Numerical modeling of flow in a phosphogypsum stack. Case of salt-marshes, Huelva, SW Spain.

Phosphogypsum is a waste that results when fertilizer is obtained from phosphate through a wet chemical process. Phosphogypsum waste can entail negative consequences for the environment and human health since it is enriched in radionuclides from U-decay series and metal impurities. Phosphogypsum wastes are commonly accumulated in large stockpiles that are exposed to weathering processes. These stockpiles are located near the plants where phosphate is processed, which are usually located in coastal areas. This is the case of a phosphogypsum stack on the western side of the Tinto River estuary (Huelva, SW Spain), where the piles were directly settled on the marshland without using any isolation from 1968 to 2010. Here, in addition to the potential environmental impacts, the effect of the phosphogypsum wastes on human health are a source of concern since the piles are located near the city of Huelva (Spain). In this context, it is of paramount importance to assess the phosphogypsum leachate percolation into underlaid aquifer systems and the release of pollutants to the Tinto River.

This investigation aims at building a complex coupled hydro-chemical numerical model accounting with variable density to quantify how the pollutants are released to the environment. The first step has consisted in developing the flow numerical model that has been calibrated by fitting the piezometric head oscillations as a result of recharge processes and sea tide oscillations. The good fitting obtained during the calibration process (normalized RMS when comparing simulated and observed piezometric heads is less than the 10%) allows affirming that the estimated hydraulic parameters are accurate, and are consistent with the literature reviewed. Furthermore, the numerically calculated mass balance is consistent with the conceptually estimated one, the differences were as expected. Thus, the model allows simulating the flow processes and modelling predictive scenarios. The next steps will consist in implementing variable density and hydro-chemical, and possibly, hydromechanical processes.

This study, which uses numerical modelling, is intended to be useful for future work related to restoration measures and provides new insights into the water balance along with the complex processes occurring at the site.