THE MYSTERIES OF MERCURY IN CASTAIC LAKE STATE RECREATION AREA, CASTAIC LAKE, CALIFORNIA

Castaic Lake, located ~70 km north of Los Angeles, is one of 12 major water reservoirs in California. It is an integral component of the California State Water Project, which includes a chain of reservoirs and aqueducts that transport potable drinking water to the LA basin. It also functions as an energy source to local communities via a hydroelectric dam and is a popular recreation area for watersports, including fishing. However, Castaic Lake is also one of 71 California reservoirs listed as impared due to elevated mercury concentrations in fish. Despite posted fish advisories, mercury cycling and toxicity in this reservoir remain unaddressed in published literature.

Mercury exists in a variety of chemical forms, with monomethylmercury (MeHg) posing the greatest risk to humans and other biota because it (1) is biologically available; (2) biomagnifies up the food chain; and (3) is a toxin that impacts neurological development. MeHg is produced by anaerobic bacteria in low oxygen environments and its impacts are more pronounced in human-made reservoirs, such as Castaic Lake, relative to natural lakes. This is because water level fluctuations are more pronounced in reservoirs and sediment transport is altered, which impacts processes such as carbon transport and redox potential. Additionally, most reservoirs are deeper and narrower than lakes, impeding water column mixing and reducing dissolved oxygen concentrations at depth, thereby enhancing MeHg production. Thus, human-made reservoirs affect the distribution, bioavailability, and toxicity of mercury.

To identify within-lake sources of mercury species in Castaic Lake, we are quantifying total mercury and MeHg within the water column and bottom sediments. Additionally, we are using atmospheric deposition estimates, along with mercury concentrations in tributaries, to quantify watershed inputs. By describing the lake’s physical and biological dynamics, as well as its geochemical trends, we aim to illustrate how mercury moves through this system. Our study is an initial step toward evaluating the importance of in situ sources of organic mercury versus watershed and atmospheric inputs in a West Coast reservoir. Furthermore, we are establishing a baseline for contaminant cycling within this ecosystem, as climate change will alter future conditions.