Abstract. The Toxic Substance Control Act (TSCA) of 1976 authorizes the Environmental Protection Agency (EPA) to obtain production and test data from industries on selected substances and mixtures, and to regulate these substances when needed. Currently, phosphogypsum, a waste byproduct of phosphate ore processing, is being regulated under TSCA. To obtain a better understanding of the environmental effects caused by phosphogypsum stacks, the Tuscaloosa Research Center investigated the effects of a synthetic rainwater and a process cooling pond water on the mobility of metal and nonmetal ions through an inactive phosphogypsum stack in Florida. Twenty-one groundwater monitoring wells were installed in a 160 acre, 30 year old inactive phosphogypsum stack in the Central Florida Phosphate Mining District. The wells were placed in the stack in clusters of three, with the wells being 17, 27, and 37 ft deep, respectively. ·well core samples were taken every 5 ft for characterization studies and column leaching tests. These wells were monitored monthly with samples analyzed for pH and metal and nonmetal ion concentrations. Two leach solutions, a synthetic rainwater to simulate natural rainfall and a phosphate plant process cooling pond water were used in the laboratory column leaching experiments. Leachate from these column leaching tests using phosphogypsum from the well cores was found to contain metal and nonmetal ions. Such tests indicate that metal and nonmetal ions contained in the phosphogypsum have the potential for migrating into surrounding surface waters and groundwater.

Phosphogypsum is a major waste byproduct of wet-process phosphate acid production and is currently regulated under the Toxic Substance Control Act (TSCA) of 1976. For each metric ton of phosphoric acid produced, approximately 5 mt of phosphogypsum is produced and must be stockpiled. The U.S. Bureau of Mines Tuscaloosa Research Center installed monitoring wells in an active 195.4-ha phosphogypsum stack. This stack is approximately 30.5 m high and has been active for 22 yr. The wells cover the entire stack perimeter and consist of five clusters, with three wells per cluster; these wells are 7.6, 12.2, and 16.7 m deep. Core drill samples were taken every 1.5 m and leached with process cooling pond water and artificial rainwater. The wells were monitored monthly, with samples being analyzed for pH and metal and nonmetal ion concentrations. In addition to site studies, pore volumes were collected from column leach tests and analyzed for metal and nonmetal ion concentrations. Initial results from the column leaching tests show that a number of metal and nonmetal ions are migrating out of the phosphogypsum. These results indicate that metal and nonmetal ions contained in the phosphogypsum have the potential for migrating into surface waters and ground water.Additional