CONSIDERING SOURCES OF BIOACCUMULATIVE MERCURY IN A SOUTHERN CALIFORNIA RESERVOIR: CASTAIC LAKE, LOS ANGELES COUNTY

Mercury (Hg) bioaccumulation and biomagnification have been identified as a pervasive threat to reservoirs throughout California (CA), with 131 CA reservoirs currently listed as impaired due to mercury concentrations in fish. In aquatic environments, such as reservoirs, transformation of inorganic Hg to organic bioaccumulative monomethylmercury (MeHg) is facilitated by microbes predominantly associated with low oxygen settings. The focus of our study is Castaic Lake, a reservoir affiliated with the CA State Water Project, which transports fresh water throughout the state. Castaic Lake is located ~65 km northwest of Los Angeles and is the county’s largest water storage facility (volume of ~0.4 km³ or 323,700 acre-feet), making it a critical component of southern California's water supply infrastructure, as well as a popular destination for fishing and recreational activities. Although there is a mercury fish consumption advisory at Castaic Lake, historical water quality data have focused on surface waters, where humans recreate and water is oxygenated. To gain a better understanding of Hg sources to the reservoir, we characterized the distribution and concentration of total Hg (HgT) and MeHg both down the water column (ranging from oxic to possible anoxic conditions at depth) and in water along the lake perimeter (oxic conditions). While we hypothesized that MeHg was produced in the water column, even at the reservoir’s greatest depth MeHg concentrations exhibited no increase and dissolved oxygen concentrations remained >3 mg/L, indicating poor conditions for methylation. Our data also confirm that tributary inputs were negligible, with HgT and MeHg concentrations notably lower than the California Toxics Rule Criteria and EPA Drinking Water Standards. Our data suggest the source of MeHg to the Castaic Lake food web is not the pelagic water column nor inflow from the watershed. Hence, current research aims to evaluate Hg concentrations upstream at Pyramid Lake, while future work is set to focus on HgT and MeHg concentrations in water and sediments within litoral vegetated coves of Castaic Lake.