Sustaining Irrigation Supplies through Immobilization of Groundwater Arsenic In Situ.

Geogenic arsenic (As) in groundwater is widespread, affecting drinking water and irrigation supplies globally, with food security and safety concerns on the rise. Here, we present push-pull tests that demonstrate field-scale As immobilization through the injection of small amounts of ferrous iron (Fe) and nitrate, two readily available agricultural fertilizers. Such injections into an aquifer with As-rich (200 ± 52 µg/L) reducing groundwater led to the formation of a regenerable As reactive filter in situ, producing 15 m3 of groundwater meeting the irrigation water quality standard of 50 µg/L. Concurrently, sediment magnetic properties were markedly enhanced around the well screen, pointing to neo-formed magnetite-like minerals. A reactive transport modeling approach was used to quantitatively evaluate the experimental observations and assess potential strategies for larger-scale implementation. The modeling results demonstrate that As removal was primarily achieved by adsorption onto neo-formed minerals and that an increased adsorption site density coincides with the finer-grained textures of the target aquifer. Up-scaled model simulations with 80-fold more Fe-nitrate reactants suggest that enough As-safe water can be produced to irrigate 1000 m2 of arid land for one season of water-intense rice cultivation at a low cost without causing undue contamination in surface soils that threatens agricultural sustainability.

Prevalence of Methylated Arsenic and Microbial Arsenic Methylation Genes in Paddy Soils of the Mekong Delta.

Microbially mediated inorganic-methylated arsenic (As) transformation in paddy soil is crucial to rice safety; however, the linkages between the microbial As methylation process and methylated As species remain elusive. Here, 62 paddy soils were collected from the Mekong River delta of Cambodia to profile As-related functional gene composition involved in the As cycle. The soil As concentration ranged from <1 to 16.6 mg kg-1, with average As contents of approximately 81% as methylated As and 54% as monomethylarsenate (MMAs(V)) in the phosphate- and oxalate-extractable fractions based on As sequential extraction analysis. Quantitative PCR revealed high arsenite-methylating gene (arsM) copy numbers, and metagenomics identified consistently high arsM gene abundance. The abundance of As-related genes was the highest in bacteria, followed by archaea and fungi. Pseudomonas, Bradyrhizobium, Burkholderia, and Anaeromyxobacter were identified as bacteria harboring the most genes related to As biotransformation. Moreover, arsM and arsI (As demethylation) gene-containing operons were identified in the metagenome-assembled genomes (MAGs), implying that arsM and arsI could be transcribed together. The prevalence of methylated As and arsM genes may have been overlooked in tropical paddy fields. The As methylation-demethylation cycle should be considered when manipulating the methylated As pool in paddy fields for rice safety.

Cross-sectional associations between drinking water arsenic and urinary inorganic arsenic in the United States: NHANES 2003-2014.

BACKGROUND: Inorganic arsenic is a potent carcinogen and toxicant associated with numerous adverse health outcomes. The contribution of drinking water from private wells and regulated community water systems (CWSs) to total inorganic arsenic exposure is not clear. OBJECTIVES: To determine the association between drinking water arsenic estimates and urinary arsenic concentrations in the 2003-2014 National Health and Nutrition Examination Survey (NHANES). METHODS: We evaluated 11,088 participants from the 2003-2014 NHANES cycles. For each participant, we assigned private well and CWS arsenic levels according to county of residence using estimates previously derived by the U.S. Environmental Protection Agency and U.S. Geological Survey. We used recalibrated urinary dimethylarsinate (rDMA) to reflect the internal dose of estimated water arsenic by applying a previously validated, residual-based method that removes the contribution of dietary arsenic sources. We compared the adjusted geometric mean ratios and corresponding percent change of urinary rDMA across tertiles of private well and CWS arsenic levels, with the lowest tertile as the reference. Comparisons were made overall and stratified by census region and race/ethnicity. RESULTS: Overall, the geometric mean of urinary rDMA was 2.52 (2.30, 2.77) µg/L among private well users and 2.64 (2.57, 2.72) µg/L among CWS users. Urinary rDMA was highest among participants in the West and South, and among Mexican American, Other Hispanic, and Non-Hispanic Other participants. Urinary rDMA levels were 25% (95% confidence interval (CI): 17-34%) and 20% (95% CI: 12-29%) higher comparing the highest to the lowest tertile of CWS and private well arsenic, respectively. The strongest associations between water arsenic and urinary rDMA were observed among participants in the South, West, and among Mexican American and Non-Hispanic White and Black participants. DISCUSSION: Both private wells and regulated CWSs are associated with inorganic arsenic internal dose as reflected in urine in the general U.S.

Surface Flooding as a Key Driver of Groundwater Arsenic Contamination in Southeast Asia.

Chronic exposure to groundwater contaminated with geogenic arsenic (As) poses a significant threat to human health worldwide, especially for those living on floodplains in South and Southeast (S-SE) Asia. In the alluvial and deltaic aquifers of S-SE Asia, aqueous As concentrations vary sharply over small spatial scales (10-100 m), making it challenging to identify where As contamination is present and mitigate exposure. Improved mechanistic understanding of the factors that control groundwater As levels is essential to develop models that accurately predict spatially variable groundwater As concentrations. Here we demonstrate that surface flooding duration and interannual frequency are master variables that integrate key hydrologic and biogeochemical processes that affect groundwater As levels in S-SE Asia. A machine-learning model based on high-resolution, satellite-derived, long-term measures of surface flooding duration and frequency effectively predicts heterogeneous groundwater As concentrations at fine spatial scales in Cambodia, Vietnam, and Bangladesh. Our approach can be reliably applied to identify locations of safe and unsafe groundwater sources with sufficient accuracy for making management decisions by solely using remotely sensed information. This work is important to evaluate levels of As exposure, impacts to public health, and to shed light on the underlying hydrogeochemical processes that drive As mobilization into groundwater.

In situ arsenic immobilisation for coastal aquifers using stimulated iron cycling: Lab-based viability assessment.

Arsenic (As) is one of the most harmful and widespread groundwater contaminants globally. Besides the occurrence of geogenic As pollution, there is also a large number of sites that have been polluted by anthropogenic activities, with many of those requiring active remediation to reduce their environmental impact. Cost-effective remedial strategies are however still sorely needed. At the laboratory-scale in situ formation of magnetite through the joint addition of nitrate and Fe(II) has shown to be a promising new technique. However, its applicability under a wider range of environmental conditions still needs to be assessed. Here we use sediment and groundwater from a severely polluted coastal aquifer and explore the efficiency of nitrate-Fe(II) treatments in mitigating dissolved As concentrations. In selected experiments >99% of dissolved As was removed, compared to unamended controls, and maintained upon addition of lactate, a labile organic carbon source. Pre- and post experimental characterisation of iron (Fe) mineral phases suggested a >90% loss of amorphous Fe oxides in favour of increased crystalline, recalcitrant oxide and sulfide phases. Magnetite formation did not occur via the nitrate-dependent oxidation of the amended Fe(II) as originally expected. Instead, magnetite is thought to have formed by the Fe(II)-catalysed transformation of pre-existing amorphous and crystalline Fe oxides. The extent of amorphous and crystalline Fe oxide transformation was then limited by the exhaustion of dissolved Fe(II). Elevated phosphate concentrations lowered the treatment efficacy indicating joint removal of phosphate is necessary for maximum impact. The remedial efficiency was not impacted by varying salinities, thus rendering the tested approach a viable remediation method for coastal aquifers.

Highly bioavailable dust-borne iron delivered to the Southern Ocean during glacial periods.

Changes in bioavailable dust-borne iron (Fe) supply to the iron-limited Southern Ocean may influence climate by modulating phytoplankton growth and CO2 fixation into organic matter that is exported to the deep ocean. The chemical form (speciation) of Fe impacts its bioavailability, and glacial weathering produces highly labile and bioavailable Fe minerals in modern dust sources. However, the speciation of dust-borne Fe reaching the iron-limited Southern Ocean on glacial-interglacial timescales is unknown, and its impact on the bioavailable iron supply over geologic time has not been quantified. Here we use X-ray absorption spectroscopy on subantarctic South Atlantic and South Pacific marine sediments to reconstruct dust-borne Fe speciation over the last glacial cycle, and determine the impact of glacial activity and glaciogenic dust sources on bioavailable Fe supply. We show that the Fe(II) content, as a percentage of total dust-borne Fe, increases from ∼5 to 10% in interglacial periods to ∼25 to 45% in glacial periods. Consequently, the highly bioavailable Fe(II) flux increases by a factor of ∼15 to 20 in glacial periods compared with the current interglacial, whereas the total Fe flux increases only by a factor of ∼3 to 5. The change in Fe speciation is dominated by primary Fe(II) silicates characteristic of glaciogenic dust. Our results suggest that glacial physical weathering increases the proportion of highly bioavailable Fe(II) in dust that reaches the subantarctic Southern Ocean in glacial periods, which represents a positive feedback between glacial activity and cold glacial temperatures.

Evaluation of a field kit for testing arsenic in paddy soil contaminated by irrigation water.

Rice is the primary crop in Bangladesh and rice yield is diminished due to the buildup of arsenic (As) in soil from irrigation with high-As groundwater. Soil testing with an inexpensive kit could help farmers target high-As soil for mitigation or decide to switch to a different crop that is less sensitive to As in soil. A total of 3,240 field kit measurements of As in 0.5 g of fresh soil added to 50 mL of water were compared with total soil As concentrations measured on oven-dried homogenized soil by X-ray fluorescence (XRF). For sets of 12 soil samples collected within a series of rice fields, the average of kit As measurements was a linear function of the average of XRF measurements (r2=0.69). Taking into account that the kit overestimates water As concentrations by about a factor of two, the relationship suggests that about a quarter of the As in paddy soil is released in the kit's reaction vessel. Using the relationship and considering XRF measurements as the reference, the 12-sample average determined correctly whether soil As was above or below a 30 mg/kg threshold in 86% of cases where soil As was above the threshold and in 79% of cases where soil As was below the threshold. We also used a Bayesian approach using 12 kit measurements to estimate the probability that soil As was above a given threshold indicated by XRF measurements. The Bayesian approach is theoretically optimal but was only slightly more accurate than the linear regression. These results show that rice farmers can identify high-As portions of their fields for mitigation using a dozen field kit measurements on fresh soil and base their decisions on this information.

Retardation of arsenic transport through a Pleistocene aquifer.

Groundwater drawn daily from shallow alluvial sands by millions of wells over large areas of south and southeast Asia exposes an estimated population of over a hundred million people to toxic levels of arsenic. Holocene aquifers are the source of widespread arsenic poisoning across the region. In contrast, Pleistocene sands deposited in this region more than 12,000 years ago mostly do not host groundwater with high levels of arsenic. Pleistocene aquifers are increasingly used as a safe source of drinking water and it is therefore important to understand under what conditions low levels of arsenic can be maintained. Here we reconstruct the initial phase of contamination of a Pleistocene aquifer near Hanoi, Vietnam. We demonstrate that changes in groundwater flow conditions and the redox state of the aquifer sands induced by groundwater pumping caused the lateral intrusion of arsenic contamination more than 120 metres from a Holocene aquifer into a previously uncontaminated Pleistocene aquifer. We also find that arsenic adsorbs onto the aquifer sands and that there is a 16-20-fold retardation in the extent of the contamination relative to the reconstructed lateral movement of groundwater over the same period. Our findings suggest that arsenic contamination of Pleistocene aquifers in south and southeast Asia as a consequence of increasing levels of groundwater pumping may have been delayed by the retardation of arsenic transport.

Quantifying Riverine Recharge Impacts on Redox Conditions and Arsenic Release in Groundwater Aquifers Along the Red River, Vietnam.

Widespread contamination of groundwater with geogenic arsenic is attributed to microbial dissolution of arsenic-bearing iron (oxyhydr)oxides minerals coupled to the oxidation of organic carbon. The recharge sources to an aquifer can influence groundwater arsenic concentrations by transport of dissolved arsenic or reactive constituents that affect arsenic mobilization. To understand how different recharge sources affect arsenic contamination-in particular through their influence on organic carbon and sulfate cycling-we delineated and quantified recharge sources in the arsenic affected region around Hanoi, Vietnam. We constrained potential end-member compositions and employed a novel end-member mixing model using an ensemble approach to apportion recharge sources. Groundwater arsenic and dissolved organic carbon concentrations are controlled by the dominant source of recharge. High arsenic concentrations are prevalent regardless of high dissolved organic carbon or ammonium levels, indicative of organic matter decomposition, where the dominant recharge source is riverine. In contrast, high dissolved organic carbon and significant organic matter decomposition are required to generate elevated groundwater arsenic where recharge is largely nonriverine. These findings suggest that in areas of riverine recharge, arsenic may be efficiently mobilized from reactive surficial environments and carried from river-aquifer interfaces into groundwater. In groundwaters derived from nonriverine recharge areas, significantly more organic carbon mineralization is required to obtain equivalent levels of arsenic mobilization within inland sediments. This method can be broadly applied to examine the connection between hydrology, geochemistry and groundwater quality.

Argentophilic Interactions in Solution: An EXAFS Study of Silver(I) Nitrene Transfer Catalysts.

Silver(I) catalysts have been developed for nitrene transfer reactions such as aziridination and C-H insertion. For some catalysts, structures determined by X-ray crystallography reveal dimers with silver-silver interactions, leading to mechanistic speculation about the potential role of dinuclear silver complexes in catalysis. However, it is often unclear if the silver-silver interactions persist in solution. Here we use EXAFS to directly interrogate the solution-phase structures of several silver(I) nitrene transfer catalysts. Retention or loss of the silver-silver interaction in solution can be clearly observed.

Tungsten Speciation and Solubility in Munitions-Impacted Soils.

Considerable questions persist regarding tungsten geochemistry in natural systems, including which forms of tungsten are found in soils and how adsorption regulates dissolved tungsten concentrations. In this study, we examine tungsten speciation and solubility in a series of soils at firing ranges in which tungsten rounds were used. The metallic, mineral, and adsorbed forms of tungsten were characterized using X-ray absorption spectroscopy and X-ray microprobe, and desorption isotherms for tungsten in these soils were used to characterize its solid-solution partitioning behavior. Data revealed the complete and rapid oxidation of tungsten metal to hexavalent tungsten(VI) and the prevalence of adsorbed polymeric tungstates in the soils rather than discrete mineral phases. These polymeric complexes were only weakly retained in the soils, and porewaters in equilibrium with contaminated soils had 850 mg L-1 tungsten, considerably in excess of predicted solubility. We attribute the high solubility and limited adsorption of tungsten to the formation of polyoxometalates such as W12SiO404-, an α-Keggin cluster, in soil solutions. Although more research is needed to confirm which of such polyoxometalates are present in soils, their formation may not only increase the solubility of tungsten but also facilitate its transport and influence its toxicity.

Identifying and Quantifying the Intermediate Processes during Nitrate-Dependent Iron(II) Oxidation.

Microbially driven nitrate-dependent iron (Fe) oxidation (NDFO) in subsurface environments has been intensively studied. However, the extent to which Fe(II) oxidation is biologically catalyzed remains unclear because no neutrophilic iron-oxidizing and nitrate reducing autotroph has been isolated to confirm the existence of an enzymatic pathway. While mixotrophic NDFO bacteria have been isolated, understanding the process is complicated by simultaneous abiotic oxidation due to nitrite produced during denitrification. In this study, the relative contributions of biotic and abiotic processes during NDFO were quantified through the compilation and model-based interpretation of previously published experimental data. The kinetics of chemical denitrification by Fe(II) (chemodenitrification) were assessed, and compelling evidence was found for the importance of organic ligands, specifically exopolymeric substances secreted by bacteria, in enhancing abiotic oxidation of Fe(II). However, nitrite alone could not explain the observed magnitude of Fe(II) oxidation, with 60-75% of overall Fe(II) oxidation attributed to an enzymatic pathway for investigated strains: Acidovorax ( A.) strain BoFeN1, 2AN, A. ebreus strain TPSY, Paracoccus denitrificans Pd 1222, and Pseudogulbenkiania sp. strain 2002. By rigorously quantifying the intermediate processes, this study eliminated the potential for abiotic Fe(II) oxidation to be exclusively responsible for NDFO and verified the key contribution from an additional, biological Fe(II) oxidation process catalyzed by NDFO bacteria.

Model-Based Analysis of Arsenic Immobilization via Iron Mineral Transformation under Advective Flows.

Recent laboratory studies have demonstrated that coinjection of nitrate and Fe(II) (as ferrous sulfate) to As-bearing sediments can produce an Fe mineral assemblage containing magnetite capable of immobilizing advected As under a relatively wide range of aquifer conditions. This study combined laboratory findings with process-based numerical modeling approaches, to quantify the observed Fe mineral (trans)formation and concomitant As partitioning dynamics and to assess potential nitrate-Fe(II) remediation strategies for field implementation. The model development was guided by detailed solution and sediment data from our well-controlled column experiment. The modeling results demonstrated that the fate of As during the experiment was primarily driven by ferrihydrite formation and reductive transformation and that different site densities were identified for natural and neoformed ferrihydrite to explain the observations both before and after nitrate-Fe(II) injection. Our results also highlighted that when ferrihydrite was nearing depletion, As immobilization ultimately relied on the presence of magnetite. On the basis of the column model, field-scale predictive simulations were conducted to illustrate the feasibility of the nitrate-Fe(II) strategy for intercepting advected As from a plume. The predictive simulations, which suggested that long-term As immobilization was feasible, favored a scenario that maintains high dissolved Fe(II) concentration during injection periods and thereby converts ferrihydrite to magnetite.

Simultaneously Quantifying Ferrihydrite and Goethite in Natural Sediments Using the Method of Standard Additions with X-ray Absorption Spectroscopy.

The presence of ferrihydrite in sediments/soils is critical to the cycling of iron (Fe) and many other elements but difficult to quantify. Extended X-ray absorption fine structure (EXAFS) spectroscopy has been used to speciate Fe in the solid phase, but this method is thought to have difficulties in distinguishing ferrihydrite from goethite and other minerals. In this study, both conventional EXAFS linear combination fitting (LCF) and the method of standard-additions are applied to the same samples in attempt to quantify ferrihydrite and goethite more rigorously. Natural aquifer sediments from Bangladesh and the United States were spiked with known quantities of ferrihydrite, goethite and magnetite, and analyzed by EXAFS. Known mineral mixtures were also analyzed. Evaluations of EXAFS spectra of mineral references and EXAFS-LCF fits on various samples indicate that ferrihydrite and microcrystalline goethite can be distinguished and quantified by EXAFS-LCF but that the choice of mineral references is critical to yield consistent results. Conventional EXAFS-LCF and the method of standard-additions both identified appreciable amount of ferrihydrite in Bangladesh sediments that were obtained from a low-arsenic Pleistocene aquifer. Ferrihydrite was also independently detected by sequential extraction and 57Fe MÓ§ssbauer spectroscopy. These observations confirm the accuracy of conventional EXAFS-LCF and demonstrate that combining EXAFS with additions of reference materials provides a more robust means of quantifying short-range-ordered minerals in complex samples.

Field Study of Rice Yield Diminished by Soil Arsenic in Bangladesh.

Rice was traditionally grown only during the summer (aman) monsoon in Bangladesh but more than half is now grown during the dry winter (boro) season and requires irrigation. A previous field study conducted in a small area irrigated by a single high-arsenic well has shown that the accumulation of arsenic (As) in soil from irrigating with high-As groundwater can reduce rice yield. We investigated the effect of soil As on rice yield under a range of field conditions by exchanging the top 15 cm of soil between 13 high-As and 13 low-As plots managed by 16 different farmers, and we explore the implications for mitigation. Soil As and rice yields were measured for soil replacement plots where the soil was exchanged and adjacent control plots where the soil was not exchanged. Differences in yield (ranging from +2 to -2 t/ha) were negatively correlated to the differences in soil As (ranging from -9 to +19 mg/kg) between adjacent replacement and control plots during two boro seasons. The relationship between soil As and yield suggests a boro rice yield loss over the entire country of 1.4-4.9 million tons annually, or 7-26% of the annual boro harvest, due to the accumulation of As in soil over the past 25 years.

Reversible adsorption and flushing of arsenic in a shallow, Holocene aquifer of Bangladesh.

The spatial heterogeneity of dissolved arsenic (As) concentrations in shallow groundwater of the Bengal Basin has been attributed to transport of As (and reactive carbon) from external sources or to the release of As from within grey sand formations. We explore the latter scenario in this detailed hydrological and geochemical study along a 300 m transect of a shallow aquifer extending from a groundwater recharge area within a sandy channel bar to its discharge into a nearby stream. Within the 10-20 m depth range, groundwater ages along the transect determined by the 3H-3He method increase from <10 yr in the recharge area to a maximum of 40 yr towards the stream. Concentrations of groundwater As within the same grey sands increase from 10 to 100 to ∼500 µg/L along this transect. Evidence of reversible adsorption of As between the groundwater and sediment was obtained from a series of push-pull experiments, traditional batch adsorption experiments, and the accidental flooding of a shallow monitoring well. Assuming reversible adsorption and a distribution coefficient, Kd, of 0.15-1.5 L/kg inferred from these observations, a simple flushing model shows that the increase in As concentrations with depth and groundwater age at this site, and at other sites in the Bengal and Red River Basins, can be attributed to the evolution of the aquifer over 100-1000 years as aquifer sands are gradually flushed of their initial As content. A wide range of As concentrations can thus be maintained in groundwater with increases with depth governed by the history of flushing and local recharge rates, without external inputs of reactive carbon or As from other sources.

In Situ Magnetite Formation and Long-Term Arsenic Immobilization under Advective Flow Conditions.

In situ precipitation of magnetite and other minerals potentially sequesters dissolved arsenic (As) in contaminated aquifers. This study examines As retention and transport in aquifer sediments using a multistage column experiment in which magnetite and other minerals formed from added nitrate and ferrous iron (Fe). Sediments were collected from the Dover Municipal Landfill Superfund site. Prior to nitrate-Fe(II) addition, As was not effectively retained within the sediments in the column. The combination of nitrate (10 mM) and Fe(II) (4 mM), resulted in mineral precipitation and rapidly decreased effluent As concentrations to <10 µg L(-1). Mineralogical analyses of sectioned replicate columns using sequential extractions, magnetic susceptibility and X-ray absorption spectroscopy indicate that magnetite and ferrihydrite formed in the column following nitrate-Fe(II) addition. This magnetite persisted in the column even as conditions became reducing, whereas ferrihydrite was transformed to more stable Fe oxides. This magnetite incorporated As into its structure during precipitation and subsequently adsorbed As. Adsorption to the minerals kept effluent As concentrations <10 µg L(-1) for more than 100 pore volumes despite considerable Fe reduction. The results indicate that it should be feasible to produce an in situ reactive filter by nitrate-Fe(II) injection.

Advection of surface-derived organic carbon fuels microbial reduction in Bangladesh groundwater.

Chronic exposure to arsenic (As) by drinking shallow groundwater causes widespread disease in Bangladesh and neighboring countries. The release of As naturally present in sediment to groundwater has been linked to the reductive dissolution of iron oxides coupled to the microbial respiration of organic carbon (OC). The source of OC driving this microbial reduction--carbon deposited with the sediments or exogenous carbon transported by groundwater--is still debated despite its importance in regulating aquifer redox status and groundwater As levels. Here, we used the radiocarbon ((14)C) signature of microbial DNA isolated from groundwater samples to determine the relative importance of surface and sediment-derived OC. Three DNA samples collected from the shallow, high-As aquifer and one sample from the underlying, low-As aquifer were consistently younger than the total sediment carbon, by as much as several thousand years. This difference and the dominance of heterotrophic microorganisms implies that younger, surface-derived OC is advected within the aquifer, albeit more slowly than groundwater, and represents a critical pool of OC for aquifer microbial communities. The vertical profile shows that downward transport of dissolved OC is occurring on anthropogenic timescales, but bomb (14)C-labeled dissolved OC has not yet accumulated in DNA and is not fueling reduction. These results indicate that advected OC controls aquifer redox status and confirm that As release is a natural process that predates human perturbations to groundwater flow. Anthropogenic perturbations, however, could affect groundwater redox conditions and As levels in the future.

In Situ Oxalic Acid Injection to Accelerate Arsenic Remediation at a Superfund Site in New Jersey.

Arsenic is a prevalent contaminant at a large number of US Superfund sites; establishing techniques that accelerate As remediation could benefit many sites. Hundreds of tons of As were released into the environment by the Vineland Chemical Co. in southern New Jersey during its manufacturing lifetime (1949-1994), resulting in extensive contamination of surface and subsurface soils and sediments, groundwater, and the downstream watershed. Despite substantial intervention at this Superfund site, sufficient aquifer cleanup could require many decades if based on traditional pump and treat technologies only. Laboratory column experiments have suggested that oxalic acid addition to contaminated aquifer solids could promote significant As release from the solid phase. To evaluate the potential of chemical additions to increase As release in situ and boost treatment efficiency, a forced gradient pilot scale study was conducted on the Vineland site. During spring/summer 2009, oxalic acid and bromide tracer were injected into a small portion (~50 m2) of the site for 3 months. Groundwater samples indicate that introduction of oxalic acid led to increased As release. Between 2.9 and 3.6 kg of As were removed from the sampled wells as a result of the oxalic acid treatment during the 3-month injection. A comparison of As concentrations on sediment cores collected before and after treatment and analyzed using X-ray fluorescence spectroscopy suggested reduction in As concentrations of ~36% (median difference) to 48% (mean difference). While further study is necessary, the addition of oxalic acid shows potential for accelerating treatment of a highly contaminated site and decreasing the As remediation time-scale.