The longevity of minewater pollution: a basis for decision-making.

Pollution from abandoned mines is a serious, widespread and increasingly common cause of surface water degradation in the LOIS study region. Indeed, in some areas of the UK it is now the single greatest cause of freshwater pollution. Remediation of such pollution in the long-term is probably best achieved by passive treatment, which has the advantage of concentrating expenditure in capital costs, with only modest financial commitments for long-term maintenance. To plan such remediation, however, it is important to have an understanding of the physical and chemical processes governing pollutant release over scales of many decades. By analysing water quality records for major abandoned mine discharges in Scotland, Wales and Cornwall, long-term acidity generation is shown to have two components: "vestigial' and "juvenile'. The vestigial component relates to the geochemical trauma which occurs as abandoned mine voids are allowed to fill with water for the first time, taking ferrous and ferric hydroxysulphate salts (intermediate products of pyrite oxidation) into solution. Dissipation of vestigial acidity is primarily controlled by the hydraulic retention time of the flooded mine system, and will generally be accomplished in less than 40 years. Juvenile acidity arises from on-going pyrite oxidation in the zone of water table fluctuation within the mined system, and can be expected to continue for many hundreds of years, until the supply of pyrite is finally exhausted. Rational planning for the remediation of minewater pollution should be based on intensive treatment of discharges while the vestigial acidity is being depleted, followed by long-term passive treatment of the juvenile acidity.