LESSONS OF CLIMATE CHANGE, DROUGHT, AND RESERVOIR MANAGEMENT FROM A 10-YEAR INVESTIGATION OF A SMALL OAKLAND WATERSHED-LAKE PAIR

Climate change, historic mines, and urban development are continuing pressures on Lake Merritt and other Oakland watersheds and associated reservoirs that drain to the San Francisco Bay (SFB). Ephemeral reservoirs are important controls on pollutant cycling and downstream water quality in urban watersheds. We studied a small Oakland SFB watershed/reservoir system (Leona Creek/Lake Aliso on the Mills College at Northeastern University campus) 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 (e.g., Mn, Ni, Cu, Zn, As, Cd, Pb) levels from water in the reservoir, reservoir inlet, and reservoir outlet on a bimonthly basis from 2012 to 2022. We found that inlet metal concentrations (e.g., Mn, Ni, Cu) were generally higher than outlet concentrations. Reservoir stratification varied over the course of a year and led to reducing conditions prevailing during warm dry summer months and oxidizing conditions dominating during cool wet winter months. With drought lowering flows, reducing conditions expanded during summer months. Because reducing conditions mobilize many metals, summertime/drought reservoir management style is likely most critical for downstream SFB water quality, particularly for reservoirs with large acid mine drainage loads. Interestingly, we found that inlet metal concentrations were similar for wet and dry seasons, but outlet metal concentrations were significantly lower during dry low flow seasons (lake present) than wet high flow seasons (lake absent). Therefore, it’s likely that the presence/absence of the reservoir, the redox state of the reservoir, and other factors such as amount of organic material determine whether metals are mobilized downstream to the SFB or retained by the reservoir.