WATER QUALITY IN A EUTROPHIC SLUDGE DISPOSAL LAKE

Mitchell Lake is a 600-acre water body located on the south side of San Antonio, Texas. From the mid 1930’s to 1987, the Riling Road Wastewater Treatment Plant discharged waste activated sludge and treated effluents into the lake, which resulted in, among other negative environmental conditions, extremely odorous algal blooms and poor water quality. Lake depth has been reduced to less than 2 meters on average due to continuous accumulation of sewage sludge. A pilot-scale water quality study, which was conducted in 1997 revealed that Mitchell Lake is severely eutrophic, and predicted the bottom sediments to be the major source of nutrients causing the excessive algal growth. This study also identified phosphorus as the limiting nutrient. The primary objectives of the reported study are to determine spatial and temporal variability in selected water quality parameters and also to determine the extent to which phosphorus is released due to wind induced sediment resuspension. A preliminary grab sampling event was conducted in October 2001; water from two depths (secchi disc depth and sediment/water interface) and sediment samples from the upper 10 cm were collected in field triplicates. Since June 2003, water samples are being collected bimonthly in field triplicates for a period of one year. Water temperature, pH, and dissolved oxygen (DO) are being measured in the field. In addition to total salinity, total alkalinity and total dissolved solids (TDS), water samples are also being analyzed for total phosphorus (TP), soluble reactive phosphorus (SRP), total nitrogen (TN), nitrate-N, ammonium-N, and chlorophyll-a contents. So far, no significant difference in TP and SRP has been noticed, indicating that phosphorus in the surface water is in soluble inorganic form, which is readily available for algal uptake. Nitrate-N is the primary component of TN in Mitchell Lake. Dried sediment samples were shaken at two different speeds (simulating weak and strong wind velocities) and SRP release were measured. Results indicate the majority of SRP was released within the first 6-12 hours at both wind speeds and that SRP release was a function of wind velocity.