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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.

Context Tidal dynamics and high evaporative rates in hypersaline tidal flat (HTF) soils favour a range of clay reactions and the formation of complex clay assemblages. HTFs have a geochemical environment conducive to mineral reactions, and therefore, clay mineral alteration controls essential nutrients in coastal wetlands. Aims The crystallochemical characteristics of clay minerals from two HTFs were studied to provide insights into the complexity of fine clay assemblages in HTF soils and their effects on Si biogeochemistry. Methods The fine clay fraction ( Key results The results have indicated the presence of endmembers and R0 mixed-layered minerals such as kaolinite, smectite, illite, kaolinite–smectite (K–S), kaolinite–illite (K–I), illite–smectite (I–S), and illite–vermiculite (I–V). In general, K–S was the dominant mineral in the samples, occurring in a range of kaolinite layers. Alteration of detrital kaolinite to other mixed-layered minerals occurs in soils, leading to silicon removal from the soil solution because of mineral genesis. There is Mg enrichment at the expense of Al in the samples, and bio-opal appears to be the source of Si for the kaolinite alteration process. Conclusions Our findings indicate that clay authigenesis is an important factor controlling Si dynamics in HTF soils, acting as a sink of Si during the formation of new clay phases. Implications Chemical reactions involving clay minerals are critical for deepening our understanding of the biogeochemistry of wetland soils.

As coastal ecosystems become widely recognized for their capacity to sequester carbon (blue carbon), standard accounting methodologies for the generation of carbon credits are being developed. To ensure the applicability of these standards across blue carbon ecosystems, we investigated organic carbon provenance and burial in salt marshes and seagrass meadows of an arid, upwelling‐dominated Eastern Pacific lagoon. We found low carbon density in benthic sediments of Bahía de San Quintín (5.9 ± 0.5 mg C cm−3), only marginally higher in Zostera marinabeds (6.9 ± 0.5 mg C cm−3), likely due to remineralization and hydrodynamically driven export of seagrass material, resulting in low carbon burial rates (4.5 ± 2.5 g C m−2yr−1). Sediment organic carbon is mainly controlled by the fraction of fine sediment and its source is largely allochthonous, although sources differ spatially. Salt marshes at San Quintín derive 40% of their organic matter from autochthonous material and exhibit higher carbon burial rates (up to 414.7 ± 28.6 g C m−2yr−1) and sediment carbon densities (32.0 ± 0.7 mg C cm−3) compared to benthic sediments. This study emphasizes the connectivity of blue carbon habitats with marsh plant detritus supplementing benthic carbon burial and incorporation of detrital eelgrass in marsh sediments. Our findings highlight the importance of allochthonous organic matter for carbon sequestration in blue carbon habitats, suggesting standard accounting practices that deduct allochthonous organic matter would miss the full potential for carbon burial.

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–1due to mounting evidence that VVexposure 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 VIVand birnessite (MnO2), a well-characterized oxidant and a scavenger of many trace metals. In our findings, birnessite quickly oxidized sparingly soluble VIVspecies such as häggite [V2O3(OH)2] into highly mobile and toxic vanadate (HnVO4(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 VIVoccurs in two stages, which are both rapid relative to the measured dissolution rate of the VIVsolid. Concomitantly, the reduction of birnessite during VIVoxidation generated soluble MnII, which led to the formation of the MnIIIoxyhydroxide feitknechtite (β-MnOOH) upon back-reaction with birnessite. XAS analysis confirmed a bidentate-mononuclear edge-sharing complex formed between VVand birnessite, although retention of VVwas minimal relative to the aqueous quantities generated. In summary, we demonstrate that Mn oxides are effective oxidants of VIVin the environment with the potential to increase dissolved V concentrations in aquifers subject to redox oscillations.

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.

Oxidative decomposition of soil organic matter determines the proportion of carbon that is either stored or emitted to the atmosphere as CO2. Full conversion of organic matter to CO2requires 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.

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 O2arsenic 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 O2and 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, SO42–, Na, Ca, Sr), Colorado Alluvium(U, Ca), Local Alluvium(Al, Fe, Ti), Agricultural Burning(K, PO43–), Sea Spray(Na, Cl–, Se), Anthropogenic Trace Metals(Sb, As, Zn, Cd, Pb, Na), and Anthropogenic Copper(Cu). All sources except Local Alluviumare 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–2yr–1) were less than, but approaching the scale of, those observed at Owens Lake playas in the late 20th century. Playaemissions 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 SO42–.

Core IdeasIron and Mn movement followed by oxidation impose strong control on soil transformations.Sulfidization is not an active process in hypersaline tidal flat (HTF) soils.Coprecipitation of Mn and ferrihydrite controls the geochemistry of Mn in HTF soil. Hypersaline tidal flats (HTFs) represent transitional ecosystems where the ocean, land, and freshwater meet. These places are globally widespread and their soils are an important environmental component controlling chemical reactions in wetland ecosystems. Here, we present a pedogenic study of two HTFs on the Brazilian coast (northeast and southeast) based on the solid‐phase geochemistry of Fe and Mn and morphological studies. Broken and irregular topographies and strong expression of redoximorphic concentrations (e.g., mottles, nodules, and pore linings) are present throughout the soil profiles. Although lepidocrocite is the most abundant Fe fraction in these soils (more than 50% of all extractions), deeper horizons show pyrite as the main Fe fraction caused by the presence of buried mangrove soils. The more oxidizing conditions in these deeper horizons indicate that pyrite is not stable and is undergoing degradation. Iron released in this process is moving upward and precipitating as Fe oxides in more oxidized portions of the soil profile. Coprecipitation of Mn and ferrihydrite seems to be an important process controlling the geochemistry of Mn in HTFs. Our data indicate that deeper horizons have been transforming, driven mainly by water table oscillation and, consequently, redox processes. This process allows translocation of Mn2+and Fe2+throughout the soil profile, followed by oxidation and precipitation of Mn3+/4+and Fe3+oxides in preferential sites.

Ceramic water filters (CWFs) provide a low-cost and effective means of pathogen removal for drinking water and are an attractive water treatment option because they can usually be manufactured without the need for imported materials. However, variation in the chemical composition of materials used for CWF production, especially in source clay, provides a potential hazard to CWF users through leaching of arsenic into filtered water. We determined the chemical composition of source clay material, and then evaluated the extent of arsenic leaching from CWFs manufactured in Kandal Province, Cambodia. Arsenic leaching from CWFs is rapid and non-linear, resulting in initial filtrate arsenic concentrations more than 100 times the World Health Organization recommended limit for safe drinking water (10 μg L−1). Under typical operating conditions in Cambodia, we determined that each CWF will leach approximately 43.5 mg of As, but that 80% of the As is released in the first 100 L of water. Arsenic desorption tests performed on source clay indicated that the ceramic firing process drastically increases the leachability of arsenic compared to unfired clay in the region. Testing of fired CWF material, not only source materials, is essential to predict arsenic leaching. Competitive desorption did not increase the release of arsenic from fired CWF materials as phosphate-dosed water in contact with CWF material showed no significant effect on arsenic release. We conclude that CWF arsenic leaching poses a potential exposure to end-users but because leaching occurs rapidly, design of controlled leaching systems prior to product distribution can mitigate arsenic exposure.

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 Shewanellasp. 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 Shewanella, 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.

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–1of 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.