UC Riverside

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.