BIOGEOCHEMISTRY OF PHOSPHORUS IN AN EUTROPHIC LAKE

Mitchell Lake is a major surface water body (600 acres) in the city of San Antonio, TX. From the mid 1930’s until 1987, the Rilling Road Wastewater Plant discharged waste activated sludge into the lake, resulting in, among other negative environmental conditions, extremely odiferous algal blooms and poor water quality. Years of discharge of sewage sludge have also reduced the depth of the lake to approximately 8 feet in average. A pilot-scale water quality study conducted in 1997 revealed that the lake is severely eutrophic, and predicted the bottom sediments to be the major source of nutrients causing the excessive algal growth. The study also identified phosphorus (P) as the limiting nutrient. The objective of the current study is to develop an empirical model illustrating the biogeochemical cycling of P in the lake aquatic system. A grab sampling event was conducted; water from three depths and sediment samples from the top six inches were collected in triplicates. In addition to pH, EC and alkalinity, water samples were analyzed for total (TP) and reactive phosphorus (RP). There was no discernable difference between TP and RP, indicating that all the P in the lake water is in soluble, inorganic form, readily available for uptake by aquatic fauna and flora, algae in particular. The sediments are characterized for particle size, pH, EC, moisture content, organic matter content, carbonate content, total Ca, Mg, Fe, Al, and oxalate extractable Fe and Al (representing the reactive fractions of amorphous Fe-Al oxides, one of the most common environmental scavengers of P). The sediments are analyzed for total and “bioavailable” fractions of P for correlation with the aforementioned sediment properties to identify the geochemical factors that have the greatest control on P retention-release characteristics. The geochemical forms of sediment-P are also being characterized via a sequential extraction procedure that will lead to operational identification of the following P-pools: soluble/exchangeable P, Al- and Fe-oxides bound P, and Ca- and Mg-oxides bound P. Experiments are in progress to study the variation in P release from sediments with wind velocity as a function of time. Results obtained from this study will be instrumental in future development of appropriate remedial action plans to improve Mitchell Lake water quality.