Phosphogypsum weathering and implications for pollutant discharge into an estuary

Abstract

Approximately 100 million tons of phosphogypsum were stockpiled onto marsh soils of the Tinto River estuary (Huelva, SW Spain). This study focuses on the hydraulic response of the phosphogypsum stack to the different weathering agents, as well as on the hydrochemical behavior of highly acidic and polluted effluents from its leaching during different hydrological conditions. To address these issues, a CTD-Diver was installed in a bore-hole within a phosphogypsum stack profile, which recorded the variations in the water-table of the stack, and edge outflows samples were collected around the perimeter of the stack during four sampling periods in different seasons. During dry periods, the water-table of the stack remains almost static and is controlled only by the tide oscillations. However, during rainy events this water-level rapidly increases up to 20 cm and subsequently decreases, defining peaks that coincide with the rainfalls. Having a hydraulic connection to the sea and groundwater flow in conduits, the phosphogypsum stack behaves as an anthropogenic karstic-coastal aquifer. Regarding the hydrochemical behavior of the edge outflows, the concentrations of most pollutants (e.g., PO4, Al, As, Cd or U) showed a slight decrease from the dry-warm to the rainy period. These leachates releases high concentrations of contaminants to the estuary, e.g., PO4, As and U (average values of 5000, 6.9 and 3.0 t/yr, respectively). The results obtained in this study could contribute to the development of effective treatment systems for leachates from phosphogypsum stack of Huelva and to minimize their impact on the surrounding estuarine environment.