MERCURY DYNAMICS IN CASTAIC LAKE STATE RECREATION AREA, CALIFORNIA

Castaic Lake, located ~65 km northwest of Los Angeles, is the third largest storage facility associated with the California (CA) State Water Project, which transports fresh water throughout the state via the CA Aqueduct. It is the largest reservoir in LA county, ~0.4 km³ (323,700 acre-feet), and is regularly utilized for fishing and recreation. Castaic Lake is also one of 71 CA reservoirs listed as impaired due to mercury (Hg) concentrations in fish. In spite of the existing mercury fish consumption advisory, no studies investigating mercury dynamics in Castaic Lake have been published in the peer reviewed literature. To address this deficiency, we are evaluating mercury cycling in Castaic Lake by characterizing the concentration and distribution of inorganic Hg and bioaccumulative organic monomethylmercury (MeHg) in both water column depth profiles and surface water samples along the lake perimeter. Although MeHg is the form of mercury that enters the food web, the source of this neurotoxin to the reservoir remains uncertain. MeHg is produced by anaerobic bacteria in low oxygen environments, with Hg methylation typically more pronounced in the profundal zone of lakes and human-made reservoirs. Therefore, we anticipate that Castaic Lake anoxic bottom waters will be enriched with MeHg. In addition to quantifying mercury species in the reservoir, we are sampling lake tributaries during the wet and dry seasons to assess stream inputs, while utilizing National Atmospheric Deposition Program (NADP) mercury deposition maps to provide context for these watershed data. Our results provide a first step towards developing a mercury mass balance for Castaic Lake, by synthesizing data from in-lake and tributary sampling. Furthermore, by documenting current water quality conditions, we provide a baseline to assess the impacts of climate change on reservoir systems.