BIOAVAILABILITY OF HEAVY METALS IN SEDIMENTS OF A SLUDGE DISPOSAL LAKE

Century-long disposal of sewage sludge in Mitchell Lake at San Antonio, Texas has resulted in hyper-eutrophic conditions and excess accumulation of toxic metals in the lake system. Mitchell Lake is currently closed to the public as a human health hazard due to its severe state of pollution. On the other hand, due to its shallow depth and extended mudflats highly enriched in nutrients, the lake is one of the main stops in the central migratory flyway of over 300 exotic species of birds. Therefore, San Antonio Water Systems, the current owner of Mitchell Lake has developed a Master Plan to transform the lake and its surrounding 3000 acres of watershed into a national wildlife refuge; improvement of water quality has been identified as the number one priority in the plan. To identify the most suitable remedial measure for Mitchell Lake, a baseline study of metal geochemistry in the lake water and the sediments is currently in progress. The primary objectives of this study are to determine the concentrations of heavy metals in the water and the sediments of Mitchell Lake. Metals in the sediments are being analyzed with respect to their total concentrations as well as their bioavailable concentration, i.e. the amount that is potentially available for uptake by plants and other aquatic species. To fulfill the study objectives, sediment and water samples were collected from twelve strategic locations within the lake. The samples were characterized for their physico-chemical properties and were analyzed for total concentrations of common sludge metals, such as Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb, and Zn. To quantify the amount of potentially bioavailable heavy metal concentrations, two widely used chemical extraction schemes, namely, the Mehlich III method and the Olsen method were used. The extracts were analyzed for metals using an ICP-MS and the results compared with total metal concentrations in lake sediments. Total and bioavailable concentrations were correlated with the sediment physico-chemical properties. Preliminary results indicate that the bioavailability of heavy metals in the contaminated sediments of Mitchell Lake is largely controlled by the properties of the sediment-water interface.