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Emerging evidence points out that the responses of soil organic carbon (SOC) to nitrogen (N) addition differ along the soil profile, highlighting the importance of synthesizing results from different soil layers. Here, using a global meta-analysis, we found that N addition significantly enhanced topsoil (0-30 cm) SOC by 3.7% (±1.4%) in forests and grasslands. In contrast, SOC in the subsoil (30-100 cm) initially increased with N addition but decreased over time. The model selection analysis revealed that experimental duration and vegetation type are among the most important predictors across a wide range of climatic, environmental, and edaphic variables. The contrasting responses of SOC to N addition indicate the importance of considering deep soil layers, particularly for long-term continuous N deposition. Finally, the lack of depth-dependent SOC responses to N addition in experimental and modeling frameworks has likely resulted in the overestimation of changes in SOC storage under enhanced N deposition., (© 2024 The Authors. Global Change Biology published by John Wiley & Sons Ltd.)

Manganese (Mn) is currently regulated as a secondary contaminant in California, USA; however, recent revisions of the World Health Organization drinking water guidelines have increased regulatory attention of Mn in drinking water due to increasing reports of neurotoxic effects in infants and children. In this study, Mn concentrations reported to California's Safe Drinking Water Information System were used to estimate the potentially exposed population within California based on system size. We estimate that between 2011 and 2021, over 525,000 users in areas with reported Mn data are potentially exposed to Mn concentrations exceeding the WHO health-based guideline (80 μg L -1 ), and over 34,000 users are potentially exposed to Mn concentrations exceeding the U.S. Environmental Protection Agency health-advisory limit (300 μg L -1 ). Water treatment significantly decreased Mn concentrations compared to intake concentrations for all system sizes. However, smaller water systems have a wider range and a higher skew of Mn concentrations in finished water than larger systems. Additionally, higher Mn concentrations were found in systems above the maximum contaminant levels for chromium and arsenic. The treatment of these primary contaminants appears to also remove Mn. Lastly, data missingness remains a barrier to accurately assess public exposure to Mn in very small, small, and medium community water system-delivered water., Competing Interests: The authors declare no competing financial interest., (© 2023 The Authors. Published by American Chemical Society.)

Over 1.3 million Californians rely on unmonitored domestic wells. Existing probability estimates of groundwater Mn concentrations, population estimates, and sociodemographic data were integrated with spatial data delineating domestic well communities (DWCs) to predict the probability of high Mn concentrations in extracted groundwater within DWCs in California's Central Valley. Additional Mn concentration data of water delivered by community water systems (CWSs) were used to estimate Mn in public water supply. We estimate that 0.4% of the DWC population (2342 users) rely on groundwater with predicted Mn > 300 μg L -1 . In CWSs, 2.4% of the population (904 users) served by small CWSs and 0.4% of the population (3072 users) served by medium CWS relied on drinking water with mean point-of-entry Mn concentration >300 μg L -1 . Small CWSs were less likely to report Mn concentrations relative to large CWSs, yet a higher percentage of small CWSs exceed regulatory standards relative to larger systems. Modeled calculations do not reveal differences in estimated Mn concentration between groundwater from current regional domestic well depth and 33 m deeper. These analyses demonstrate the need for additional well-monitoring programs that evaluate Mn and increased access to point-of-use treatment for domestic well users disproportionately burdened by associated costs of water treatment.

Anthropogenic emissions of vanadium (V) into terrestrial and aquatic surface systems now match those of geogenic processes, and yet, the geochemistry of vanadium is poorly described in comparison to other comparable contaminants like arsenic. In oxic systems, V is present as an oxyanion with a +5 formal charge on the V center, typically described as H x VO 4 (3- x )- , but also here as V(V). Iron (Fe) and manganese (Mn) (oxy)hydroxides represent key mineral phases in the cycling of V(V) at the solid-solution interface, and yet, fundamental descriptions of these surface-processes are not available. Here, we utilize extended X-ray absorption fine structure (EXAFS) and thermodynamic calculations to compare the surface complexation of V(V) by the common Fe and Mn mineral phases ferrihydrite, hematite, goethite, birnessite, and pyrolusite at pH 7. Inner-sphere V(V) complexes were detected on all phases, with mononuclear V(V) species dominating the adsorbed species distribution. Our results demonstrate that V(V) adsorption is exergonic for a variety of surfaces with differing amounts of terminal -OH groups and metal-O bond saturations, implicating the conjunctive role of varied mineral surfaces in controlling the mobility and fate of V(V) in terrestrial and aquatic systems., Competing Interests: The authors declare no competing financial interest., (© 2022 The Authors. Published by American Chemical Society.)

Vanadium is a redox-active metal that has been added to the EPA's Contaminant Candidate List with a notification level of 50 μg L -1 due to mounting evidence that V V exposure can lead to adverse health outcomes. Groundwater V concentration exceeds the notification level in many locations, yet geochemical controls on its mobility are poorly understood. Here, we examined the redox interaction between V IV and birnessite (MnO 2 ), a well-characterized oxidant and a scavenger of many trace metals. In our findings, birnessite quickly oxidized sparingly soluble V IV species such as häggite [V 2 O 3 (OH) 2 ] into highly mobile and toxic vanadate (H n VO 4 (3- n )- ) in continuously stirred batch reactors under neutral pH conditions. Synchrotron X-ray absorption spectroscopic (XAS) analysis of in situ and ex situ experiments showed that oxidation of V IV occurs in two stages, which are both rapid relative to the measured dissolution rate of the V IV solid. Concomitantly, the reduction of birnessite during V IV oxidation generated soluble Mn II , which led to the formation of the Mn III oxyhydroxide feitknechtite (β-MnOOH) upon back-reaction with birnessite. XAS analysis confirmed a bidentate-mononuclear edge-sharing complex formed between V V and birnessite, although retention of V V was minimal relative to the aqueous quantities generated. In summary, we demonstrate that Mn oxides are effective oxidants of V IV in the environment with the potential to increase dissolved V concentrations in aquifers subject to redox oscillations.

Ceramic water filters (CWFs) are produced globally using local clay sources and can effectively remove bacterial pathogens during point-of-use water treatment. The ceramic production process involves firing clay mixed with burnout material at temperatures of 800-1100 °C, which induces mineralogical changes leading to increased arsenic (As) leaching from CWF material compared to source clay. Unfired clay and fired CWFs from Cambodia, Canada, and Mexico, CWF from Laos, and test-fired clay from the United States were analyzed to determine the extent of As leaching from CWFs that range in As (<1 to 16 mg kg -1 ) and iron (Fe) (0.6 to 5%) content. Deionized water, NaOH, HCl, and oxalate extractions showed that firing increased As solubility and decreased Fe solubility compared to unfired clay, with up to 8 mg kg -1 of water-soluble As in Cambodian CWFs. X-ray absorption spectra of the Cambodian clay and CWF showed a decrease in the Fe-O distance from 2.01 to 1.91 Å and decreased Fe coordination number from 6.3 to 4.6 after firing, indicating a decrease in Fe-O coordination. Arsenic(V) was the dominant species in Cambodia clay and CWF, existing primarily as a surface complex with average As-Fe distance of 3.28 Å in clay while in CWF As was either an outer-sphere As(V) phase or a discrete arsenate phase with no significant As-Fe scattering contribution within the resolution of the data. Improved understanding of molecular-scale processes that cause increased As leaching from CWFs provides a basis for assessing As leaching potential prior to CWF factory capital investment as well as engineered solutions (e.g., modified firing temperature, material amendments, and leaching prior to distribution) to mitigate As exposure from CWFs.

Though thallium (Tl) is usually present in trace amounts in natural environments, its biotoxicity is extremely high. With the development of mining, the metallurgy industry, and the growing application of Tl in high-tech fields, the threat of Tl to ecological environments and human health is increasing. Natural minerals, such as clay minerals, iron oxides, and manganese oxides, are natural Tl adsorbents due to their mineralogy and crystal structures. In this review, we discuss the mechanisms of Tl adsorption by various natural minerals, compare the adsorption capacities of common soil minerals for Tl, and describe the limitations of traditional sequential extraction methods for identifying the chemical states of Tl on minerals and source of Tl. We also provide suggestions on future directions needed in Tl research including a) additional in-depth studies on the competitive adsorption of Tl by minerals; b) more direct comparison of Tl adsorption behavior from lab-based experiments with field observations to clarify the mechanisms of Tl adsorption by minerals under environmental conditions; c) more research data are needed to support the establishment and improvement of relevant research methods based on modern leading-edge technologies such as synchrotron radiation. Further, we suggest further research is needed in adsorption technologies used for Tl treatment. This is the first review on the research progress of Tl adsorption by natural minerals with the purpose of helping understanding the mechanisms of Tl migration and transformation controlled by natural minerals, and providing theoretical supports for the development of Tl adsorbents and the treatments of Tl pollution., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2021 Elsevier B.V. All rights reserved.)

The role of Fe oxyhydroxides dynamic on metal bioavailability was studied in the Rio Doce estuary after the largest mining disaster in the world. Soon after the disaster in 2015, metals were associated with Fe oxyhydroxides under a redox-active estuarine environment. Our results indicate that organic matter inputs from plant colonization on deposited tailings over estuarine soils led to a reductive dissolution of Fe oxyhydroxides within two years. Soil pseudo-total Fe content decreased by 70% between 2015 and 2017, while the total metal contents (Cr, Cu, Ni, Pb, and Zn) decreased by 79% in the soil. The losses of Fe and metals coupled to changes in Fe oxides crystallinity reveal a future ephemeral control of Fe oxyhydroxides over metal immobilization. Our results suggest a potential chronic contamination at the estuary and points to an aggravating scenario for the following years due to the increasing dominance of poorly crystalline Fe oxyhydroxides., (Copyright © 2021 Elsevier Ltd. All rights reserved.)

Population growth and climate variability highlight the need to enhance freshwater security and diversify water supplies. Subsurface storage of water in depleted aquifers is increasingly used globally to alleviate disparities in water supply and demand often caused by climate extremes including floods and droughts. Managed aquifer recharge (MAR) stores excess water supplies during wet periods via infiltration into shallow underlying aquifers or direct injection into deep aquifers for recovery during dry seasons. Additionally, MAR can be designed to improve recharge water quality, particularly in the case of soil aquifer treatment and riverbank filtration. While there are many potential benefits to MAR, introduction of recharge water can alter the native geochemical and hydrological conditions in the receiving aquifer, potentially mobilizing toxic, naturally occurring (geogenic) contaminants from sediments into groundwater where they pose a much larger threat to human and ecosystem health. On the basis of the present literature, arsenic poses the most widespread challenge at MAR sites due to its ubiquity in subsurface sediments and toxicity at trace concentrations. Other geogenic contaminants of concern include fluoride, molybdenum, manganese, and iron. Water quality degradation threatens the viability of some MAR projects with several sites abandoning operations due to arsenic or other contaminant mobilization. Here, we provide a critical review of studies that have uncovered the geochemical and hydrological mechanisms controlling mobilization of arsenic and other geogenic contaminants at MAR sites worldwide, including both infiltration and injection sites. These mechanisms were evaluated based on site-specific characteristics, including hydrological setting, native aquifer geochemistry, and operational site parameters (e.g., source of recharge water and recharge/recovery cycling). Observed mechanisms of geogenic contaminant mobilization during MAR via injection include shifting redox conditions and, to a lesser extent, pH-promoted desorption, mineral solubility, and competitive ligand exchange. The relative importance of these mechanisms depends on various site-specific, operational parameters, including pretreatment of injection water and duration of injection, storage, and recovery phases. This critical review synthesizes findings across case studies in various geochemical, hydrological, and operational settings to better understand controls on arsenic and other geogenic contaminant mobilization and inform the planning and design of future MAR projects to protect groundwater quality. This critical review concludes with an evaluation of proposed management strategies for geogenic contaminants and identification of knowledge gaps regarding fate and transport of geogenic contaminants during MAR.

Manganese (Mn) is an abundant element in terrestrial and coastal ecosystems and an essential micronutrient in the metabolic processes of plants and animals. Mn is generally not considered a potentially toxic element due to its low content in both soil and water. However, in coastal ecosystems, the Mn dynamic (commonly associated with the Fe cycle) is mostly controlled by redox processes. Here, we assessed the potential contamination of the Rio Doce estuary (SE Brazil) by Mn after the world's largest mine tailings dam collapse, potentially resulting in chronic exposure to local wildlife and humans. Estuarine soils, water, and fish were collected and analyzed seven days after the arrival of the tailings in 2015 and again two years after the dam collapse in 2017. Using a suite of solid-phase analyses including X-ray absorption spectroscopy and sequential extractions, our results indicated that a large quantity of Mn II arrived in the estuary in 2015 bound to Fe oxyhydroxides. Over time, dissolved Mn and Fe were released from soils when Fe III oxyhydroxides underwent reductive dissolution. Due to seasonal redox oscillations, both Fe and Mn were then re-oxidized to Fe III , Mn III , and Mn IV and re-precipitated as poorly crystalline Fe oxyhydroxides and poorly crystalline Mn oxides. In 2017, redox conditions (Eh: -47 ± 83 mV; pH: 6.7 ± 0.5) favorable to both Fe and Mn reduction led to an increase (~880%) of dissolved Mn (average for 2015: 66 ± 130 µg L -1 ; 2017: 582 ± 626 µg L -1 ) in water and a decrease (~75%, 2015: 547 ± 498 mg kg -1 ; 2017: 135 ± 80 mg kg -1 ) in the total Mn content in soils. The crystalline Fe oxyhydroxides content significantly decreased while the fraction of poorly ordered Fe oxides increased in the soils limiting the role of Fe in Mn retention. The high concentration of dissolved Mn found within the estuary two years after the arrival of mine tailings indicates a possible chronic contamination scenario, which is supported by the high levels of Mn in two species of fish living in the estuary. Our work suggests a high risk to estuarine biota and human health due to the rapid Fe and Mn biogeochemical dynamic within the impacted estuary., (Copyright © 2020. Published by Elsevier Ltd.)

Oxidative decomposition of soil organic matter determines the proportion of carbon that is either stored or emitted to the atmosphere as CO 2 . Full conversion of organic matter to CO 2 requires oxidative mechanisms that depolymerize complex molecules into smaller, soluble monomers that can be respired by microbes. Current models attribute oxidative depolymerization largely to the activity of extracellular enzymes. Here we show that reactive manganese (Mn) and iron (Fe) intermediates, rather than other measured soil characteristics, best predict oxidative activity in temperate forest soils. Combining bioassays, spectroscopy, and wet-chemical analysis, we found that oxidative activity in surface litters was most significantly correlated to the abundance of reactive Mn(III) species. In contrast, oxidative activity in underlying mineral soils was most significantly correlated to the abundance of reactive Fe(II/III) species. Positive controls showed that both Mn(III) and Fe(II/III) species are equally potent in generating oxidative activity, but imply conventional bioassays have a systematic bias toward Fe. Combined, our results highlight the coupled biotic-abiotic nature of oxidative mechanisms, with Mn-mediated oxidation dominating within Mn-rich organic soils and Fe-mediated oxidation dominating Fe-rich mineral soils. These findings suggest microbes rely on different oxidative strategies depending on the relative availability of Fe and Mn in a given soil environment.

Mining rare earth elements (REEs) can release large amounts of metal(loid)-rich dust, which can pose significant health risks to local residents. However, compared to other types of particulates, toxicity of mining dust has been largely overlooked. To provide experimental evidence on toxicity of REE mine dust, the study assessed the oxidative stress potential and genotoxicity of inhalable particles collected in a REE mining area, and associated toxicological response with source compositions. Both source types (i.e., mine and tailing area) and distances from source (i.e., industrial and residential areas) were considered when selecting the 44 sampling sites. The particle samples contained 2.3-3.5 folds higher concentrations of tested metal(loid)s than background concentrations in soil. Specially, elevated Fe, REEs, Cd, Pb were found. In spite of low cytotoxicity in lung epithelial A549 cells, there was increased cellular ROS production by of particle exposure. Samples with higher mining-originated source contributions (Provenance Index <0.3) had higher cellular ROS production (1.72 fold, 95%CI: 1.66-1.79 fold) than samples with lower mining contributions (1.58 fold, 95%CI: 1.52-1.65 fold). The factors soil (~46%), mine (~22%), and heavy metal (~20%) sources were recognized by source apportionment analysis as the main contributors to cellular ROS production; importantly, mine and heavy metal sources counted more in industrial samples. While samples generated genotoxicity, there were no differences in DNA damage between the location groups of sampling. Collectively, the results indicate that particles in mining areas may cause ROS production and DNA damage in lung cells depending on mine dust. Coupled with the long-range transportation potential of mine dust, safety measures on open pit and dust disposal sites should be adopted., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020 Elsevier B.V. All rights reserved.)

Increased human-derived nitrogen (N) deposition to terrestrial ecosystems has resulted in widespread phosphorus (P) limitation of net primary productivity. However, it remains unclear if and how N-induced P limitation varies over time. Soil extracellular phosphatases catalyze the hydrolysis of P from soil organic matter, an important adaptive mechanism for ecosystems to cope with N-induced P limitation. Here we show, using a meta-analysis of 140 studies and 668 observations worldwide, that N stimulation of soil phosphatase activity diminishes over time. Whereas short-term N loading (≤5 years) significantly increased soil phosphatase activity by 28%, long-term N loading had no significant effect. Nitrogen loading did not affect soil available P and total P content in either short- or long-term studies. Together, these results suggest that N-induced P limitation in ecosystems is alleviated in the long-term through the initial stimulation of soil phosphatase activity, thereby securing P supply to support plant growth. Our results suggest that increases in terrestrial carbon uptake due to ongoing anthropogenic N loading may be greater than previously thought., (© 2020 John Wiley & Sons Ltd.)

Manganese and arsenic both threaten groundwater quality globally, but their chemical behavior leads to both co-contamination and separation of these contaminants from individual well to regional scales. Here we tested manganese and arsenic retention under conditions commonly found within aquifer redox fluctuating and transition zones where both arsenic and iron phases are present in oxidized forms, but manganese persists as reduced and soluble Mn(II). Analysis of column aqueous breakthrough data and characterization of solid-phase products using X-ray photoelectron (XPS) and absorption spectroscopies (XAS) show that the addition of bicarbonate increased manganese retention but decreased arsenic retention, while the presence of manganese and arsenic together increased both arsenic and manganese retention. In the presence of O 2 arsenic remained oxidized as arsenate under all conditions measured; however, reduced Mn(II) was oxidized to an average Mn oxidation state of ∼3 in the absence of arsenate. The presence of arsenate partially inhibited Mn(II) oxidation likely by blocking ferrihydrite surfaces needed to catalyze Mn(II) oxidation by O 2 and by stabilizing Mn(II) via ternary complex formation. These results highlight the interactions between reduced and oxidized contaminants that can contribute to the co-occurrence or physical separation of manganese and arsenic in groundwater systems under changing or stratified redox conditions.

The Salton Sea Basin in California suffers from poor air quality, and an expanding dry lakebed (playa) presents a new potential dust source. In 2017-18, depositing dust was collected approximately monthly at five sites in the Salton Sea Basin and analyzed for total elemental and soluble anion content. These data were analyzed with Positive Matrix Factorization (PMF). The PMF method resolved seven dust sources with distinct compositional markers: Playa (Mg, SO 4 2- , Na, Ca, Sr), Colorado Alluvium (U, Ca), Local Alluvium (Al, Fe, Ti), Agricultural Burning (K, PO 4 3- ), Sea Spray (Na, Cl - , Se), Anthropogenic Trace Metals (Sb, As, Zn, Cd, Pb, Na), and Anthropogenic Copper (Cu). All sources except Local Alluvium are influenced or caused by current or historic anthropogenic activities. PMF attributed 55 to 80% of the measured dust flux to these six sources. The dust fluxes at the site where the playa source was dominant (89 g m -2 yr -1 ) were less than, but approaching the scale of, those observed at Owens Lake playas in the late 20th century. Playa emissions in the Salton Sea region were most intense during the late spring to early summer and contain high concentrations of evaporite mineral tracers, particularly Mg, Ca, and SO 4 2- .

In an effort to combat the threat of drought, China constructed the South-to-North Water Transfer Project (SNWTP), the biggest water transfer project in terms of volume with the largest beneficiary population in the world. Reports have shown that massive water diversion projects have had detrimental environmental consequences including water quality decline and freshwater habitat degradation. However, few reports have assessed the impact of the transfer project on sediment quality, which is highly susceptible to allogenic and local anthropogenic pollution. We examined the distribution characteristics of Cd, Cr, Cu, Ni, Pb and Zn in surface sediment of the largest reservoir along the East Route of SNWTP, Nansihu Lake, followed by positive matrix factorization (PMF) to determine their potential sources. We utilized enrichment factor, multiple sediment quality guidelines (SQGs), and potential ecological risk index (RI) to determine metal accumulation or pollution risk. The results show the mean concentrations of Cr, Cu, Pb, Zn were slightly lower than in samples collected in 2003, 2010 and 2012, while the mean concentrations of Cr and Ni were significantly higher than samples from previous years. Among the six metals, Cr, Cu and Ni are of higher ecological risk according to SQGs; but Cd is of higher ecological risk according to RI. PMF analysis shows that industrial production and shipping are important sources of Cr, Cu, and Ni. PMF analysis also shows that a considerable amount of trace metals, especially Cd, Cr, Pb and Zn, mainly comes from the use of pesticide fertilizers and biomass sources in farmland, and may partly enter Nansihu Lake from SNWTP. This study reveals the possible sources of trace metals to the Nansihu Lake which is part of SNWTP; the results of the study may serve as a reference for better understanding the impact of future water diversion projects on metals distribution., (Copyright © 2019 Elsevier B.V. All rights reserved.)

In an effort to optimize soil management practices that can help mitigate terrestrial carbon emissions, biochar has been applied to a wide range of soil environments to examine its effect on soil greenhouse gas emissions. Such studies have shown that the soil methane (CH4) flux response can vary widely leading to both increase and decrease in CH4 flux upon biochar amendment. To address this discrepancy, multiple meta-analysis studies have been performed in recent years to determine the key factors that may control the direction of CH4 flux upon biochar treatment. However, even comparing across conclusions from meta-analyses reveals disagreement upon which factors ultimately determine the change in direction and magnitude of CH4 flux due to biochar addition. Furthermore, using multiple observations from a single study can lead to misinterpretation of the influence of a factor within a meta-analysis due to non-independence. In this study, we use a multivariate meta-regression approach that allows factor interactions to investigate which biochar, soil, and management practice factors in combination or individually best explain the CH4 flux response in past biochar amendment studies. Our results show that the interaction of multiple soil factors (i.e., water saturation, soil texture, and soil organic carbon content) best explains the soil CH4 flux response to biochar addition (minimum deviance information criterion (DIC) value along with lowest heterogeneity) as compared to all models utilizing individual factors alone. These findings provide insight into the specific soil factors that should be taken into account simultaneously when optimizing the CH4 flux response to biochar amendments and building empirical models to quantitatively predict soil CH4 flux.

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.

The composition of ambient particulate matter (PM) and its sources were investigated at the Salton Sea, a shrinking saline lake in California. To investigate the influence of playa exposure on PM composition, PM samples were collected during two seasons and at two sites around the Salton Sea. To characterize source composition, soil samples were collected from local playa and desert surfaces. PM and soil samples were analyzed for 15 elements using mass spectrometry and X-ray diffraction. The contribution of sources to PM mass and composition was investigated using Al-referenced enrichment factors (EFs) and source factors resolved from positive matrix factorization (PMF). Playa soils were found to be significantly enriched in Ca, Na, and Se relative to desert soils. PMF analysis resolved the PM 10 data with four source factors, identified as Playa-like, Desert-like, Ca-rich, and Se. Playa-like and desert-like sources were estimated to contribute to a daily average of 8.9% and 45% of PM 10 mass, respectively. Additionally, playa sources were estimated to contribute to 38-68% of PM 10 Na. PM 10 Se concentrations showed strong seasonal variations, suggesting a seasonal cycle of Se volatilization and recondensation. These results support the importance of playas as a source of PM mass and a controlling factor of PM composition.

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.

Reduction of As(V) and Fe(III) is commonly the dominant process controlling the fate and transport of As in soils and sediments. However, the physical structure of such environments produces complex heterogeneity in biogeochemical processes controlling the fate and transport of As. To resolve the role of soil and sediment physical structure on the distribution of biogeochemical processes controlling the fate and transport of As, we examined the biogeochemical transformations of As and Fe within constructed aggregates-a fundamental unit of soil structure. Spherical aggregates were made with As(V)- or As(III)-bearing, ferrihydrite-coated quartz that was fused with agarose and placed in a cylindrical reactor; advective flow of anoxic solutes was then initiated around the aggregates to examine As release from a dual-pore domain system. To examine the impact of biotic As(V) and Fe(III) reduction, constructed aggregates having As(V)-bearing, ferrihydrite-coated quartz inoculated with sp. ANA-3 were placed in flow-through reactors under anoxic and aerated advective flow. Consistent with desorption from advective columns, As(III) is released to advecting water more prevalently than As(V) within abiotic aggregate systems, indicating a greater lability and concomitant enhanced propensity for transport of As(III) relative to As(V). During reaction with , As release to advecting water was similar between anoxic and aerated systems for the first 20 d; thereafter, the anoxic advecting solutes increased As release relative to the aerated counterpart. With aerated advecting solutes, Fe remained oxidized (or was oxidized) in the aggregate exterior, forming a protective barrier that limited As release to the advective channel. However, anaerobiosis within the aggregate interior, even with aerated advective flow, promotes internal repartitioning of As to the exterior region., (Copyright © by the American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America, Inc.)

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.