Hexavalent Chromium Generation within Naturally Structured Soils and Sediments

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_0.25Fe_0.75(OH)₃-coated quartz grains and either mixed with synthetic birnessite or inoculated with the Mn­(II)-oxidizing bacterium <i>Leptothrix cholodnii</i>. 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 (<i>x</i> = 0 to <i>x</i> = 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

Silicate Mineral Impacts on the Uptake and Storage of Arsenic and Plant Nutrients in Rice (Oryza sativa L.)

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

Arsenic in the Multi-aquifer System of the Mekong Delta, Vietnam: Analysis of Large-Scale Spatial Trends and Controlling Factors

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

Quantifying Cr(VI) Production and Export from Serpentine Soil of the California Coast Range

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/km² reservoir of Cr­(VI) in soil. Here, Cr­(VI) is produced at a rate of 0.3 to 4.8 kg Cr­(VI)/km²/yr and subsequently flushed from soil during water infiltration, exporting 0.01 to 3.9 kg Cr­(VI)/km²/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

Hexavalent Chromium Sources and Distribution in California Groundwater

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

Arsenic Concentrations in Paddy Soil and Rice and Health Implications for Major Rice-Growing Regions of Cambodia

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

Morphological Adaptations for Digging and Climate-Impacted Soil Properties Define Pocket Gopher (<i>Thomomys</i> spp.) Distributions

<div><p>Species ranges are mediated by physiology, environmental factors, and competition with other organisms. The allopatric distribution of five species of northern Californian pocket gophers (<i>Thomomys</i> spp.) is hypothesized to result from competitive exclusion. The five species in this environmentally heterogeneous region separate into two subgenera, <i>Thomomys</i> or <i>Megascapheus</i>, which have divergent digging styles. While all pocket gophers dig with their claws, the tooth-digging adaptations of subgenus <i>Megascapheus</i> allow access to harder soils and climate-protected depths. In a Northern Californian locality, replacement of subgenus <i>Thomomys</i> with subgenus <i>Megascapheus</i> occurred gradually during the Pleistocene-Holocene transition. Concurrent climate change over this transition suggests that environmental factors – in addition to soil – define pocket gopher distributional limits. Here we show 1) that all pocket gophers occupy the subset of less energetically costly soils and 2) that subgenera sort by percent soil clay, bulk density, and shrink-swell capacity (a mineralogical attribute). While clay and bulk density (without major perturbations) stay constant over decades to millennia, low precipitation and high temperatures can cause shrink-swell clays to crack and harden within days. The strong yet underappreciated interaction between soil and moisture on the distribution of vertebrates is rarely considered when projecting species responses to climatic change. Furthermore, increased precipitation alters the weathering processes that create shrink-swell minerals. Two projected outcomes of ongoing climate change—higher temperatures and precipitation—will dramatically impact hardness of soil with shrink-swell minerals. Current climate models do not include factors controlling soil hardness, despite its impact on all organisms that depend on a stable soil structure.</p></div

Geochemical Triggers of Arsenic Mobilization during Managed Aquifer Recharge

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 Ca²⁺ and Mg²⁺ 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 Ca²⁺ and Mg²⁺, these cations are not present at sufficient concentrations during recharge of highly purified recycled water. Subsequently, the absence of dissolved Ca²⁺ and Mg²⁺ 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 Ca²⁺ and Mg²⁺ ions and subsequently decreases the release of As during infiltration

Water Supply Planning in the Face of Drought and Ecosystem Flows: Examining the Impact of the Bay-Delta Plan on Bay Area Water Supply

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 m3) 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

Stable Isotopes and Iron Oxide Mineral Products as Markers of Chemodenitrification.

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