Pollutant pathways from a phosphogypsum disposal area to an estuarine environment: An insight from geochemical signatures

Abstract

This paper reports about the contamination potential of the phosphogypsum stack from fertilizer industry on the Estuary of Huelva (SW Spain). Inside the waste pile (ca. 1200 ha and 100 Mt), highly polluted acid porewaters flows up to the edge of the stack, emerging as small fluvial courses, known as edge outflows, which discharge directly into the estuary. The disposal area is divided into four zones; two unrestored zones with surface ponds of industrial process water and two a priori already-restored zones. Preliminary restoration actions and those planned for the future prioritize removal of ponded process water and cover of the phosphogypsum with artificial topsoil. These actions presuppose that the ponded process water percolates through the porous medium towards the edge up to reach the estuary. Accordingly, the already-restored zones would not produce pollution on the ecological receptors. However, this study reveals by using geochemical tracers such as rare earth elements (REE) and Cl/Br ratios that this statement is far from reality. An extensive sampling of edge outflows conducted in the perimeter of the four phosphogypsum zones demonstrates the high potential of contamination of the whole stack, including those zones that were supposedly restored. These solutions are characterized, on average, by a pH value of 1.94 and concentrations of 6111 mg/L for P, 1967 mg/L for S, 617 mg/L for F, 205 mg/L for NH4 +, 125 mg/L for Fe, 28.5 mg/L for Zn, 16.6 mg/L for As, 9.52 mg/L for U, 7.02 mg/L for Cr, 4.27 mg/L for Cu, 3.62 mg/L for Cd, among others. These findings should lead to an immediate change of future restoration plans and improve the preliminary already taken mitigation measures.