Date of Award

12-2025

Document Type

Thesis

Degree Name

Master of Science in Earth and Environmental Sciences

Department

Geological Sciences

First Reader/Committee Chair

Beyersdorf, Andreas

Abstract

This study investigates the distribution of naturally occurring arsenic in groundwater within the Oro Grande Wash in San Bernardino County, California. Depth-discrete groundwater quality data were collected over multiple years using nested and Westbay monitoring wells. Water quality results showed higher levels of arsenic in the deeper aquifers, ranging between 600 feet and 1,000 feet below ground surface. Arsenic levels in these regions reached as high as 66 µg/L. At roughly 800 feet and below, water quality data from the wells reveal that arsenic concentrations exceeded the 10 µg/L maximum contaminant level (MCL) in 87% of cases. These data provide a baseline for evaluating the potential impacts of artificial recharge at the Amethyst Basin, a facility constructed between 2017 and 2019 as Phase 2 of the Mojave Water Agency’s (MWA) Basin Recharge Plan. The basin, operated by the San Bernardino County Flood Control District in collaboration with regional agencies, was designed for both groundwater recharge and flood mitigation and is connected to the California Aqueduct.

Previous pilot recharge tests in the area, conducted by MWA and the U.S. Geological Survey, demonstrated the feasibility of artificial recharge. However, concerns remain regarding the mobilization or redistribution of arsenic, which may pose risks to groundwater quality and public health. Arsenic concentrations exceeding regulatory limits have been observed at certain depths, particularly under reducing conditions that may be affected by recharge activities.

Understanding arsenic behavior in response to recharge is critical for protecting drinking water supplies. As MWA prepares for future recharge operations in the Oro Grande Wash, continued monitoring and geochemical evaluation are essential to assess risks, inform management decisions, and ensure the long-term sustainability and safety of the groundwater resources.

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