SPATIAL AND STATISTICAL ANALYSIS OF GROUNDWATER CONTAMINANTS IN CALIFORNIA’S CENTRAL VALLEY

California’s Central Valley is a water-stressed region where groundwater pumping remains a primary resource for agriculture, industry, and municipal drinking water. Groundwater throughout the valley is subject to contamination of an array of anthropogenic and geogenic chemicals. Contaminants originate in aquifer sediments, mobilized into groundwater due to changing pH, redox conditions, and/or reaction with anthropogenically sourced chemicals, such as nitrate (NO₃^-) in agricultural fertilizer. Common groundwater contaminants include: As, Cr, Fe, Mn, and U posing health hazards and financial burdens to municipal water treatment centers. In this research we used spatial and statistical analyses of data from the Groundwater Ambient Monitoring and Assessment (GAMA) program to characterize and trace the geochemical changes that mobilize contaminants to concentrations exceeding safe drinking water thresholds. Multivariate statistical analyses suggest Fe, Mn, and As are mobilized concomitantly and are anticorrelated with both NO₃^- and dissolved oxygen, suggesting that they are mobilized from sediments in reducing redox environments. Conversely, U and Cr(VI) tend to correlate more strongly with NO₃^- and dissolved oxygen and are likely oxidatively dissolved from sediments. Furthermore, the presence of elevated bicarbonate appears to stabilize U in groundwater via formation of uranyl-carbonate complexes. Using GIS, we explored the spatial correlation of 16 analytes in shallow and deep aquifers throughout the Central Valley to track regional trends and relationships in contaminants. In the north valley Fe, Mn, and As are correlated with reducing conditions, whereas in the south As is correlated with over-pumping and subsidence in the deep aquifer. U and Cr(VI) are present in higher concentrations the shallow aquifer correlated with more oxic recharge conditions. The mobilization of geogenic groundwater contaminants in the Central Valley is a complex function of variable natural redox conditions combined with anthropogenic factors such as nitrate application and over-pumping. However, taking these factors into account will help identify regions susceptible to contamination and inform remediation strategies.