TRACE METAL CONTRIBUTIONS TO SAN FRANCISCO BAY UNDER DROUGHT (2012-2016) AND POST-DROUGHT (2016-2021) CONDITIONS FROM A SMALL ALAMEDA COUNTY WATERSHED-EPHEMERAL RESERVOIR PAIR

Small reservoirs in the urban San Francisco Bay (SFB) area provide a variety of services including flood control, recreation, and irrigation water storage; however, major freeways, historic mines, climate change, and continued development are changing contaminant loads. Watersheds and associated reservoirs in the SFB area were severely stressed due to the 2012-2016 California drought. We investigated a small Alameda County SFB watershed/reservoir system (Leona Creek/Lake Aliso on the Mills College campus in Oakland) to assess how reservoir management may mitigate or exacerbate contaminant discharge to coastal urban environments during drought (2012-2016) and post-drought (2016-2021) conditions.

We collected and analyzed standard geochemistry, nutrients, and trace element (Mn, Fe, Ni, Cu, Zn, As, Cd, Pb) levels from water in the reservoir inlet and the outlet on a bimonthly basis for Water Years (WY) 2012 through 2022. We measured bimonthly lake depth profiles of pH, conductivity, temperature, and dissolved oxygen, and collected and analyzed water and sediments from the reservoir for nutrient and trace element concentrations.

Ephemeral reservoirs are important controls on pollutant cycling and downstream water quality in urban watersheds. Both the presence/absence of the reservoir as well as the redox state of the reservoir when present determines whether metals and nutrients are mobilized or retained by sediments. Reservoir stratification varies over the course of a year and leads to reducing conditions prevailing during warm summer months and oxidizing conditions dominating during cool winter months. With drought lowering flows, reducing conditions expand during summer months. Because reducing conditions mobilize many metals and retain nutrients, summertime reservoir management style is most critical for downstream SFB water quality, particularly for reservoirs with large acid mine drainage loads. We assess whether drought effects linger beyond drought years as evidenced by the geochemistry, nutrient levels, and trace metals. Trade-offs will have to be made between in situ watershed/reservoir water supply for recreation or irrigation and downstream water quality for SFB health.