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