Long-term changes in the quality of polluted minewater discharges from abandoned underground coal workings in Scotland

Long-term trends of changes in the quality of water discharging from abando ned coal mines have been studied for 32 long established discharges in the Midland Valley of Scotland. Six discharges have been studied in greater det ail. providing insights into discharge quality evolution over more than a c entury. It has been found that minewater pollution is most severe in the fi rst few decades after a discharge begins (the 'first flush'), and that even the largest systems settle down to a lower level of pollution (particularl y in terms of iron concentration) within 40 years. Long-term iron concentra tions of less than 30 mg/l are typical, and many are less than 10 mg/l. Low pH values (which might justify the over-used term 'acid mine drainage') do not generally persist, due to the rapid buffering of localized acidic wate rs by carbonates (both natural carbonates, and those introduced as rock pow der for fire precautions during mining). This is corroborated by alkalinity concentrations, which tend to he highest in the early years of a discharge . While the pyrite content of the worked sequence strongly influences initi al water quality (in terms of pH, iron and sulphate) during the 'first flus h', there appears to be no correlation between long-term iron concentration s of discharges and pyrite content of local strata. Rather, higher levels a re found in any sequence where there is scope for fluctuations of the water table in worked ground near to the discharge. A scientific approach to min ewater remediation should allow for active treatment of discharges for the first decade or two, followed by long-term passive treatment after asymptot ic pollutant concentrations are attained.