HEAVY METAL GEOCHEMISTRY IN SLUDGE-AFFECTED SEDIMENTS OF MITCHELL LAKE, TEXAS

Mitchell Lake in the San Antonio metropolitan area is a natural lake of 600 acres that was used as a sewage sludge disposal lagoon for almost the entire of last century. The lake is closed to the general public because of its current state of pollution, resulting in, among other negative environmental conditions, extremely odorous algal blooms and poor water quality. Improvement of water quality has been identified as the number one priority in a master implementation plan to develop the lake and its adjacent 3000 acres of forested land into a world-class wildlife refuge. The primary objectives of the study were to determine the spatial variability of heavy metals in the water and sediments of the lake and to investigate the geochemical fate of heavy metals in the lake sediments to quantify the extent of metal availability for plant uptake. Sediments from the top 15 cm of the sediment-water interface were sampled in triplicates from twelve strategic locations in May 2003. The sediments are currently being analyzed for both total and bioavailable concentrations of heavy metals typical to sewage sludge, namely Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb, and Zn. Porewater samples as well water samples collected from the surface of the lake in November 2003 are also being analyzed. Selective physico-chemical properties of the sediments (texture, pH, salinity, cation exchange capacity, Fe/Al oxide fraction, organic matter content, Ca/Mg content, carbonate percentage) are being evaluated to determine the factors controlling the geochemical fate, and hence, heavy metal remobilization potential in the lacustrine system. Although the sediments typically contain high concentrations of heavy metals, so far, only a minor percentage of the total metal concentration have been observed to be resolubilized from the sediments into the overlying water column under typical environmental conditions of the lake. This indicates that the heavy metals are irreversibly bound to the sediments, and hence, are unavailable for plant uptake.