., Kent B D and Fuller C C (1998) Application of the surface complexation concept to complex mineral assemblages Environ Sci Technol 32, 2820–2828 Dowling C B., Poreda R J., Basu A R., Peters S L and Aggarwal P K (2002) Geochemical study of arsenic release mechanisms in the Bengal Basin groundwater Water Resour Res 38, 1173 http://dx.doi.org/10.1029/2001WR000968 Dzombak D A and Morel F M M (1990) Surface Complexation Modeling-Hydrous Ferric Oxide Wiley, New York Fendorf S., Michael H A and van Geen A (2010) Spatial and temporal variations of groundwater arsenic in south and southeast Asia Science 328, 1123–1127 Harvey C F., Swartz C H., Badruzzaman A B M., Keon-Blute N., Yu W., Ali A., Jay J., Beckie R., Niedan V., Brabander D., Oates P M., Ashfaque K N., Islam S., Hemond H F and 200 S Jessen et al / Geochimica et Cosmochimica Acta 98 (2012) 186–201 Ahmed (2002) Arsenic mobility and groundwater extraction in Bangladesh Science 298, 1602–1606 Hiemstra T and Van Riemsdijk W H (1996) A surface structural approach to ion adsorption: the charge distribution (CD) model J Colloid Interf Sci 179, 488–508 Hiemstra T and van Riemsdijk W H (2007) Adsorption and surface oxidation of Fe(II) on metal (hydr)oxides Geochim Cosmochim Acta 71, 5913–5933 Hiemstra T., Barnett M O and van Riemsdijk W H (2007) Interaction of silicic acid with goethite J Colloid Interf Sci 310, 8–17 Hiemstra T., Antelo J., Rahnemaie R and Van Riemsdijk W H (2010) Nanoparticles in natural systems I: the effective reactive surface area of the natural oxide fraction in field samples Geochim Cosmoschim Acta 74, 41–58 Horneman A., van Geen A., Kent D V., Mathe P E., Zheng Y., Dhar R K., O’Connel S., Hoque M A., Aziz Z., Shamsudduha M., Seddique A A and Ahmed K M (2004) Decoupling of As and Fe release to Bangladesh groundwater under reducing conditions: Part I Evidence from sediment profiles Geochim Cosmochim Acta 68, 3459–3473 Itai T., Takahashi Y., Seddique A A., Maruoka T and Mitamura M (2010) Variations in the redox state of As and Fe measured by X-ray absorption spectroscopy in aquifers of Bangladesh and their effect on As adsorption Appl Geochem 25, 34–47 Langmuir D L., Mahoney J and Rowson J (2006) Solubility products of amorphous ferric arsenate and crystalline scorodite (FeAsO4Á2H2O) and their application to arsenic behavior in buried mine tailings Geochim Cosmochim Acta 70, 2942– 2956 Larsen F., Pham Q N., Dang D N., Postma D., Jessen S., Pham H V., Nguyen B T., Duc T H., Luu T T., Nguyen H., Chambon J., Nguyen V H., Hoang D H., Nguyen T M H., Mai T D and Refsgaard J C (2008) Controlling geological and hydrogeological processes in an arsenic contaminated aquifer on the Red River flood plain Vietnam Appl Geochem 23, 3099–3115 Liger E., Charlet L and van Cappellen P (1999) Surface catalysis of Uranium(VI) reduction by iron(II) Geochim Cosmochim Acta 63, 2939–2955 Luxton T P., Eick M J and Tadanier C J (2006) Mobilization of arsenite by competitive interaction with silicic acid Soil Sci Soc Am J 70, 204–214 Lin Z and Puls R W (2000) Adsorption, desorption and oxidation of arsenic affected by clay minerals and aging process Environ Geol 39, 753–759 McArthur J M., Ravenscroft P., Safiulla S and Thirlwall M F (2001) Arsenic in groundwater: testing pollution mechanisms for sedimentary aquifers in Bangladesh Water Resour Res 37, 109–117 Meng X and Wang W (1998) Speciation of arsenic by disposable cartridges In Proceedings of the 3rd Internat Conf on Arsenic Exposure and Health Effects Soc Environ Geochem Health University of Colorado, Denver Nath B., Chakraborty S., Burnol A., Stuăben D., Chatterjee D and Charlet L (2009) Mobility of arsenic in the sub-surface environment: An integrated hydrogeochemical study and sorption model of the sandy aquifer materials J Hydrol 364, 236– 248 Neumann R B., Ashfaque1 K N., Badruzzaman A B M., Ali M A., Shoemaker J K and Harvey C F (2009) Anthropogenic influences on groundwater arsenic concentrations in Bangladesh Nat Geosci http://dx.doi.org/10.1038/NGEO685 Nickson R., McArthur J., Burgess W., Ahmed K M., Ravenscroft P and Rahman M (1998) Arsenic poisoning of Bangladesh groundwater Nature 395, 338 Nickson R T., McArthur J M., Ravenscroft P., Burgess W G and Ahmed K M (2000) Mechanism of arsenic release to groundwater, Bangladesh and West Bengal Appl Geochem 15, 403–413 Parkhurst D.L and Appelo C.A.J (1999) User’s guide to PHREEQC (Version 2) – a computer program for speciation, reaction-path, 1D-transport, and inverse geochemical calculations USGS Water Resour Inv Rep 99–4259 Polizzotto M L., Harvey C F., Li G., Badruzzman B., Ali A., Newville M., Sutton S and Fendorf S (2006) Solid-phases and desorption processes of arsenic within Bangladesh sediments Chem Geol 228, 97–111 Polizzotto M L., Kocar B D., Benner S G., Sampson M and Fendorf S (2008) Near-surface wetland sediments as a source of arsenic release to ground water in Asia Nature 454, 505–509 Postma D., Larsen F., Nguyen T M H., Mai T D., Pham H V., Pham Q N and Jessen S (2007) Arsenic in groundwater of the Red River floodplain, Vietnam: Controlling geochemical processes and reactive transport modeling Geochim Cosmochim Acta 71, 5054–5071 Postma D., Jessen S., Nguyen T M H., Mai T D., Koch C B., Pham H V., Pham Q N and Larsen F (2010) Mobilization of arsenic and iron from Red River floodplain sediments Vietnam Geochim Cosmochim Acta 74, 3367–3381 Radu T., Subacz J L., Phillippi J M and Barnett M O (2005) Effects of dissolved carbonate on arsenic adsorption and mobility Environ Sci Technol 39, 7875–7882 Rowland H A L., Gault A G., Lythgoe P and Polya D A (2008) Geochemistry of aquifer sediments and arsenic-rich groundwaters from Kandal Province Cambodia Appl Geochem 23, 3029–3046 Seddique A A., Masuda H., Mitamura M., Shinoda K., Yamanaka T., Itai T., Maruoka T., Uesugi K., Ahmed K M and Biswas D K (2008) Arsenic release from biotite into a Holocene groundwater aquifer in Bangladesh Appl Geochem 23, 2236–2248 Smedley P L and Kinniburgh D G (2002) A review of the source, behaviour and distribution of arsenic in natural waters Appl Geochem 17, 517–568 Spadini L., Schindler P W., Charlet L., Manceau A and Ragnarsdottir K V (2003) Hydrous ferric oxide: evaluation of Cd-HFO surface complexation models combining Cd-K EXAFS data, potentiometric titration results, and surface site structures identified from mineralogical knowledge J Colloid Interf Sci 266, 1–18 Stachowicz M., Hiemstra T and van Riemsdijk W H (2006) Surface speciation of As(III) and As(V) in relation to charge distribution J Colloid Interf Sci 302, 62–75 Stachowicz M., Hiemstra T and van Riemsdijk W H (2007) Arsenic-bicarbonate interaction on goethite particles Environ Sci Technol 41, 5620–5625 Stachowicz M., Hiemstra T and van Riemsdijk W H (2008) Multi-competitive interaction of As(III) and As(V) oxyanions with Ca2+, Mg2+, PO43À and CO32À ions on goethite J Colloid Interf Sci 320, 400–414 Stollenwerk K G., Breit G N., Welch A H., Yount J C., Whitney J W., Forster A L., Uddin M N., Majumder R K and Ahmed N (2007) Arsenic attenuation by oxidized sediments in Bangladesh Sci Total Environ 379, 133–150 Stookey L L (1970) Ferrozine – a new spectrophotometric reagent for iron Anal Chem 42, 779–781 Stumm W and Morgan J J (1981) Aquatic Chemistry, second ed Wiley & Sons, New York Stute M., Zheng Y., Schlosser P., Horneman A., Dhar R K., Datta S., Hoque M A., Seddique A A., Shamsudduha M., Ahmed K M and van Geen A (2007) Hydrological control of As S Jessen et al / Geochimica et Cosmochimica Acta 98 (2012) 186–201 concentrations in Bangladesh groundwater Water Resour Res 43, W09417 http://dx.doi.org/10.1029/2005WR004499 Swartz C H., Blute N K., Badruzzman B., Ali A., Brabander D., Jay J., Besancon J., Islam S., Hemond H F and Harvey C F (2004) Mobility of arsenic in a Bangladesh aquifer: Inferences from geochemical profiles, leaching data, and mineralogical characterization Geochim Cosmochim Acta 68, 4539–4557 Swedlund P J and Webster J G (1999) Adsorption and polymerization of silicic acid on ferrihydrite and its effect on arsenic adsorption Water Res 33, 3413–3422 Sø H U., Postma D., Jakobsen R and Larsen F (2008) Sorption and desorption of arsenate and arsenite on calcite Geochim Cosmochim Acta 72, 5871–5884 van der Zee C., Roberts D R., Rancourt D G and Slomp C P (2003) Nanogoethite is the dominant reactive oxyhydroxide phase in lake and marine sediments Geology 31, 993–996 van Geen A., Robertson A P and Leckie J O (1994) Complexation of carbonate species at the goethite surface Implications for adsorption of metal ions in natural waters Geochim Cosmochim Acta 58, 2073–2086 201 van Geen A., Zheng Y., Goodbred, Jr., S., Horneman A., Aziz Z., Cheng Z., Stute M., Mailloux B., Weinman B., Hoque M A., Seddique A A., Hossain M S., Chowdhury S H and Ahmed K M (2008) Flushing history as a hydrogeological control on the regional distribution of arsenic in shallow groundwater of the Bengal Basin Environ Sci Technol 42, 2283–2288 Waltham C A and Eick M J (2002) Kinetics of arsenic adsorption on goethite in the presence of sorbed silicic acid Soil Sci Soc Am J 66, 818–825 Wang F Y., Chen J S and Forsling W (1997) Modeling sorption of trace metals on natural sediments by surface complexation model Env Sci Technol 31, 448–453 Zachara J M., Girvin D C., Schmidt R L and Resch C T (1987) Chromate adsorption on amorphous iron oxyhydroxide in the presence of major groundwater ions Environ Sci Technol 21, 589–594 Associate Editor: Karen Johannesson ... Ca2+ and Mg2+ as the main cations that are charge-balanced by alkalinity The channel water is oxic, contains no As and has a low EC of 168 lS/cm (average for first days of pumping) In terms of. .. variation indicates that the system has a high dispersivity Rather than a simple displacement of groundwater by the intruding channel water, it appears that chan- Fig Changes in water chemistry in. .. first appearance of Table The composition of pristine groundwater (boring N1 (Fig 1b) on day after initiation of pumping) and of channel water (average for first days) Parameter Groundwater (N1, day