Abstract
In California, over 10 percent of groundwater aquifers used for public supply have arsenic concentrations that exceed regulatory thresholds, yet the processes controlling arsenic mobilization are not fully understood. This thesis presents geochemical and groundwater flow modeling to investigate arsenic mobilization from the dissolution of and desorption from iron hydroxides along a groundwater flow path in the Kings alluvial fan in the San Joaquin Valley of California. Groundwater sample data, previously collected and analyzed as part of the state of California Groundwater Ambient Monitoring and Assessment Program Priority Basin Project, were used to develop and calibrate a PHREEQC kinetic geochemical model simulating reactions between aquifer sediments and groundwater along the Kings alluvial fan flow path for 25,000 years. The geochemical results indicate that reductive dissolution of amorphous iron hydroxides has a 40 percent greater effect on arsenic mobilization than pH controlled sorption of arsenic to iron hydroxides. Reducing conditions alter dissolved As(V) to As(III) and thus increase its affinity to sorb at alkaline pH; however, this is outweighed by the redox-driven dissolution of iron hydroxides and resulting decrease in available sorption sites that lowers the total amount of sorbed arsenic and increases the dissolved arsenic concentration. These processes occur in the high pH anoxic waters towards the center of the valley created by water and sediment reactions, such as dissolution of primary minerals and precipitation of clays, occurring along the groundwater flow path. Groundwater flow model particle analysis shows lateral and vertical flow paths are controlled by clay lenses throughout the study area. The flow model also confirms that there is recharge from the Kings River and subsurface flows from outside of the model area. Modelled flow paths indicate there may not be a direct path from recharge in the Sierra Nevada foothills to the valley axis due to anthropogenic influence. Recharge from the observed sources may provide oxic water with low arsenic concentration to the aquifer system. To investigate the potential impact of this type of recharge, PHREEQC was used to simulate the mixing of oxic recharge water into high pH anoxic groundwater containing a high concentration of arsenic. Results indicate that mixing may be a possible mitigation technique due to arsenic concentration decreasing to a level lower than that in either initial mixing solution. Future work could include coupling the geochemical and flow models to better understand how flow through clay lenses affects dissolved arsenic concentration. Laboratory analyses to quantify the concentration of sorbed arsenic or rate of iron hydroxide dissolution in sediments from the Kings alluvial fan would also improve the accuracy of this and future studies.