What are the water quality concerns at mines?

Although regulated mines have greatly improved their environmental performance, the use of liquid mercury in informal or artisanal mining (ASM) continues to pose a serious threat to water quality in some areas of the world. [7] ASM produces approximately 20-30% of the world’s gold (500-800 tonnes per year). [9] At least 650 to 1000 tonnes of mercury are released into the environment every year by ASM as a result of poor management practices, which accounts for about a third of all human-released mercury. [9] Other environmental problems associated with ASM include the direct dumping of tailings and effluents into rivers, improperly constructed tailings dams, acid rock drainage from mining areas, and absent or improper mine closure. [10] Monitoring environmental violations and enforcing the rules that combat those violations has been difficult due to a lack of resources and the widely scattered and inaccessible nature of ASM. [10]
http://www.miningfacts.org/Environment/What-are-the-water-quality-concerns-at-mines-/