The Spatial Variability of Total and Bioavailable Metal Concentrations in the Sediments of Mitchell Lake

Department of Earth and Environmental Science Environmental Geochemistry Laboratory The University of Texas at San Antonio ________________________________________________________________________ Abstract Century-long disposal of sewage sludge in Mitchell Lake at San Antonio, Texas has resulted in hyper-eutrophic conditions and an excess accumulation of toxic metals in the lake system. To identify the most suitable remedial measure for Mitchell Lake, a baseline study of metal geochemistry in lake water and sediments is currently in progress. The sediment samples were collected from twelve strategic locations, close to possible sources of additional contamination, throughout the lake. Three locations were chosen close to the Leon Creek Effluent Pipeline and from the center of the lake. Two locations were chosen by the Mission Del Lago Golf Course and the SAPD Academy Gun Range. One location was chosen by Polder Pond and the Dam. Samples were tested for their total concentrations of nine 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, a widely used chemical extraction scheme, namely, the Olsen method was utilized. Due to the alkalinity of the lake environment, the Olsen method was determined to be an appropriate method for determining the true bioavailability of the metals in the sediments. An ArcGIS map of the lake was created showing the sample locations, their associated total and bioavailable metal fractions, and the elevations. The Geostatistical Analyst exploration tools were used to become familiar with the data. Also, the Geostatistical Analyst tool, Inverse Distance Weighting (IDW), was used to interpolate and to access the spatial variability of the metal concentrations surrounding the sample locations.