In California, up to 46% of the state's total water supply is from groundwater and its geogenic contamination may affect a large area, number of wells, and number of people throughout the state. The presence of geogenic contaminants, such as chromium (Cr) and manganese (Mn), in groundwater is due to the interplay between complex processes (i.e., biogeochemical reaction networks) involving mineral dissolution, diffusion, redox transformation, sorption, and precipitation. To effectively evaluate the safety of our drinking water it is critical that we understand the network of biogeochemical reactions that control geogenic metals cycling. The overarching goal of this dissertation work is to explore how fluctuating redox conditions from micron- to field-scale may influence the release of geogenic metals and threaten drinking water sources. To first address this goal on the micro-scale, we investigated how the co-occurrence of iron (Fe) and Cr minerals in reduced zones of soil aggregates may alter the redox potential of Mn oxides within oxidized zones at diffusion-controlled soil interfaces. Altering the oxidation state and structure of Mn oxides can impact the oxidation of Cr(III), a non-toxic mineral, to Cr(VI), a mobile and highly toxic groundwater contaminant. Simultaneously, the oxidation of Cr minerals by Mn oxides will also release soluble Mn, a secondary contaminant, through reductive dissolution. On a state-wide scale, we then investigated the groundwater contamination by redox controlled groundwater contaminants, such as Mn, within public water systems relying on groundwater resources. Larger systems that treated for primary contaminants often saw associated benefits of less Mn, however, this was not observed within small systems or systems without treated groundwater. Further, we investigated differences in Mn contaminant exposure between private, shallow well users and deeper, public well users in California’s Central Valley. We found that, despite predicted differences in redox status groundwater based on well-depth, this led to no differences in Mn concentration in groundwater. However, lack of monitoring or treatment of private wells and the high associated costs may lead to exposure to concentrations of Mn exceeding regulatory standards.

Exposure to vanadium (V) results in adverse health outcomes in humans and a plethora of species in the environment across trophic levels. V is released from rock, slag and mine tailings, where it is able to oxidize to vanadate (VV) (HnVO4 −3+n), which poses the greatest risk to human health. Vanadate is highly soluble unlike its VIII or VIV, and is readily taken up by well systems, resulting in human exposure in areas where water treatment is minimal. However, the geochemical controls that determine the mobility of V in the environment are poorly understood, making the behavior of V in the subsurface difficult to predict. This dissertation addresses this knowledge gap by investigating the abiotic reactions of V with common iron and manganese mineral phases. These mineral phases are already known to play an important role in the fate and transport of similar contaminants, but many aspects of their specific reactivity towards V are unknown. To address this knowledge gap, their surface capacity, adsorption affinity and their ability to retain vanadate via surface complexation is investigated. The vanadate adsorption capacity and surface affinity are both found to be inversely proportional to the crystallinity of the oxide examined. One manganese oxide, birnessite is also known to promote the oxidation of many trace metal contaminants. The oxidative capacity of birnessite towards VIV is much greater than its surface capacity for VV. At the aggregate scale, birnessite, V and Fe oxides coexist in close spatial proximity where rates of diffusion limit the transport of solutes. In such an environment, birnessite effectively oxidizes VIV and retains more VV than adjacent Fe oxide phases. Further, FeII-bearing oxides can reduce VV back to VIV. However, this pathway is slow compared to the oxidation rate of VIV by birnessite. This suggests that surface complexation of vanadate, even by birnessite is a greater attenuation pathway than abiotic FeII-mediated reduction. The result of this research emphasizes the role of abiotic surface processes in constraining the mobility of V within the soil matrix which will improve the reliability of models implemented in the management and reclamation of V contaminated sites.

Vanadium (V) is a ubiquitous redox active transition metal found in soil systems, which can have adverse effects on the neurological development of children when ingested. Human exposure to vanadium can occur via consumption of contaminated groundwater containing V that is mobilized from soils and sediments under redox-fluctuating conditions. V is predominantly present in the more mobile V(V) form under oxidizing conditions; however, specific biological and geochemical processes contributing to vanadium redox cycling and subsequent change in its mobility into groundwater systems is not well understood. Past studies have demonstrated that permanganate is a strong oxidant of V, rapidly oxidizing V(IV) to V(V) upon contact. Therefore, we hypothesize that naturally-occurring manganese (Mn) oxides, such as birnessite, could be a potential oxidant of V in soils and sediment systems. Furthermore, iron (Fe) and manganese oxides are important components of soil systems as possible sorbents of vanadium. To understand the fate and transport of V, a Donnan reactor that simulates diffusion-limited environments within soils was used to examine V(IV) oxidation and subsequent sorption of V(V) on Mn and Fe oxides. V sorption and oxidation was monitored over time by measuring the concentration of aqueous V using ICP-OES in conjunction with Fe speciation determined by ferrozine assay. X-ray absorption spectroscopy (EXAFS and XANES analysis) and SEM-EDX imaging was used to determine changes in Mn oxide structure and speciation of adsorbed V over time. The oxidation of V resulting from contact with birnessite, with subsequent reductive dissolution of the oxidant, will affirm the potential role of V transformation from V(IV) to V(V) by Mn oxides. The results of this study demonstrate that the presence of naturally-occurring Fe and Mn oxides in soils and sediments can impact the fate and transport of V in natural systems and their role in controlling V mobilization into water resources.

Arsenic groundwater contamination is a worldwide issue that impacts more than 200 million people across the globe, according to the World Health Organization. We look at arsenic groundwater contamination in Coachella Valley, CA using publicly available environmental data and two different mapping methods, spatial interpolation via inverse distance weighting and modeling via random forest classification. We find that within Coachella Valley, the majority of arsenic contamination is found within the eastern portion of the region. More groundwater sampling should be done to better understand the extent of this issue and the people being impacted by this exposure.

Drying-rewetting cycles are ubiquitous across natural and managed ecosystems. These cycles are known to mobilize carbon (C) in soils producing dramatic pulses in microbial respiration. While many factors contribute to these pulses, how the drying-rewetting history of soils affects these emissions remains unclear, especially in irrigated soils where soil moisture fluctuations are much more repetitive than natural, seasonally influenced soils, and where the volume of soil that water dominates is on much smaller spatiotemporal scales. To understand the controls of repeated wet-up and dry-down effects on agricultural soils, in the first study, we took a systems-level approach to examine the cross section of a furrow irrigated orchard in a semi-arid Mediterranean climate to delineate the C dynamics in the two contrasting regions of water availability: in furrows used to deliver water to tree crops where soils are temporarily but repeatedly inundated versus in berms where trees are planted and soils only receive water during infrequent rain events. Overall, our findings show that the high degree of heterogeneity in soil moisture across the landscape of a furrow-irrigated field results in contrasting gaseous release of C as CO2, microbial selectivity of substrates, and dominant mechanisms for C stabilization. In the same heterogenous landscape, we characterized the temporal variation in composition and concentration of soil C pools over the course of a year. We found that within this timeframe, soils that are typically dry but have large amount of litter and root inputs did not vary significantly in total C concentrations but exhibited increase in the active C pool after seasonal rain events. We also found that sustained anoxic conditions that dominate furrow soils throughout the year limit decomposition, regardless of season. The seasonal trends captured in this study can inform ecological models of semi-arid irrigated soils to better predict C sequestration. Lastly, in the third study, we estimated the potential changes to CO2 flux and soluble C chemistry if a legacy furrow-irrigated soil was converted to precision irrigation, hypothesizing that large releases of C would occur when lower-volume irrigation method is adopted. However, we found that low-volume irrigation leads to water limitations that inhibit CO2 pulse release from berm soils.

High arsenic (As) concentrations can be found in drinking water around the world, the consumption of which can lead to increased incidences of skin cancers, arsenicosis, and cardiovascular disease. The source of arsenic in contaminated regions is geogenic, where its fate and transport is affected by other naturally-occurring mineral constituents within the soil solid phase. Manganese (Mn) oxides within soil profiles are strong oxidants of many metals and metalloids found in soil, including As, where the more mobile and more toxic arsenite [As(III)] form can be oxidized to the less mobile and less toxic arsenate [As(V)]. Within redox transition zones in soil profiles, including the diffusion-limited matrix of soil aggregates, Fe(II) can come into contact with Mn oxides, altering the reactivity of the Mn oxides. This change in reactivity of Mn oxides may therefore alter the impact of Mn oxides on the fate and transport of arsenic in these interfacial zones. In the current study, we quantitatively demonstrate the impact of the formation of a potentially passivating Fe(III)-oxide coat on Mn-oxide mediated contaminant oxidation. By simulating diffusion-limited transport using a Donnan cell reactor, we investigated the oxidation of As(III) by Mn-oxides after exposure to varying concentrations of Fe(II). Our findings show that Fe(II) treatment of birnessite, a poorly-crystalline Mn oxide, leads to the formation a combination of Fe(III) oxides with a range of crystallinities as determined by Fe EXAFS analysis. Concomitant reduction of birnessite leads to formation of hausmannite, a Mn(II, III) oxide. Reaction of the resultant Fe-Mn oxide with As(III) shows decreased rate of As oxidation as compared to systems without Fe. Additionally, increasing Fe(II) concentrations exposed to birnessite by two folds leads to a five fold decreases in As(III) oxidation rate. During As(III) oxidation, hausmannite is rapidly reductively dissolved, resulting in increased aqueous Mn concentrations; continued As(III) oxidation is then mediated by birnessite. Furthermore, our results show that a higher concentration of arsenic partitions into the solid phase at high Fe(II) treatment due to the tendency of arsenic to bind preferentially to Fe oxides as compared to Mn oxides.