WATER COLUMN GEOCHEMISTRY IN LOS ANGELES COUNTY RESERVOIRS: CASTAIC LAKE AND PYRAMID LAKE PUBLIC RECREATION AREAS

The California State Water Project distributes water via a series of aqueducts, reservoirs, and pumping stations. The western branch passes through Pyramid Lake before ending at Castaic Lake, an emergency drinking water reservoir in Los Angeles County. Although Castaic Lake and Pyramid Lake have fish consumption advisories for mercury (Hg) and polychlorinated biphenyls (PCBs), little is known about the geochemistry of their water columns, which reach depths of ~90 m (300 ft) and experience seasonal density-driven water column stratification. During summer months, surface water warms and a thermocline develops, separating warmer low density surface water from colder denser deep water. When a lake stratifies, bottom water loses contact with the atmosphere. As oxygen is consumed by biological processes, it cannot be replenished and anaerobic respiration begins to influence water chemistry. For example, reducing conditions can enhance the mobility of metals (e.g., Cu, Fe, Pb, Zn), causing them to desorb from sediments and organic matter. The release of metals from lake bottom sediments is a concern because, in aqueous environments, >90% of metals can be associated with particulate matter. We are therefore characterizing changes in water column geochemistry and trace metals concentrations over a range of seasons to determine the extent to which stratification alters water quality in these Southern California reservoirs. This study also establishes a baseline for current water quality conditions that can be used to assess climate-induced changes, including the impact of prolonged droughts and wildfires, which are persistent threats to watersheds in the Western USA. As global warming persists, deep water bodies like Castaic Lake and Pyramid Lake will experience greater temperature gradients between surface water and deep water, intensifying the changes associated with water column stratification.