Temporal trends of pollution in an estuary affected by phosphogypsum leachates and acid mine drainage: Waste management implications

The Huelva estuary has been historically impacted by multiple pollution sources, including acid mine drainage (AMD), industrial emissions, and discharges from phosphogypsum (PG) piles. This study analyses the concentrations of heavy metals, major and trace elements, and radionuclides from the ²³⁸U series in two sediment cores collected from the Tinto River estuary: one adjacent to the PG piles (Core 1) and the other on the opposite riverbank (Core 2). A reference core from the Piedras River was used to establish natural background levels. Core 1 provides a clear stratigraphic record of industrial activity, showing three contamination phases. The open-discharge period (60-30 cm) is characterized by extremely high values of P (∼4 %), ²³⁸U (∼1200 Bq·kg^-1), ²²⁶Ra (∼800 Bq·kg^-1), and metals such as As (∼4700 μg·g^-1) and Pb (∼3000 μg·g^-1). Contamination indices (EF, Igeo, PERI) peak in this interval, confirming severe ecological risk. A decline from 30 to 10 cm reflects the shift to closed-circuit waste management, although U and P persist due to PG leaching. The surface layer (0-10 cm) records a marked reduction, but concentrations remain above background, evidencing long-term legacy pollution. The sedimentation rate (sr = 1.2 cm/yr) provides high temporal resolution. In contrast, Core 2 shows attenuated contamination restricted to the upper 20 cm, with lower radionuclide activities (²²⁶Ra ∼ 80 Bq·kg^-1, ²¹⁰Pb ∼ 90 Bq·kg^-1) and metals, and a reduced sr = 0.4 cm/yr. Profiles suggest diffuse AMD influence rather than direct PG input. PCA differentiates contamination sources, confirming PG-dominated inputs in Core 1 and AMD-related contributions in Core 2.