This lecture explores case studies of Decazeville and Guadiamar, focusing on water quality and biogeochemical processes. It covers topics such as mine waste oxidation, AMDs, metal contamination, bacterial interactions, and metal fate in estuaries and oceans. The Decazeville case study delves into the historical industrial center's impact on the Lot River, while the Guadiamar incident highlights the consequences of a massive sulfidic sludge spill. The presentation also discusses the release of heavy metals into the ocean, the hazard for mussels and oysters, and the transformation of metal ions along the contamination pathway. Overall, the lecture emphasizes the complex interplay between geochemistry, biology, and environmental contamination.