Pollution at former minesites is locally a serious problem in parts of Wales and I have been involved in several case studies and remedial projects. Issues met with have included acid mine drainage in which the unstable iron sulphides pyrite and marcasite react with moist air. The result from this process is dilute sulphuric acid with a heavy toxic metal loading which causes damage to aquatic life in nearby watercourses. The photographs below (mouse over for captions) show the marcasite-rich and problematic Cwmrheidol mines.

Potentially catastrophic incidents may occur where a tunnel entrance has become dammed-up by collapsed debris. The photograph below (L) is the entrance to Cwmrheidol No 9 Adit in late 1992. It was dammed by soil and scree. Had the dam failed, over 500,000 gallons of ochre-laden water with a pH of around 2.5 would have suddenly entered Afon Rheidol. That would have been wipeout! I was involved, with Simon Hughes, in the controlled dewatering of the adit and removal of the dam - a successful if messy job commisioned by the National Rivers Authority, back in 1993. Click HERE for the full story!