Paper No. 8
Presentation Time: 10:45 AM
THE FATE OF TRACE METALS IN AN URBAN WATERSHED IMPACTED BY ACID MINE DRAINAGE
MCDANIEL, Gabriella R.1, RADEMACHER, Laura K.
1, FAUL, Kristina L.
2 and BURMEISTER, Kurtis C.
3, (1)Department of Earth and Environmental Sciences, University of the Pacific, 3601 Pacific Avenue, Stockton, CA 95211, (2)Department of Chemistry and Physics, Mills College, 5000 MacArthur Blvd, Oakland, CA 94613, (3)Department of Geological & Environmental Sciences, University of the Pacific, 3601 Pacific Avenue, Stockton, CA 95211, g_mcdaniel@u.pacific.edu
Acid mine drainage (AMD) from the former Leona Heights Sulfur mine in Oakland, Ca, contributes elevated levels of trace metals to the Lion Creek Watershed via Leona Creek. Monthly water samples collected since July of 2009 from the three tributaries (Horseshoe, Leona and Lion Creek) and the inlet and outlet of Lake Aliso reveal that although Leona Creek contributes the smallest volume of water, it has the highest metal concentrations of the three tributaries. Toxic levels of Cu, Cd, and Zn exist in Leona Creek but are rarely observed after the tributaries have merged. In addition, toxic levels of Cu and Cd are occasionally observed at the lake inlet and correlate to elevated toxicity in Leona Creek due to increased mobilization associated with precipitation events. Tributary mixing calculations suggest that Leona Creek contributes the largest metal load to Lake Aliso of the three tributaries despite being the smallest contributor volumetrically.
Since 1990 Lake Aliso has been filled during the dry season (July-October) and returned to creek form during the wet season (November-June). Lake depth profiles show that Lake Aliso is stratified during the summer when it is full. Seasonal changes in the dominant redox reactions were not observed at the lake outlet site even though Lake Aliso undergoes seasonal changes. Observed concentration differences between the lake inlet and outlet suggest that Mn, Zn, Cd, Cu and Ni are removed from the water while As, Pb and Fe are mobilized when the lake is full. Geochemical modeling using PhreeqcI suggests the deposition of minerals containing the metals that are being removed from the water. Understanding contaminant transport in this watershed and lake is important as Lion Creek discharges into San Francisco Bay. Further research will focus on analyzing sediment cores from Lake Aliso to obtain a longer history of metal cycling in the lake. Also, water collection from one-meter depth intervals within Lake Aliso will provide insight into the redox conditions and the fate of metals in the lake. Finally, analysis of microbial community structure for water and sediment samples will help explain the processes controlling the transport and fate of metals in this watershed.
