SEDIMENT CORE DERIVED HISTORY OF METAL CYCLING THROUGH A SMALL, ACID MINE DRAINAGE IMPACTED RESERVOIR IN OAKLAND, CA

Small upstream reservoir systems may play an important role in removing pollutants from urban streams. Continued urbanization in the San Francisco Bay area and a long mining legacy have left many watershed systems contaminated. The Lake Aliso/Lion Creek system in Oakland, California is a good example of an acid mine drainage polluted urban stream-reservoir pair. The goal of this research is to unravel the history of metals cycling in Lake Aliso through the analysis of sediment core records. Sediment cores were collected from Lake Aliso using a vibracoring system. These sediment cores were initially analyzed for color, grain size, and bulk density. Cores collected near the inlet exhibit nearly homogenous grain size (primarily silt and clay) and color throughout the core. Cores collected near the center of the lake, but outside the stream channel exhibit some variation in grain size (sand to silt with some clay) and color (alternating bands of orange and black/dark grey). These results suggest that the redox conditions of Lake Aliso may not be laterally uniform throughout the reservoir and that the lake cycles through reducing and oxidizing environments over time. These interpretations are supported by lake depth profiles of dissolved oxygen, pH, and temperature, which indicate that redox conditions of the lake are not spatially or temporally uniform. Preliminary scanning x-ray fluorescence (XRF) measurements of sediment cores suggest that metal concentrations in the lake varied through time and that higher concentrations of Zn, Cu, and Ni may correlate with periods of reducing conditions and higher concentrations of Fe and Mn correspond to oxidizing conditions in the lakes. Further XRF analysis of the sediment cores will be combined with Pb-210 analysis to produce a detailed history of metals cycling in Lake Aliso. This analysis will provide insight into whether Lake Aliso acts as a sink for pollutants or allows pollutants to pass through the system to ultimately discharge in the ocean. Results from this research will be useful in developing reservoir management strategies in the San Francisco Bay area.