Branom, John R.¹, Dibyendu Sarkar¹, Rupali Datta¹
(1) University of Texas at San Antonio, San Antonio, TX

ABSTRACT: Geochemical Forms of Phosphorus in Sediments of a Sludge Disposal Lake

Mitchell Lake is a major surface water body (600 acres) in the city of San Antonio that lies on a karst system. From 1901 until 1987, sewage sludge and/or sewage effluents were discharged into the lake, resulting in, among other negative environmental conditions, extremely odorous algal blooms and poor water quality. Sludge disposal reduced the depth of the lake to less than 8 feet in average. A pilot-scale water quality study revealed that the lake is severely eutrophic, and predicted bottom sediments to be the major source of nutrients causing the excessive algal growth. This study also identified phosphorus as the limiting nutrient. However, the entire amount of phosphorus in Mitchell Lake sediments is generally not available for uptake by aquatic plants, such as algae. Hence, total phosphorus is not a good indicator of bioavailable phosphorus, which depends on solubility and desorption potential of the various species of sediment phosphorus. A study is underway to identify the major geochemical forms of phosphorus in Mitchell Lake sediments, collected in triplicates from twelve strategic locations. Geochemical speciation of phosphorus is being performed using a sequential method to identify the following forms: soluble/exchangeable phosphorus, Fe/Al-bound phosphorus, and Ca/Mg-bound phosphorus. Preliminary results indicate that majority of phosphorus is bound to the Ca/Mg fraction, indicating the influence of karst geochemistry in phosphorus speciation. These “operationally defined” forms of phosphorus will be correlated with the bioavailable fractions of sediment phosphorus to identify those species with highest plant-uptake potential.

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.