Resume/ Abstract
The Ria of Huelva estuary is a severely Acid mine drainage (AMD)-affected system that receives the inputs of two strongly metal-polluted rivers, the Odiel and Tinto, which drain the Iberian Pyrite Belt (SW Spain). This estuary experiences a water pH gradient ranging from acidic values (2.5 to 3.5) to alkaline values typical of the seawater. In addition, enormous concentrations of potentially toxic elements (PTE) are transported through the estuary to the Atlantic Ocean. The contaminants transported from the mining areas exhibit different behaviors: Fe, Al, and Cu are removed from the water column through mineral precipitation during the AMD neutralization, becoming part of the particulate matter and finally being deposited in the bottom sediments. Conversely, conservative elements like Zn, Mn, Ni, and Co remain in solution throughout the estuary, reaching the Atlantic Ocean. Other elements such as As suffer from processes of adsorption or mineral co-precipitation and later released back to the water column, also reaching the ocean along with conservative elements. In this context, the precipitation of dissolved metals and subsequent sedimentation of particulate matter within the estuary are influenced by natural processes such as tidal cycles and variations in river discharges. Therefore, the estuarine sediments act as sinks of metals as well as a natural acidity buffer system. However, changing environmental conditions (e.g., sea level rise) might induce metal re-release leading to increasing bioaccessibility and toxicity.