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Karst terrains provide drinking water for about 25% of the people on our planet, particularly in the southwest of China. Pollutants such as arsenic (As) in the soil can infiltrate groundwater through sinkholes and bedrock fractures in karst terrains. Despite this, the underlying mechanisms responsible for As release from karst soils under redox changes remain largely unexplored. Here, we used multiple synchrotron-based spectroscopic analyses to explore As mobilization and sequestration in As-polluted karstic soil under biogeochemical conditions that mimic field-validated redox conditions. We observed that As in the soil exists primarily as As(V), which is mainly associated with Fe(oxyhydr)oxides. The concentration of the dissolved As was high (294 μM) and As(III) was dominant (∼95%) at low E h (≤-100 mV), indicating the high risk of As leaching under reducing conditions. This As mobilization was attributed to the fact that the dissolution of ferrihydrite and calcite promoted the release and reduction of associated As(V). The concentration of the dissolved As was low (17.0 μM) and As(V) was dominant (∼68%) at high E h (≥+100 mV), which might be due to the oxidation and/or sequestration of As(III) by the recrystallized ferric phase. Our results showed that the combined effects of the reductive release of As(V) from both ferric and nonferric phases, along with the recrystallization of the ferric phase, govern the redox-induced mobilization and potential leaching of As in soils within karst environments.

Floodplain soils are vast reservoirs of organic carbon often attributed to anaerobic conditions that impose metabolic constraints on organic matter degradation. What remains elusive is how such metabolic constraints respond to dynamic flooding and drainage cycles characteristic of floodplain soils. Here we show that microbial depolymerization and respiration of organic compounds, two rate-limiting steps in decomposition, vary spatially and temporally with seasonal flooding of mountainous floodplain soils (Gothic, Colorado, USA). Combining metabolomics and -proteomics, we found a lower abundance of oxidative enzymes during flooding coincided with the accumulation of aromatic, high-molecular weight compounds, particularly in surface soils. In subsurface soils, we found that a lower oxidation state of carbon coincided with a greater abundance of chemically reduced, energetically less favorable low-molecular weight metabolites, irrespective of flooding condition. Our results suggest that seasonal flooding temporarily constrains oxidative depolymerization of larger, potentially plant-derived compounds in surface soils; in contrast, energetic constraints on microbial respiration persist in more reducing subsurface soils regardless of flooding. Our work underscores that the potential vulnerability of these distinct anaerobic carbon storage mechanisms to changing flooding dynamics should be considered, particularly as climate change shifts both the frequency and extent of flooding in floodplains globally.

Anoxic microsites are potentially important but unresolved contributors to soil organic carbon (C) storage. How anoxic microsites vary with soil management and the degree to which anoxic microsites contribute to soil C stabilization remain unknown. Sampling from four long-term agricultural experiments in the central United States, we examined how anoxic microsites varied with management (e.g., cultivation, tillage, and manure amendments) and whether anoxic microsites determine soil C concentration in surface (0-15 cm) soils. We used a novel approach to track anaerobe habitat space and, hence, anoxic microsites using DNA copies of anaerobic functional genes over a confined volume of soil. No-till practices inconsistently increased anoxic microsite extent compared to conventionally tilled soils, and within one site organic matter amendments increased anaerobe abundance in no-till soils. Across all long-term tillage trials, uncultivated soils had ∼2-4 times more copies of anaerobic functional genes than their cropland counterparts. Finally, anaerobe abundance was positively correlated to soil C concentration. Even when accounting for other soil C protection mechanisms, anaerobe abundance, our proxy for anoxic microsites, explained 41% of the variance and 5% of the unique variance in soil C concentration in cropland soils, making anoxic microsites the strongest management-responsive predictor of soil C concentration. Our results suggest that careful management of anoxic microsites may be a promising strategy to increase soil C storage within agricultural soils.

In California, recent Bay-Delta Plan legislation attempts to balance water supply and ecosystem protection by requiring 40% of the flow to remain in-stream in the Tuolumne River from February through June. Serious questions remain about what this means for the Bay Area water supply, especially during drought. Our work develops a new approach to analyze how in-stream flow policy coupled with climate change could impact regional water supply over the coming decades. Results show that the new in-stream flow demand would exceed urban water deliveries in a typical year. In wet years, water supply performance is minimally impacted, but in drought, the policy can lead to less water in storage, delayed reservoir recovery, and increased time at critically low storage. Storage impact exceeding 50 000 acre-feet (60 million m 3 ) is anticipated with at least 18% frequency, demonstrating that, climate uncertainty notwithstanding, this impact must be planned for and managed to ensure a reliable future water supply.

Soil zinc contamination is a major threat to water quality and sensitive ecosystems. While Zn itself is not redox-active in soils, transitions in soil redox conditions may promote mobilization of Zn from common Zn hosts, including Mn(IV)/Fe(III)-(hydr)oxides and sulfide precipitates, leading to elevated concentrations of dissolved Zn in surface and groundwater and thus a potential increase in Zn transport and uptake. Here, we examined the impacts of hydrologic fluctuations and coupled redox transitions on Zn partitioning in contaminated riparian soil in a mountain watershed. We found that oxygenation of the soil profile during low water conditions caused a spike in porewater Zn concentrations, driven by oxidative dissolution of amorphous ZnS and weak partitioning of Zn to Fe(III)-(hydr)oxides, hydroxy-interlayer vermiculite, and vermiculite. In contrast to Pb, released Zn did not immediately adsorb to Fe(III)-(hydr)oxides or particulate organic matter due to less-favorable sorption of Zn than that of Pb and, further, decreased Zn sorption at slightly acidic pH. As aridification intensifies and groundwater levels decline throughout the western United States, contaminated floodplain soils in mountain watersheds may be frequently oxygenated, leading to increased mobilization of dissolved Zn, which will amplify the threat Zn poses to water quality and ecosystem health.

Increased periods of bottom water anoxia in deep temperate lakes due to decreasing frequency and depth of water column mixing in a warming climate may result in the reductive dissolution of iron minerals and increased flux of nutrients from the sediment into the water column. Here, we assessed the sediment properties and reactivities under depleted oxygen concentrations of Lake Tahoe, a deep ultraoligotrophic lake in the Sierra Nevada mountain range. Using whole-core incubation experiments, we found that a decrease in dissolved oxygen concentration in the top 2 cm of the sediment resulted in an extension of the microbial iron reduction zone from below 4.5 to below 1.5 cm depth. Concentrations of reactive iron generally decreased with sediment depth, and microbial iron reduction seemingly ceased as concentrations of Fe(II) approximated concentrations of reactive iron. These findings suggest that microorganisms preferentially utilized reactive iron and/or iron minerals became less reactive due to mineral transformation and surface passivation. The estimated release of iron mineral-associated phosphorus is not expected to change Lake Tahoe's trophic state but will likely contribute to increased phytoplankton productivity if mixed into surface waters.

Lead adulteration of spices, primarily via Pb chromate compounds, has been documented globally as a growing public health concern. Currently, Pb detection in spices relies primarily on expensive and time-consuming laboratory analyses. Advancing rapid Pb detection methods, inclusive of their accuracy and precision, would improve field assessments by food safety inspectors, stakeholders, and the public in the hope of reducing Pb exposure risks at its source. Here, we present two field procedures for Pb detection: portable X-ray fluorescence analysis (pXRF) and a simple colorimetric test. We assess their efficacy to detect Pb and its chemical form in seven spice types, including powders, spice-salt mixtures, and dried roots, compared to the proven laboratory technique, inductively coupled plasma mass spectrometry (ICP-MS). Lead concentrations measured using pXRF and ICP-MS were within 5% of each other for spice powders and 24% for dried roots. By pXRF, spice samples were analyzed within collection plastic bags without preparation, resulting in a detection limit of 2 mg Pb/kg for spice powders, which is comparable to national food standards. The colorimetric test utilized here targets hexavalent chromium, making the method selective to Pb chromate adulteration assuming that this is its dominant source in spices. Color development, and thus detection, was observed when Pb concentrations exceeded approximately 5-70 mg/kg in dried turmeric roots and 1000 mg/kg in spice powders; however, it was ineffective for the spice-salt mixture. We show that pXRF analysis and a colorimetric assay provide information that may improve field decisions about Pb adulteration in a range of spice types, helping to minimize Pb exposure.

Mountain floodplain soils often show spatiotemporal variations in redox conditions that arise due to changing hydrology and resulting biogeochemistry. Under oxygen-depleted conditions, solid phase terminal electron acceptors (TEAs) can be used in anaerobic respiration. However, it remains unclear to what degree the redox properties of solid phases limit respiration rates and hence organic matter degradation. Here, we assess such limitations in soils collected across a gradient in native redox states from the Slate River floodplain (Colorado, U.S.A.). We incubated soils under anoxic conditions and quantified CO 2 production and microbial Fe(III) reduction, the main microbial metabolic pathway, as well as the reactivity of whole-soil solid phase TEAs toward mediated electrochemical reduction. Fe(III) reduction occurred together with CO 2 production in native oxic soils, while neither Fe(II) nor CO 2 production was observed in native anoxic soils. Initial CO 2 production rates increased with increasing TEA redox reactivity toward mediated electrochemical reduction across all soil depths. Low TEA redox reactivity appears to be caused by elevated Fe(II) concentrations rather than crystallinity of Fe(III) phases. Our findings illustrate that the buildup of Fe(II) in systems with long residence times limits the thermodynamic viability of dissimilatory Fe(III) reduction and thereby limits the mineralization of organic carbon.

Aquifer groundwater quality is largely controlled by sediment composition and physical heterogeneity, which commonly sustains a unique redox gradient pattern. Attenuation of heavy metals within these heterogeneous aquifers is reliant on multiple factors, including redox conditions and redox-active species that can further influence biogeochemical cycling. Here, we simulated an alluvial aquifer system using columns filled with natural coarse-grained sediments and two domains of fine-grained sediment lenses. Our goal was to examine heavy metal (Ni and Zn) attenuation within a complex aquifer network and further explore nitrate-rich groundwater conditions. The fine-grained sediment lenses sustained reducing conditions and served as a sink for Ni sequestration─in the form of Ni-silicates, Ni-organic matter, and a dominant Ni-sulfide phase. The silicate clay and sulfide pools were also important retention mechanisms for Zn; however, Ni was associated more extensively with organic matter compared to Zn, which formed layered double hydroxides. Nitrate-rich conditions promoted denitrification within the lenses that was coupled to the oxidation of Fe(II) and the concomitant precipitation of an Fe(III) phase with higher structural distortion. A decreased metal sulfide pool also resulted, where nitrate-rich conditions generated an average 20% decrease in solid-phase Ni, Zn, and Fe. Ultimately, nitrate plays a significant role in the aquifer's biogeochemical cycling and the capacity to retain heavy metals.

Sediment interfaces in alluvial aquifers have a disproportionately large influence on biogeochemical activity and, therefore, on groundwater quality. Previous work showed that exports from fine-grained, organic-rich zones sustain reducing conditions in downstream coarse-grained aquifers beyond the influence of reduced aqueous products alone. Here, we show that sustained anaerobic activity can be attributed to the export of organic carbon, including live microorganisms, from fine-grained zones. We used a dual-domain column system with ferrihydrite-coated sand and embedded reduced, fine-grained lenses from Slate River (Crested Butte, CO) and Wind River (Riverton, WY) floodplains. After 50 d of groundwater flow, 8.8 ± 0.7% and 14.8 ± 3.1% of the total organic carbon exported from the Slate and Wind River lenses, respectively, had accumulated in the sand downstream. Furthermore, higher concentrations of dissolved Fe(II) and lower concentrations of dissolved organic carbon in the sand compared to total aqueous transport from the lenses suggest that Fe(II) was produced in situ by microbial oxidation of organic carbon coupled to iron reduction. This was further supported by an elevated abundance of 16S rRNA and iron-reducing ( gltA ) gene copies. These findings suggest that organic carbon transport across interfaces contributes to downstream biogeochemical reactions in natural alluvial aquifers.

Lead contamination in soils and sediments is a major threat to water quality. In surface and near-surface environments, Pb is not redox active; however, common Pb hosts, including Fe(III)-(hydr)oxides and sulfides, dissolve and precipitate as redox conditions change. Dissolution of Pb hosts may release Pb to porewater, leading to spikes in dissolved Pb concentrations and potential transport into surface or groundwater. Here, we examine the impacts of hydrologically coupled redox transitions on Pb partitioning in contaminated floodplain soils. We find that the affinity of Pb for particulate organic matter (POM), inclusive of mineral-associated organic matter, ensures that across redox transitions Pb is retained in the solid phase, despite host-phase (Fe(III)-(hydr)oxide and sulfide) dissolution. As seasonal hydrologic dynamics shift porewater redox conditions, Pb-bearing Fe(III)-(hydr)oxides (Pb-HFO) and sulfides (PbS) are dissolved and (re)precipitated. However, despite these shifts in redox conditions and associated host-phase transformations, Pb retention on POM, coupled with the formation of PbS and Pb-HFO, maintains dissolved Pb concentrations below 17 μg L -1 . Importantly, the predominance of Pb adsorbed on POM alongside low dissolved Pb concentrations indicates that Pb released from HFO and PbS is retained by POM. Thus, despite host-phase dissolution during redox transitions, partitioning of Pb to the aqueous phase is minimal and, therefore, transport of dissolved Pb is unlikely.

Alluvial aquifers serve as one of the main water sources for domestic, agricultural, and industrial purposes globally. Groundwater quality, however, can be threatened by naturally occurring and anthropogenic metal contaminants. Differing hydrologic and biogeochemical conditions between predominantly coarse-grained aquifer sediments and embedded layers or lenses of fine-grained materials lead to variation in metal behavior. Here, we examine processes controlling Zn partitioning within a dual-pore domain-reconstructed alluvial aquifer. Natural coarse aquifer sediments from the Wind River-Little Wind River floodplain near Riverton, WY, were used in columns with or without fine-grained lenses to examine biogeochemical controls on Zn concentrations, retention mechanisms, and transport. Following the introduction of Zn to the groundwater source, Zn preferentially accumulated in the fine-grained lenses, despite their small volumetric contributions. While the clay fraction dominated Zn retention in the sandy aquifer, the lenses supported additional reaction pathways of retention-the reducing conditions within the lenses resulted in ZnS precipitation, overriding the contribution of organic matter. Zinc concentration in the groundwater controlled the formation of Zn-clays and Zn-layered double hydroxides, whereas the extent of sulfide production controlled precipitation of ZnS. Our findings illustrate how both spatial and compositional heterogeneities govern the extent and mechanisms of Zn retention in intricate groundwater systems, with implications for plume behavior and groundwater quality.

With the increasing global need for groundwater resources to fulfill domestic, agricultural, and industrial demands, we face the threat of increasing concentrations of naturally occurring contaminants in water sources and a consequential need to improve our predictive capacity. Here, we combine machine learning and geochemical modeling to reveal the biogeochemical controls on regional groundwater uranium contamination within the Central Valley, California. We use 23 environmental parameters from a statewide groundwater geochemical database and publicly available maps of soil and aquifer physicochemical properties to predict groundwater uranium concentrations by random forest regression. We find that groundwater calcium, nitrate, and sulfate concentrations, soil pH, and clay content (weighted average between 0 and 2 m depths) are the most important predictors of groundwater uranium concentrations. By pairing multivariate partial dependence and accumulated local effect plots with modeled aqueous uranium speciation and surface complexation outputs, we show that regional groundwater uranium exceedances of drinking water standards, 30 μg L -1 , are dependent on the formation of uranyl-calcium-carbonato species. The geochemical conditions leading to ternary uranyl complexes within the aquifer are, in part, created by infiltration through the vadose zone, illustrating the critical dependence of groundwater quality on recharge conditions.

Managed aquifer recharge (MAR) enhances freshwater security and augments local groundwater supplies. However, geochemical and hydrological shifts during MAR can release toxic, geogenic contaminants from sediments to groundwater, threatening the viability of MAR as a water management strategy. Using reactive transport modeling coupled with aquifer analyses and measured water chemistry, we investigate the causal mechanisms of arsenic release during MAR via injection in the Orange County Groundwater Basin. Here, injection water is oxygenated, highly purified recycled water produced by advanced water treatment. Injection occurs via a well screened at several depth intervals ranging from 160-365 m, allowing recharge into multiple confined horizons (zones) of the aquifer system. However, these zones are characterized by varying degrees of prior oxidation due to historic, long-term infiltration from the overlying aquifer. The resulting sediment geochemical heterogeneity provides a critical control on the release (or retention) of arsenic. In zones with prior oxidation, As mobilization occurs via arsenate desorption from Fe-(hydr)oxides, primarily associated with shifts in pH; within zones that remain reduced prior to injection, As release is attributed to the oxidative dissolution of As-bearing pyrite. We find that As release can be attributed to various geochemical mechanisms within a single injection well owing to geochemical heterogeneity across the aquifer system.

Reduced sulfur (S) has a contrasting role in the fate of arsenic (As) in peatlands. Sulfur bridges provide efficient binding of As to organic carbon (C), but the formation of aqueous As-S species, so-called thioarsenates, leads to a low to no sorption tendency to organic C functional groups. Here, we studied how pH changes the role of reduced S in desorption and retention of presorbed As in model peat. Control desorption experiments without S addition revealed that As was mobilized, predominantly as arsenite, in all treatments with relative mobilization increasing with pH (4.5 < 7.0 < 8.5). Addition of sulfide or polysulfide caused substantial As retention at acidic conditions but significantly enhanced As desorption compared to controls at neutral to alkaline pH. Thioarsenates dominated As speciation at pH 7.0 and 8.5 (maximum, 79%) and remained in solution without (re)sorption to peat. Predominance of arsenite in control experiments and no evidence of surface-bound thioarsenates at pH 7.0 suggest mobilization to proceed via arsenite desorption, reaction with dissolved or surface-bound reduced S, and formation of thioarsenates. Our results suggest that natural or management-related increases in pH or increases in reduced S in near-neutral pH environments can turn organic matter from an As sink into a source.

Reaction conditions and mechanisms promoting or inhibiting U reduction exert a central control on U solubility and, therefore, U transport and its associated risks. Here, we vary and track common aqueous uranium species to show that a kinetic restriction inhibits homogeneous reduction of the calcium-uranyl-carbonato species (CaUO 2 (CO 3 ) 3 2- and Ca 2 UO 2 (CO 3 ) 3 ) by Fe(II) (aq) , while ferrihydrite surface-catalyzed reduction of all aqueous uranyl by Fe(II) proceeds. Using U L 3 high energy resolution fluorescence detection (HERFD) X-ray absorption near edge structure (XANES) spectroscopy, U L 3 extended X-ray absorption fine structure (EXAFS) spectroscopy, and transmission electron microscopy (TEM), we also show that U(V) is generated and incorporated into ferrihydrite formed from homogeneous U(VI) reduction by Fe(II) (aq) . Through elucidation of the mechanisms that inhibit reduction of the calcium-uranyl-carbonato species and promote stabilization of U(V), we advance our understanding of the controls on U solubility and thus improve prediction of U transport in surface and subsurface systems.

Natural or anthropogenic processes can increase the concentration of uranium (U) and arsenic (As) above the maximum contaminant levels in water sources. Bicarbonate and calcium (Ca) can have major impacts on U speciation and can affect the reactivity between U and As. We therefore investigated the reactivity of aqueous U and As mixtures with bicarbonate and Ca for acidic and neutral pH conditions. In experiments performed with 1 mM U and As mixtures, 10 mM Ca, and without added bicarbonate (pCO 2 = 3.5), aqueous U decreased to <0.25 mM at pH 3 and 7. Aqueous As decreased the most at pH 3 (∼0.125 mM). Experiments initiated with 0.005 mM As and U showed similar trends. X-ray spectroscopy (i.e., XAS and EDX) and diffraction indicated that U-As-Ca- and U-Ca-bearing solids resemble uranospinite [Ca(UO 2 ) 2 (AsO 4 ) 2 ·10H 2 O] and becquerelite [Ca(UO 2 ) 6 O 4 (OH) 6 ·8(H 2 O)]. These findings suggest that U-As-Ca-bearing solids formed in mixed solutions are stable at pH 3. However, the dissolution of U-As-Ca and U-Ca-bearing solids at pH 7 was observed in reactors containing 10 mM bicarbonate and Ca, suggesting a kinetic reaction of aqueous uranyl-calcium-carbonate complexation. Our study provides new insights regarding U and As mobilization for risk assessment and remediation strategies.

Groundwater contamination by As from natural and anthropogenic sources is a worldwide concern. Redox heterogeneities over space and time are common and can influence the molecular-level speciation of As, and thus, As release/retention but are largely unexplored. Here, we present results from a dual-domain column experiment, with natural organic-rich, fine-grained, and sulfidic sediments embedded as lenses (referred to as "reducing lenses") within natural aquifer sand. We show that redox interfaces in sulfur-rich, alkaline aquifers may release concerning levels of As, even when sediment As concentration is low (<2 mg/kg), due to the formation of mobile thioarsenates at aqueous sulfide/Fe molar ratios <1. In our experiments, this behavior occurred in the aquifer sand between reducing lenses and was attributed to the spreading of sulfidic conditions and subsequent Fe reductive dissolution. In contrast, inside reducing lenses (and some locations in the aquifer) the aqueous sulfide/Fe molar ratios exceeded 1 and aqueous sulfide/As molar ratios exceeded 100, which partitioned As(III)-S to the solid phase (associated with organics or as realgar (As 4 S 4 )). These results highlight the importance of thioarsenates in natural sediments and indicate that redox interfaces and sediment heterogeneities could locally degrade groundwater quality, even in aquifers with unconcerning solid-phase As concentrations.

Uranium contamination threatens the availability of safe and clean drinking water globally. This toxic element occurs both naturally and as a result of mining and ore-processing in alluvial sediments, where it accumulates as tetravalent U [U(IV)], a form once considered largely immobile. Changing hydrologic and geochemical conditions cause U to be released into groundwater. Knowledge of the chemical form(s) of U(IV) is essential to understand the release mechanism, yet the relevant U(IV) species are poorly characterized. There is growing belief that natural organic matter (OM) binds U(IV) and mediates its fate in the subsurface. In this work, we combined nanoscale imaging (nano secondary ion mass spectrometry and scanning transmission X-ray microscopy) with a density-based fractionation approach to physically and microscopically isolate organic and mineral matter from alluvial sediments contaminated with uranium. We identified two populations of U (dominantly +IV) in anoxic sediments. Uranium was retained on OM and adsorbed to particulate organic carbon, comprising both microbial and plant material. Surprisingly, U was also adsorbed to clay minerals and OM-coated clay minerals. The dominance of OM-associated U provides a framework to understand U mobility in the shallow subsurface, and, in particular, emphasizes roles for desorption and colloid formation in its mobilization.

Lead (Pb) exposure is a major public health problem worldwide. Although high levels of Pb in blood in Bangladesh have been documented, the dominant Pb sources contributing to human exposure in rural Bangladesh have not been determined. Here, we first obtained blood from pregnant women from three rural Bangladeshi districts who were previously assessed by a case-control and sampling study, and we then conducted semistructured in-depth interviews to understand Pb exposure behavior and finally collected samples of the suspected Pb sources. We measured the Pb isotopic composition of both potential Pb sources and 45 blood samples in order to understand which of three sources predominate: (1) food from Pb-soldered cans, (2) turmeric, or (3) geophagous materials (clay, soil, or ash). The Pb isotope ratios of the three sources are distinct ( p = 0.0001) and blood isotope ratios are most similar to turmeric. Elevated lead and chromium (Cr) concentrations in turmeric and a yellow pigment used in turmeric processing are consistent with reported consumption behavior that indicated turmeric as a primary contributor to blood Pb. The Pb isotopic composition analyses combined with a case-control and sampling approach provides evidence that turmeric adulterated with the yellow Pb-bearing pigment is the main Pb exposure source in these districts and illustrates the need to assess drivers and practices of turmeric adulteration, as well as the prevalence of adulteration across South Asia.

Peatlands and other wetlands are sinks for antimony (Sb), and solid natural organic matter (NOM) may play an important role in controlling Sb binding. However, direct evidence of Sb sequestration in natural peat samples is lacking. Here, we analyzed solid phase Sb, iron (Fe), and sulfur (S) as well as aqueous Sb speciation in three profiles up to a depth of 80 cm in a mine water impacted peatland in northern Finland. Linear combination fittings of extended X-ray absorption fine structure spectra showed that Sb binding to Fe phases was of minor importance and observed only in the uppermost layers of the peatland. Instead, the dominant (to almost exclusive) sequestration mechanism was Sb(III) binding to oxygen-containing functional groups, and at greater depths, increasingly Sb(III) binding to thiol groups of NOM. Aqueous Sb speciation was dominated by antimonate, while antimonite concentrations were low, further supporting our findings of much higher reactivity of Sb(III) than Sb(V) toward peat surfaces. Insufficient residence time for efficient reduction of antimonate to antimonite currently hinders higher Sb removal in the studied peatland. Overall, our findings imply that Sb(III) binding to solid NOM acts as an important sequestration mechanism under reducing conditions in peatlands and other high-organic matter environments.

Peatlands and other wetlands with abundant natural organic matter (NOM) are important sinks for antimony (Sb). While formation of Sb(III) sulfide phases or Sb(III) binding to NOM are discussed to decrease Sb mobility, the exact binding mechanisms remain elusive. Here, we reacted increasing sulfide concentrations with purified model peat at pH 6, forming reduced organic sulfur species, and subsequently equilibrated the reaction products with 50 μM of antimonite under anoxic conditions. Sulfur solid-phase speciation and the local binding environment of Sb were analyzed using X-ray absorption spectroscopy. We found that 85% of antimonite was sorbed by untreated peat. Sulfide-reacted peat increased sorption to 98%. Shell-by-shell fitting of Sb K-edge X-ray absorption fine structure spectra revealed Sb in untreated peat bound to carboxyl or phenol groups with average Sb-carbon distances of ∼2.90 Å. With increasing content of reduced organic sulfur, Sb was progressively coordinated to S atoms at distances of ∼2.45 Å and Sb-carbon distances of ∼3.33 Å, suggesting increasing Sb-thiol binding. Iterative target factor analysis allowed exclusion of reduced inorganic Sb-sulfur phases with similar Sb-sulfur distances. In conclusion, even when free sulfide concentrations are too low for formation of Sb-sulfur precipitates, peat NOM can sequester Sb in anoxic, sulfur-enriched environments.

Groundwater resources in California represent a confluence of high-risk factors for hexavalent chromium contamination as a result of industrial activities, natural geology, and, potentially, land use. Here, we examine state-wide links in California between groundwater Cr(VI) concentrations and chemicals that provide signatures for source attribution. In environmental monitoring wells, Cr(VI) had the highest co-occurrence and also clustered with 1,4-dioxane and several chlorinated hydrocarbons indicative of the metal plating industry. Additionally, hotspots of Cr(VI) co-occurring with bromoform result from volatile organic compound remediation using in situ chemical oxidation that inadvertently oxidizes naturally occurring Cr(III). In groundwater supply wells, which are typically free of industrial inputs, Cr(VI) correlates with dichlorodiphenyldichloroethylene (DDE), vanadium, and ammonia and clusters with nitrate and dissolved oxygen, suggesting potential links between agricultural activities and Cr(VI). Specific controls on Cr(VI) vary substantially by region: from the metal plating industry around Los Angeles and the San Francisco Bay areas to natural redox conditions along flow paths in the Mojave Desert and to correlations with agricultural practices in the Central Valley of California. While industrial uses of Cr lead to the most acute cases of groundwater Cr(VI) contamination, oxidation of naturally occurring Cr affects a larger area, more wells, and a greater number of people throughout California.

Uranium (U) contamination occurs as a result of mining and ore processing; often in alluvial aquifers that contain organic-rich, reduced sediments that accumulate tetravalent U, U(IV). Uranium(IV) is sparingly soluble, but may be mobilized upon exposure to nitrate (NO 3 - ) and oxygen (O 2 ), which become elevated in groundwater due to seasonal fluctuations in the water table. The extent to which oxidative U mobilization can occur depends upon the transport properties of the sediments, the rate of U(IV) oxidation, and the availability of inorganic reductants and organic electron donors that consume oxidants. We investigated the processes governing U release upon exposure of reduced sediments to artificial groundwater containing O 2 or NO 3 - under diffusion-limited conditions. Little U was mobilized during the 85-day reaction, despite rapid diffusion of groundwater within the sediments and the presence of nonuraninite U(IV) species. The production of ferrous iron and sulfide in conjunction with rapid oxidant consumption suggested that the sediments harbored large concentrations of bioavailable organic carbon that fueled anaerobic microbial respiration and stabilized U(IV). Our results suggest that seasonal influxes of O 2 and NO 3 - may cause only localized mobilization of U without leading to export of U from the reducing sediments when ample organic carbon is present.

Providing access to safe drinking water is a global challenge, for which groundwater is increasingly being used throughout the world. However, geogenic contaminants limit the suitability of groundwater for domestic purposes over large geographic areas across most continents. Geogenic contaminants in groundwater are often evaluated individually, but here we demonstrate the need to evaluate multiple contaminants to ensure that groundwater is safe for human consumption and agricultural usage. We compiled groundwater chemical data from three aquifer regions across the world that have been reported to have widespread As and Mn contamination including the Glacial Aquifer in the U.S., the Ganges-Brahmaputra-Mehta Basin within Bangladesh, and the Mekong Delta in Cambodia, along with newly sampled wells in the Yangtze River Basin of China. The proportion of contaminated wells increase by up to 40% in some cases when both As and Mn contaminants are considered. Wilcoxon rank-sum analysis indicates that Mn contamination consistently occurs at significantly shallower depths than As contaminated wells in all regions. Arsenic concentrations in groundwater are well predicted by redox indicators (Eh and dissolved oxygen) whereas Mn shows no significant relationship with either parameter. These findings illustrate that the number of safe wells may be drastically overestimated in some regions when Mn contamination is not taken into account and that depth may be used as a distinguishing variable in efforts to predict the presence of groundwater contaminants regionally.

Chromium(VI) produced from the oxidation of indigenous Cr(III) minerals is increasingly being recognized as a threat to groundwater quality. A critical determinant of Cr(VI) generation within soils and sediments is the necessary interaction of two low-solubility phases-Cr(III) silicates or (hydr)oxides and Mn(III/IV) oxides-that lead to its production. Here we investigate the potential for Cr(III) oxidation by Mn oxides within fixed solid matrices common to soils and sediments. Artificial aggregates were constructed from Cr(OH) 3 - and Cr 0.25 Fe 0.75 (OH) 3 -coated quartz grains and either mixed with synthetic birnessite or inoculated with the Mn(II)-oxidizing bacterium Leptothrix cholodnii. In aggregates simulating low organic carbon environments, we observe Cr(VI) concentrations within advecting solutes at levels more than twenty-times the California drinking water standard. Chromium(VI) production is highly dependent on Cr-mineral solubility; increasing Fe-substitution (x = 0 to x = 0.75) decreases the solubility of the solid and concomitantly decreases total Cr(VI) generation by 37%. In environments with high organic carbon, reducing conditions within aggregate cores (microbially) generate sufficient Fe(II) to suppress Cr(VI) efflux. Our results illustrate Cr(VI) generation from reaction with Mn oxides within structured media simulating soils and sediments and provide insight into how fluctuating hydrologic and redox conditions impact coupled processes controlling Cr and Mn cycling.

Hexavalent chromium (Cr(VI)) is generated in serpentine soils and exported to surface and groundwaters at levels above health-based drinking water standards. Although Cr(VI) concentrations are elevated in serpentine soil pore water, few studies have reported field evidence documenting Cr(VI) production rates and fluxes that govern Cr(VI) transport from soil to water sources. We report Cr speciation (i) in four serpentine soil depth profiles derived from the California Coast Range serpentinite belt and (ii) in local surface waters. Within soils, we detected Cr(VI) in the same horizons where Cr(III)-minerals are colocated with biogenic Mn(III/IV)-oxides, suggesting Cr(VI) generation through oxidation by Mn-oxides. Water-extractable Cr(VI) concentrations increase with depth constituting a 7.8 to 12 kg/km 2 reservoir of Cr(VI) in soil. Here, Cr(VI) is produced at a rate of 0.3 to 4.8 kg Cr(VI)/km 2 /yr and subsequently flushed from soil during water infiltration, exporting 0.01 to 3.9 kg Cr(VI)/km 2 /yr at concentrations ranging from 25 to 172 μg/L. Although soil-derived Cr(VI) is leached from soil at concentrations exceeding 10 μg/L, due to reduction and dilution during transport to streams, Cr(VI) levels measured in local surface waters largely remain below California's drinking water limit.

Consumption of groundwater containing >10 μg L(-1) arsenic (As) adversely impacts more than 100 million people worldwide. Multiyear trends in aquifer As concentrations have been documented, but strong seasonal variations are not commonly observed. Here we report dramatic seasonal changes in As concentrations and aquifer chemistry within the Jianghan Plain of the Yangtze River, China. At some wells, concentrations fluctuate by more than an order of magnitude within a single year (100-1200 μg L(-1)). Groundwater extraction and sustained water levels of surface channels during the dry season induces a strong downward hydraulic gradient, seasonally supplying oxidizing (oxygen, nitrate) water to the otherwise anoxic aquifer. Oxygen and/or nitrate addition promotes a transient drop in As concentrations for 1-3 months. When recharge ceases, reducing, low-arsenic conditions are reestablished by reactive, endogenous organic carbon. Temporal variability in As concentrations is especially problematic because it increases the probability of false-negative well testing during low-arsenic seasons. However, periods of low As may also provide a source of less toxic water for irrigation or other uses. Our results highlight the vulnerability and variability of groundwater resources in the Jianghan Plain and other inland basins within Asia to changing geochemical conditions, both natural and anthropogenic, and reinforce that continued monitoring of wells in high-risk regions is essential.

Millions of individuals worldwide are chronically exposed to hazardous concentrations of arsenic from contaminated drinking water. Despite massive efforts toward understanding the extent and underlying geochemical processes of the problem, numerical modeling and reliable predictions of future arsenic behavior remain a significant challenge. One of the key knowledge gaps concerns a refined understanding of the mechanisms that underlie arsenic mobilization, particularly under the onset of anaerobic conditions, and the quantification of the factors that affect this process. In this study, we focus on the development and testing of appropriate conceptual and numerical model approaches to represent and quantify the reductive dissolution of iron oxides, the concomitant release of sorbed arsenic, and the role of iron-mineral transformations. The initial model development in this study was guided by data and hypothesized processes from a previously reported,1 well-controlled column experiment in which arsenic desorption from ferrihydrite coated sands by variable loads of organic carbon was investigated. Using the measured data as constraints, we provide a quantitative interpretation of the processes controlling arsenic mobility during the microbial reductive transformation of iron oxides. Our analysis suggests that the observed arsenic behavior is primarily controlled by a combination of reductive dissolution of ferrihydrite, arsenic incorporation into or co-precipitation with freshly transformed iron minerals, and partial arsenic redox transformations.

The Rifle alluvial aquifer along the Colorado River in west central Colorado contains fine-grained, diffusion-limited sediment lenses that are substantially enriched in organic carbon and sulfides, as well as uranium, from previous milling operations. These naturally reduced zones (NRZs) coincide spatially with a persistent uranium groundwater plume. There is concern that uranium release from NRZs is contributing to plume persistence or will do so in the future. To better define the physical extent, heterogeneity and biogeochemistry of these NRZs, we investigated sediment cores from five neighboring wells. The main NRZ body exhibited uranium concentrations up to 100 mg/kg U as U(IV) and contains ca. 286 g of U in total. Uranium accumulated only in areas where organic carbon and reduced sulfur (as iron sulfides) were present, emphasizing the importance of sulfate-reducing conditions to uranium retention and the essential role of organic matter. NRZs further exhibited centimeter-scale variations in both redox status and particle size. Mackinawite, greigite, pyrite and sulfate coexist in the sediments, indicating that dynamic redox cycling occurs within NRZs and that their internal portions can be seasonally oxidized. We show that oxidative U(VI) release to the aquifer has the potential to sustain a groundwater contaminant plume for centuries. NRZs, known to exist in other uranium-contaminated aquifers, may be regionally important to uranium persistence.

Mobilization of arsenic and other trace metal contaminants during managed aquifer recharge (MAR) poses a challenge to maintaining local groundwater quality and to ensuring the viability of aquifer storage and recovery techniques. Arsenic release from sediments into solution has occurred during purified recycled water recharge of shallow aquifers within Orange County, CA. Accordingly, we examine the geochemical processes controlling As desorption and mobilization from shallow, aerated sediments underlying MAR infiltration basins. Further, we conducted a series of batch and column experiments to evaluate recharge water chemistries that minimize the propensity of As desorption from the aquifer sediments. Within the shallow Orange County Groundwater Basin sediments, the divalent cations Ca(2+) and Mg(2+) are critical for limiting arsenic desorption; they promote As (as arsenate) adsorption to the phyllosilicate clay minerals of the aquifer. While native groundwater contains adequate concentrations of dissolved Ca(2+) and Mg(2+), these cations are not present at sufficient concentrations during recharge of highly purified recycled water. Subsequently, the absence of dissolved Ca(2+) and Mg(2+) displaces As from the sediments into solution. Increasing the dosages of common water treatment amendments including quicklime (Ca(OH)2) and dolomitic lime (CaO·MgO) provides recharge water with higher concentrations of Ca(2+) and Mg(2+) ions and subsequently decreases the release of As during infiltration.

When oxygen is limiting in soils and sediments, microorganisms utilize nitrate (NO3-) in respiration--through the process of denitrification--leading to the production of dinitrogen (N2) gas and trace amounts of nitrous (N2O) and nitric (NO) oxides. A chemical pathway involving reaction of ferrous iron (Fe2+) with nitrite (NO2-), an intermediate in the denitrification pathway, can also result in production of N2O. We examine the chemical reduction of NO2- by Fe(II)--chemodenitrification--in anoxic batch incubations at neutral pH. Aqueous Fe2+ and NO2- reacted rapidly, producing N2O and generating Fe(III) (hydr)oxide mineral products. Lepidocrotite and goethite, identified by synchrotron X-ray diffraction (XRD) and extended X-ray absorption fine structure (EXAFS) spectroscopy, were produced from initially aqueous reactants, with two-line ferrihydrite increasing in abundance later in the reaction sequence. Based on the similarity of apparent rate constants with different mineral catalysts, we propose that the chemodenitrification rate is insensitive to the type of Fe(III) (hydr)oxide. With stable isotope measurements, we reveal a narrow range of isotopic fractionation during NO2- reduction to N2O. The location of N isotopes in the linear N2O molecule, known as site preference, was also constrained to a signature range. The coexistence of Fe(III) (hydr)oxide, characteristic 15N and 18O fractionation, and N2O site preference may be used in combination to qualitatively distinguish between abiotic and biogenically emitted N2O--a finding important for determining N2O sources in natural systems.

Groundwater exploitation is rising in the Mekong Delta, Vietnam, potentially exacerbating arsenic contamination from natural sources. We investigate trends and controls on contamination patterns throughout the Delta's multi-aquifer system as observed in a spatially exhaustive data set of arsenic measured in >40,000 wells, 10.5% of which exceed the WHO drinking water standard for arsenic (10 μg/L). We relate strong trends in the distribution of contamination among well samples to explanatory variables derived from 3D ancillary physicochemical data sets using logistic regression models. Parsimonious models describe much of the observed variability in arsenic occurrence, which differs considerably between subsets of wells tapping shallow versus deeper aquifer groups. In the shallowest Holocene-Pleistocene aquifers, arsenic occurrence is best described by distance to the Mekong river channels and delta front, depth, and location within fault-bounded zones of the region. The same model, however, fails to explain observations in the deeper group of Pliocene-Miocene aquifers. Among these deeper units, arsenic occurrence is rare except among older wells in near-river, heavily pumped areas. Our analysis is the first to examine both natural and anthropogenically mediated contributions to the distribution of arsenic throughout the Mekong Delta's multi-aquifer system, with implications for management of similarly affected basins throughout Southeast Asia.

Despite the global importance of As in rice, research has primarily focused on Bangladesh, India, China, and the United States with limited attention given to other countries. Owing to both indigenous As within the soil and the possible increases arising from the onset of irrigation with groundwater, an assessment of As in rice within Cambodia is needed, which offers a "base-case" comparison against sediments of similar origin that comprise rice paddy soils where As-contaminated water is used for irrigation (e.g., Bangladesh). Here, we evaluated the As content of rice from five provinces (Kandal, Prey Veng, Battambang, Banteay Meanchey, and Kampong Thom) in the rice-growing regions of Cambodia and coupled that data to soil-chemical factors based on extractions of paddy soil collected and processed under anoxic conditions. At total soil As concentrations ranging 0.8 to 18 μg g(-1), total grain As concentrations averaged 0.2 μg g(-1) and ranged from 0.1 to 0.37 with Banteay Meanchey rice having significantly higher values than Prey Veng rice. Overall, soil-extractable concentrations of As, Fe, P, and Si and total As were poor predictors of grain As concentrations. While biogeochemical factors leading to reduction of As(V)-bearing Fe(III) oxides are likely most important for predicting plant-available As, husk and straw As concentrations were the most significant predictors of grain-As levels among our measured parameters.

High concentrations of uranium are commonly observed in naturally occurring amorphous silica (including opal) deposits, suggesting that incorporation of U into amorphous silica may represent a natural attenuation mechanism and promising strategy for U remediation. However, the stability of uranium in opaline silicates, determined in part by the binding mechanism for U, is an important factor in its long-term fate. U may bind directly to the opaline silicate matrix, or to materials such as iron (hydr)oxides that are subsequently occluded within the opal. Here, we examine the coordination environment of U within opaline silica to elucidate incorporation mechanisms. Precipitates (with and without ferrihydrite inclusions) were synthesized from U-bearing sodium metasilicate solutions, buffered at pH ∼ 5.6. Natural and synthetic solids were analyzed with X-ray absorption spectroscopy and a suite of other techniques. In synthetic amorphous silica, U was coordinated by silicate in a double corner-sharing coordination geometry (Si at ∼ 3.8-3.9 Å) and a small amount of uranyl and silicate in a bidentate, mononuclear (edge-sharing) coordination (Si at ∼ 3.1-3.2 Å, U at ∼ 3.8-3.9 Å). In iron-bearing synthetic solids, U was adsorbed to iron (hydr)oxide, but the coordination environment also contained silicate in both edge-sharing and corner-sharing coordination. Uranium local coordination in synthetic solids is similar to that of natural U-bearing opals that retain U for millions of years. The stability and extent of U incorporation into opaline and amorphous silica represents a long-term repository for U that may provide an alternative strategy for remediation of U contamination.

Arsenic-contaminated rice grain may threaten human health globally. Since H₃AsO₃⁰ is the predominant As species found in paddy pore-waters, and H₄SiO₄⁰ and H₃AsO₃⁰ share an uptake pathway, silica amendments have been proposed to decrease As uptake and consequent As concentrations in grains. Here, we evaluated the impact of two silicate mineral additions differing in solubility (+Si(L), diatomaceous earth, 0.29 mM Si; +Si(H), Si-gel, 1.1 mM Si) to soils differing in mineralogy on arsenic concentration in rice. The +Si(L) addition either did not change or decreased As concentration in pore-water but did not change or increased grain-As levels relative to the (+As--Si) control. The +Si(H) addition increased As in pore-water, but it significantly decreased grain-As relative to the (+As--Si) control. Only the +Si(H) addition resulted in significant increases in straw- and husk-Si. Total grain- and straw-As was negatively correlated with pore-water Si, and the relationship differed between two soils exhibiting different mineralogy. These differing results are a consequence of competition between H₄SiO₄⁰ and H₃AsO₃⁰ for adsorption sites on soil solids and subsequent plant-uptake, and illustrate the importance of Si mineralogy on arsenic uptake.

The speciation and partitioning of arsenic (As) in surface and subsurface environments are controlled, in part, by redox processes. Within soils and sediments, redox gradients resulting from mass transfer limitations lead to competitive reduction-oxidation reactions that drive the fate of As. Accordingly, the objective of this study was to determine the fate and redox cycling of As at the interface of birnessite (a strong oxidant in soil with a nominal formula of MnO(x), where x ≈ 2) and dissimilatory As(V)-reducing bacteria (strong reductant). Here, we investigate As reduction-oxidation dynamics in a diffusively controlled system using a Donnan reactor where birnessite and Shewanella sp. ANA-3 are isolated by a semipermeable membrane through which As migrates. Arsenic(III) injected into the reaction cell containing birnessite is rapidly oxidized to As(V). Arsenic(V) diffusing into the Shewanella chamber is then reduced to As(III), which subsequently diffuses back to the birnessite chamber, undergoing oxidation, and establishing a continuous cycling of As. However, we observe a rapid decline in the rate of As(III) oxidation owing to passivation of the birnessite surface. Modeling and experimental results show that high [Mn(II)] combined with increasing [CO(3)(2-)] from microbial respiration leads to the precipitation of rhodochrosite, which eventually passivates the Mn oxide surface, inhibiting further As(III) oxidation. Our results show that despite the initial capacity of birnessite to rapidly oxidize As(III), the synergistic effect of intense As(V) reduction by microorganisms and the buildup of reactive metabolites capable of passivating reactive mineral surfaces-here, birnessite-will produce (bio)geochemical conditions outside of those based on thermodynamic predictions.

Soluble Fe(II) can reduce soluble U(VI) at rapid rates and in accordance with thermodynamic predictions. This was established by initially creating acidic aqueous solutions in which the sole oxidants were soluble U(VI) species and the sole reductants were soluble Fe(II) species. The pH of the solution was then increased by stepwise addition of OH(-), thereby increasing the potential for electron transfer from Fe(II) to U(VI). For each new pH value resulting from addition of base, values of ΔG for the Fe(II)-mediated reduction of U(VI) were calculated using the computed distribution of U and Fe species and possible half reaction combinations. For initial conditions of pH 2.4 and a molar ratio of Fe(II) to U(VI) of 5:1 (1 mM Fe(II) and 0.2 mM U(VI)), ΔG for U(VI) reduction was greater than zero, and U(VI) reduction was not observed. When sufficient OH(-) was added to exceed the computed equilibrium pH of 5.4, ΔG for U(VI) reduction was negative and soluble Fe(II) species reacted with U(VI) in a molar ratio of ∼2:1. X-ray absorption near-edge structure (XANES) spectroscopy confirmed production of U(IV). A decrease in pH confirmed production of acidity as the reaction advanced. As solution pH decreased to the equilibrium value, the rate of reaction declined, stopping completely at the predicted equilibrium pH. Initiation of the reaction at a higher pH resulted in a higher final ratio of U(IV) to U(VI) at equilibrium.

Nanoscale zerovalent iron particles (nZVI), bimetallic nanoparticles (nZVI/Pd), and nZVI/Pd impregnated activated carbon (nZVI/Pd-AC) composite particles were synthesized and investigated for their effectiveness to remove polybrominated diphenyl ethers (PBDEs) and/or polychlorinated biphenyls (PCBs). Palladization of nZVI promoted the dehalogenation kinetics for mono- to tri-BDEs and 2,3,4-trichlorobiphenyl (PCB 21). Compared to nZVI, the iron-normalized rate constants for nZVI/Pd were about 2-, 3-, and 4-orders of magnitude greater for tri-, di-, and mono-BDEs, respectively, with diphenyl ether as a main reaction product. The reaction kinetics and pathways suggest an H-atom transfer mechanism. The reaction pathways with nZVI/Pd favor preferential removal of para-halogens on PBDEs and PCBs. X-ray fluorescence mapping of nZVI/Pd-AC showed that Pd mainly deposits on the outer part of particles, while Fe was present throughout the activated carbon particles. While BDE 21 was sorbed onto activated carbon composites quickly, debromination was slower compared to reaction with freely dispersed nZVI/Pd. Our XPS and chemical data suggest about 7% of the total iron within the activated carbon was zerovalent, which shows the difficulty with in-situ synthesis of a significant fraction of zerovalent iron in the microporous material. Related factors that likely hinder the reaction with nZVI/Pd-AC are the heterogeneous distribution of nZVI and Pd on activated carbon and/or immobilization of hydrophobic organic contaminants at the adsorption sites thereby inhibiting contact with nZVI.

Soils are an aggregate-based structured media that have a multitude of pore domains resulting in varying degrees of advective and diffusive solute and gas transport. Consequently, a spectrum of biogeochemical processes may function at the aggregate scale that collectively, and coupled with solute transport, determine element cycling in soils and sediments. To explore how the physical structure impacts biogeochemical processes influencing the fate and transport of As, we examined temporal changes in speciation and distribution of As and Fe within constructed aggregates through experimental measurement and reactive transport simulations. Spherical aggregates were made with As(V)-bearing ferrihydrite-coated sand inoculated with Shewanella sp. ANA-3; aerated solute flow around the aggregate was then induced. Despite the aerated aggregate exterior, where As(V) and ferrihydrite persist as the dominant species, anoxia develops within the aggregate interior. As a result, As and Fe redox gradients emerge, and the proportion of As(III) and magnetite increases toward the aggregate interior. Arsenic(III) and Fe(II) produced in the interior migrate toward the aggregated exterior and result in coaccumulation of As and Fe(III) proximal to preferential flow paths as a consequence of oxygenic precipitation. The oxidized rind of aggregates thus serves as a barrier to As release into advecting pore-water, but also leads to be a buildup of this hazardous element at preferential flow boundaries that could be released upon shifting geochemical conditions.

Natural organic matter (NOM) can affect the behavior of arsenic within surface and subsurface environments. We used batch and column experiments to determine the effect of peat humic acids (PHA), groundwater fulvic acids (GFA), and a soil organic matter (SOM) extract on As sorption/transport in ferrihydrite-coated sand columns. A reactive transport model was used to quantitatively interpret the transport of As in flow-through column (breakthrough) experiments. We found that As(III) breakthrough was faster than As(V) by up to 18% (with OM) and 14% (without OM). The most rapid breakthrough occurred in systems containing SOM and GFA. Dialysis and ultrafiltration of samples from breakthrough experiments showed that in OM-containing systems, As was transported mostly as free (noncomplexed) dissolved As but also as ternary As-Fe-OM colloids and dissolved complexes. In OM-free systems, As was transported in colloidal form or as a free ion. During desorption, more As(III) desorbed (23-37%) than As(V) (10-16%), and SOM resulted in the highest and OM-free systems the lowest amount of desorption. Overall, our experiments reveal that (i) NOM can enhance transport/mobilization of As, (ii) different fractions of NOM are capable of As mobilization, and (iii) freshly extracted SOM (from a forest soil) had greater impact on As transport than purified GFA/PHA.

Arsenic contamination of rice is widespread, but the rhizosphere processes influencing arsenic attenuation remain unresolved. In particular, the formation of Fe plaque around rice roots is thought to be an important barrier to As uptake, but the relative importance of this mechanism is not well characterized. Here we elucidate the colocalization of As species and Fe on rice roots with variable Fe coatings; we used a combination of techniques--X-ray fluorescence imaging, μXANES, transmission X-ray microscopy, and tomography--for this purpose. Two dominant As species were observed in fine roots-inorganic As(V) and As(III) -with minor amounts of dimethylarsinic acid (DMA) and arsenic trisglutathione (AsGlu(3)). Our investigation shows that variable Fe plaque formation affects As entry into rice roots. In roots with Fe plaque, As and Fe were strongly colocated around the root; however, maximal As and Fe were dissociated and did not encapsulate roots that had minimal Fe plaque. Moreover, As was not exclusively associated with Fe plaque in the rice root system; Fe plaque does not coat many of the young roots or the younger portion of mature roots. Young, fine roots, important for solute uptake, have little to no iron plaque. Thus, Fe plaque does not directly intercept (and hence restrict) As supply to and uptake by rice roots but rather serves as a bulk scavenger of As predominantly near the root base.

Adsorption on soil and sediment solids may decrease aqueous uranium concentrations and limit its propensity for migration in natural and contaminated settings. Uranium adsorption will be controlled in large part by its aqueous speciation, with a particular dependence on the presence of dissolved calcium and carbonate. Here we quantify the impact of uranyl speciation on adsorption to both goethite and sediments from the Hanford Clastic Dike and Oak Ridge Melton Branch Ridgetop formations. Hanford sediments were preconditioned with sodium acetate and acetic acid to remove carbonate grains, and Ca and carbonate were reintroduced at defined levels to provide a range of aqueous uranyl species. U(VI) adsorption is directly linked to UO(2)(2+) speciation, with the extent of retention decreasing with formation of ternary uranyl-calcium-carbonato species. Adsorption isotherms under the conditions studied are linear, and K(d) values decrease from 48 to 17 L kg(-1) for goethite, from 64 to 29 L kg (-1) for Hanford sediments, and from 95 to 51 L kg(-1) for Melton Branch sediments as the Ca concentration increases from 0 to 1 mM at pH 7. Our observations reveal that, in carbonate-bearing waters, neutral to slightly acidic pH values ( approximately 5) and limited dissolved calcium are optimal for uranium adsorption.

Within soils, biogeochemical processes controlling elemental cycling are heterogeneously distributed owing, in large part, to the physical complexity of the media. Here we quantify how diffusive mass-transfer limitation at the soil aggregate scale controls the biogeochemical processes governing ferrihydrite reductive dissolution and secondary iron mineral formation. Artificial cm-scale aggregates made of ferrihydrite-coated sand inoculated with iron-reducing bacteria were placed in flow-through reactors, mimicking macro- and microporous soil environments. A reactive transport model was developed to delineate diffusively and advectively controlled regions, identify reaction zones and estimate kinetic parameters. Simulated iron (Fe) breakthrough-curves show good agreement with experimental results for a wide-range of flow rates and input lactate concentrations, with only a limited amount (< or =12%) of Fe lost in the reactor outflow over a 31 day period. Model simulations show substantial intra-aggregate, mm-scale radial variations in the secondary iron phase distributions, reproducing the trends observed experimentally where only limited transformation of ferrihydrite was found near the aggregate surface, whereas extensive formation of goethite/lepidocrocite and minor amounts of magnetite and/or siderite were observed toward the aggregate center. Our study highlights the important control of variations in transport intensities on microbially induced iron transformation at the soil aggregate scale.

The oxidation state of uranium plays a major role in determining uranium mobility in the environment. Under anaerobic conditions, common metal respiring bacteria enzymatically reduce soluble U(VI) to U(IV), resulting in the formation of sparingly soluble UO(2(bio)) (biogenic uraninite). The stability of biologically precipitated uraninite is critical for determining the long-term fate of uranium and is not well characterized within soils and sediments. Here, we demonstrate that biogenic uraninite oxidation by ferrihydrite, an environmentally ubiquitous, disordered Fe(III) (hydr)oxide, appears to proceed through a soluble U(IV) intermediate and results in the concomitant production of Fe(II) and dissolved U(VI). Uraninite oxidation rates are accelerated under conditions that increase its solubility and decrease uraninite surface passivation, which include high bicarbonate concentration and pH values deviating from neutrality. Thus, our results demonstrate that UO(2(bio)) oxidation by Fe(III) (hydr)oxides is controlled by the rate of uraninite dissolution and that this process may limit uranium(IV) sequestration in the presence of Fe(III) (hydr)oxides.

The form of solid phase U after Fe(II) induced anaerobic remineralization of ferrihydrite in the presence of aqueous and absorbed U(VI) was investigated under both abiotic batch and biotic flow conditions. Experiments were conducted with synthetic ground waters containing 0.168 mM U(VI), 3.8 mM carbonate, and 3.0 mM Ca2+. In spite of the high solubility of U(VI) under these conditions, appreciable removal of U(VI) from solution was observed in both the abiotic and biotic systems. The majority of the removed U was determined to be substituted as oxidized U (U(VI) or U(V)) into the octahedral position of the goethite and magnetite formed during ferrihydrite remineralization. It is estimated that between 3 and 6% of octahedral Fe(III) centers in the new Fe minerals were occupied by U. This site specific substitution is distinct from the nonspecific U coprecipitation processes in which uranyl compounds, e.g., uranyl hydroxide or carbonate, are entrapped within newly formed Fe oxides. The prevalence of site specific U incorporation under both abiotic and biotic conditions and the fact that the produced solids were shown to be resistant to both extraction (30 mM KHCO3) and oxidation (air for 5 days) suggest the potential importance of sequestration in Fe oxides as a stable and immobile form of U in the environment.

Reaction pathways resulting in uranium-bearing solids that are stable (i.e., having limited solubility) under aerobic and anaerobic conditions will limit dissolved concentrations and migration of this toxin. Here, we examine the sorption mechanism and propensity for release of uranium reacted with Fe (hydr)oxides under cyclic oxidizing and reducing conditions. Upon reaction of ferrihydrite with Fe(II) under conditions where aqueous Ca-UO2-CO3 species predominate (3 mM Ca and 3.8 mM total CO3), dissolved uranium concentrations decrease from 0.16 mM to below detection limit (BDL) after 5-15 d, depending on the Fe(II) concentration. In systems undergoing 3 successive redox cycles (14 d of reduction, followed by 5 d of oxidation) and a pulsed decrease to 0.15 mM total CO3, dissolved uranium concentrations varied depending on the Fe(II) concentration during the initial and subsequent reduction phases. U concentrations resulting during the oxic "rebound" varied inversely with the Fe(II) concentration during the reduction cycle. Uranium removed from solution remains in the oxidized form and is found adsorbed onto and incorporated into the structure of newly formed goethite and magnetite. Our results reveal that the fate of uranium is dependent on anaerobic/ aerobic conditions, aqueous uranium speciation, and the fate of iron.

Arsenic is a widespread environmental toxin having devastating impacts on human health. A transition to anaerobic conditions is a key driver for promoting As desorption through either the reduction of As(V) or the reductive dissolution of Fe(III) (hydr)oxides. However, a disparity in the reported release sequence for As and Fe to the aqueous solution hinders our ability to determine the controlling factors liberating As to the aqueous environment. Accordingly, we performed a thermodynamic analysis of Fe, using a range of Fe-(hydr)oxides, and As reduction coupled with hydrogen, acetate, and lactate oxidation for a range of relevant field conditions. The favorability of sulfate reduction is also evaluated. Our results illustrate that As reduction is favorable over a wide-range of field conditions, and Fe reduction is differentially favorable depending on the buildup of metabolites (mainly Fe2+) and the Fe (hydr)oxide being reduced; reduction of As(V) is thermodynamically favorable under most environmental conditions and almost always more favorable than goethite and hematite reduction. Sulfate reduction is favorable over a range of conditions, and may occur concomitantly with Fe reduction depending on the Fe (hydr)oxides present. Thus, on a thermodynamic basis, the general sequence of microbial reduction should be As(V) followed by Fe(III) or sulfate.

The fate and transport of arsenic is regulated, in part, by its strong affinity for iron (hydr)oxides. A transition from aerobic to anaerobic conditions resulting in concomitant reduction of both As(V) and iron (hydr)oxides can thus have a pronounced influence on As partitioning. However, it is presently unclear whether As desorption under anaerobic conditions results predominantly from a transformation from As(V) to As(III) or from mineralogical changes as a consequence of iron and manganese reduction. Here, we examine desorption of both As(III) and As(V) from ferrihydrite-, goethite-, and hematite-coated sand under hydrodynamic conditions. Furthermore, to resolve the relative role of Fe(III) and/or As(V) reduction in regulating dissolved As concentrations, we also examined As desorption from ferrihydrite- and goethite-coated sands presorbed with As(V) using wild type or mutants of Shewanella sp. ANA-3, capable of Fe(III)- and/or As(V)-reduction. We reveal substantial differences in As(III) and As(V) desorption from ferrihydrite, goethite, and hematite. Despite being adsorbed to a greater extent than As(V), As(III) is desorbed more rapidly and extensively from all oxides, suggesting weaker binding of As(III) than As(V). When As(V) and Fe(III) reduction are decoupled, As(V) reduction appears to be the dominant process controlling As release. Our results also suggest the importance of appreciating physical properties of specific Fe (hydr)oxides when predicting the potential for As desorption.

Arsenic-containing minerals mobilized during mining activities and deposited to Lake Coeur d'Alene (CDA), Idaho sediments represent a potential source of soluble As to the overlying water. Our objective was to delineate the processes controlling porewater As concentrations within Lake CDA sediments. Sediment and porewater As concentrations were determined, and solid-phase As associations were probed using X-ray absorption near-edge structure (XANES) spectroscopy. Although maximum As in the sediment porewaters varied from 8.4 to 16.2 microM, As sorption on iron oxyhydroxides at the oxic sediment-water interface prevented flux to overlying water. Floods deposit sediment containing variable amounts of arsenopyrite (FeAsS), with majorfloods depositing large amounts of sediment that bury and preserve reduced minerals. Periods of lower deposition increase sediment residence times in the oxic zone, promoting oxidation of reduced minerals, SO4(2-) efflux, and formation of oxide precipitates. Depositional events bury oxides containing sorbed As, transitioning them into anoxic environments where they undergo dissolution, releasing As to the porewater. High Fe:S ratios limit the formation of arsenic sulfides in the anoxic zone. As a result of As sequestration at the sediment-water interface and its release upon burial, decreased concentrations of porewater As will not occur unless As-bearing erosional inputs are eliminated.