THE URBANIZATION HISTORY OF AN ACID MINE DRAINAGE IMPACTED RESERVOIR IN THE SAN FRANCISCO BAY AREA, AS DETERMINED BY SEDIMENTARY TRACE METAL, PHOSPHORUS, NITROGEN, AND CARBON RECORDS

Small urban reservoirs may be sinks for trace metals, nutrients, and carbon (C). We examine the sedimentary history of an ephemeral reservoir (Lake Aliso, Oakland CA) impacted by acid mine drainage and urban pollution. Leona Creek originates in the Oakland hills, incorporates discharge from the closed Leona Heights sulfur mine, flows through Lake Aliso, and terminates in the San Francisco Bay. Beginning in the 1940s, freeways and housing developments were built in the watershed. We retrieved a 1.5 meter sediment core from Lake Aliso and collected, dried, ground, and sieved samples in 2 cm increments. We dissolved biologically available metals into solution and analyzed the samples for their concentrations of Mn, Fe, Ni, Cu, Zn, As, Cd, and Pb via Inductively Coupled Plasma Mass Spectrometry. We sequentially extracted oxide associated, organic, detrital, and authigenic phosphorus (P) and then measured them using a Lachat ion analyzer. Carbon to nitrogen (C:N) ratios and carbon (δ13C) and nitrogen (δ15N) isotopic values were measured by the UC Davis Stable Isotope Lab.

Fe and Mn concentrations within the core are an order of magnitude higher than in sediments of other local reservoirs with less anthropogenic impact (e.g., Lake Anza, Berkeley, CA). Since Fe is released during the oxidation of pyrite, it is likely that acid mine drainage is the Fe source. Typically, Cd, Cu, Zn, Pb, Ni, and As are present in both acid mine drainage and in freeway runoff. We see highly correlated increases in Cu, Zn, Cd, and As concentrations from the bottom to the top of the core, indicating greater delivery of these metals to the lake over time. The concentrations of Pb and Ni do not fit this pattern. Nutrient and C data indicate eutrophication of the lake through time. Organic P deposition increases from the bottom to the top of the core, indicating increased nutrient delivery. An increase in C/N ratio from the bottom (10) to the top (15) of the core may indicate a shift in source from algae within the lake (typical values of <10) to terrestrial organic matter being delivered to the lake (typical values > 20). The sedimentary δ13C values decrease and the δ15N values increase towards the top of the core, consistent with increasing eutrophication. The watershed history of urbanization and acid mine drainage is recorded in the sediments of Lake Aliso.