Large and usually abiotic store of a nutrient in a biogeochemical cycle. Exchanges between the reservoir pool and the active pool are typically slow by comparison with exchange within the active pool. Human activity, such as the mining of mineral resources, may profoundly alter this exchange rate, generally releasing an excess into the active pool which can be accommodated only by establishing a new equilibrium. This may in turn produce unfavourable conditions, manifested as chemical pollution, e.g. excess phosphorus in eutrophication, excess sulphur in acid rainfall, and lake acidification.