Mining discharges and environmental assessment and management in estuaries: insights from the Guadalquivir Estuary, Spain

The advancement of low-carbon energy hinges on access to energy transition minerals, driving the development of new mining projects. This review assesses the potential ecological risks of mining discharges in estuaries, with the Guadalquivir Estuary as focal study system. We considered physical, chemical, hydrodynamic, and biological characteristics. Substantial portion of discharged metals would be absorbed by particulate matter and deposited in bottom sediments. The inner estuary, a low-salinity, hypoxic, and sediment-trapping environment, is particularly vulnerable. Its confinement by a dam, coupled with reduced freshwater flow due to climate change and water overuse, exacerbates the retention of pollutants. Furthermore, a recent mining discharge from Las Cruces Mine carried out since 2009 have already compromised the estuary's sediments with high loads of ecotoxic metals. Moreover, two new authorized mining discharges could further contaminate the inner estuary and the downstream Fishing Reserve, where increased salinity may mobilize metals. Riparian forests and Spartina grasslands remain particularly vulnerable to metal pollution. Our analysis highlights the limitations of current environmental assessments, which often oversimplify the complex dynamics of the Guadalquivir Estuary. To adhere to the precautionary principle, we recommend a moratorium on new mining discharges. This aligns with the Non-Deterioration Principle of the Water Framework Directive, emphasizing the need to prevent further degradation of aquatic environments. We propose future research lines to investigate the dynamics of metal pollution and its environmental impacts. Our analysis shows the complexity of estuarine ecosystems and points to the importance of assessing metal metabolism in relation, especially, to sediments and biota in the current context of increasing mining activity.