Trace metal contamination along an estuarine gradient under increasing anthropogenic pressure: A case study from the Guadalquivir estuary (SW Iberian Peninsula).

This study investigates trace metal contamination in the Guadalquivir Estuary (SW Spain), a historically impacted system influenced by both past and ongoing mining. Over two years (2022-2023), sediment and water samples were collected at 15 stations along the estuarine gradient. The objective was to provide a comprehensive assessment of current contamination levels before the onset of discharges from newly approved mining projects. Results showed a clear spatial pattern, with concentrations of metals increasing upstream, particularly near the discharge point of the Cobre Las Cruces mine. Arsenic and zinc also exhibited secondary peaks at the estuary mouth, overlapping ecologically sensitive areas such as Doñana National Park, nursery grounds, and shellfish harvesting zones. These spatial patterns likely reflect both anthropogenic sources and hydrodynamic conditions favoring metal accumulation. Although concentrations did not exceed effects range-medium thresholds, ecological risk indices indicated considerable risk at several sites. Physicochemical parameters, particularly pH, salinity, and dissolved oxygen, seems to modulate metal mobility. In the upper estuary, hypoxia and low pH may enhance metal remobilization from sediments, especially during high freshwater discharges. Overall, the study highlights the estuary's vulnerability to metal pollution and the need for integrated, long-term monitoring including sediments, water, and biota. The results offer essential data to guide future impact assessments and inform environmental management.