EVALUATING SOURCES OF BIOACCUMULATIVE MERCURY IN SOUTHERN CALIFORNIA RESERVOIRS: CASTAIC LAKE, LOS ANGELES COUNTY

Castaic Lake, located ~65 km northwest of Los Angeles, is the third largest storage facility associated with the California State Water Project, which transports fresh water throughout the state. It is the largest reservoir in Los Angeles County (~0.4 km³; 323,700 acre-feet) and is regularly utilized for fishing and recreation. Castaic Lake is also one of 131 California reservoirs listed as impaired due to mercury (Hg) concentrations in fish. Despite the existing mercury fish consumption advisory, no studies investigating mercury dynamics in Castaic Lake have been published in the peer reviewed literature. Mercury is a local, regional, and global pollutant that regularly cycles through the environment while transitioning between different phases and species, dependent upon local environmental pressures. Monomethylmercury (MeHg) is the bioaccumulative organic form of mercury that enters the food web. But, the source of this neurotoxin to the reservoir remains uncertain. MeHg is predominantly produced by anaerobic microbes in low oxygen environments, with Hg methylation typically more pronounced in the profundal zone of lakes and human-made reservoirs. Additionally, recent reservoir studies show that fluctuating lake levels can result in increased MeHg production. We therefore anticipate elevated MeHg concentrations in reservoirs within low oxygen zones, such as bottom waters and sediments, and/or nearshore environments experiencing drying-rewetting cycles. To address this knowledge gap, we are investigating MeHg and inorganic Hg concentrations and distributions in water column depth profiles of Castaic Lake, surface water samples along the lake perimeter, and stream inputs from lake tributaries during the wet and dry seasons. Further context for these data is provided via previously collected federal and state agency data pertaining to Hg concentrations in fish, water, and sediments. Our results provide a first step towards a better understanding of Hg cycling and bioaccumulation pathways in Castaic Lake, as well as the development of a mercury mass balance for this system, by synthesizing data from in-lake and tributary sampling. Furthermore, we provide a water quality baseline to assess future reservoir conditions in a changing climate