Date of Award
Thesis open access
Brady A Ziegler
The sediment aquifer of California’s Central Valley is threatened by various contaminants including arsenic (As), iron (Fe), manganese (Mn), and uranium (U). In this region more than six million people and an agricultural industry worth over 20 billion dollars (Faunt et al., 2016) rely predominantly on groundwater, and contamination poses both health risks and financial burdens due to costly remediation. I seek to identify the physical and geochemical processes that mobilize contaminants observed at levels that far exceed both federal and state regulatory standards. Contaminants such as As and U naturally occur in sediment and can be mobilized by either anthropogenic contamination or geogenic changes in the aquifer’s geochemistry. I use data from the Groundwater Ambient Monitoring and Assessment (GAMA) program to create spatial, statistical, and geochemical models to elucidate the mobilization mechanisms of As and U. I also explored the stability of aquifer geochemistry as influenced by drought, over-pumping, and agriculture. This study provides evidence that As is strongly correlated with areas of subsidence caused by over-pumping, consistent with the findings of (Smith et al., 2018). As drought conditions in the region worsen, As mobilization will be exacerbated in the shallow aquifer with a lack of oxic recharge causing lower redox potential. U mobilization is more complex and involves a multi-step process. U appears to use NO3 - as the electron acceptor for oxidation, which oxidatively dissolves U from uraninite in the aquifer matrix. Furthermore, U appears to form aqueous calcium carbonate complexes which prevent its reabsorption onto the mineral surfaces, stabilizing it in groundwater.
Nickels, Mark G F, "Spatial and Statistical Analysis of Groundwater Contaminants in Shallow and Deep Aquifers of the Central Valley of California" (2023). Geosciences Student Honors Theses. 26.