Decrease in acid mine drainage release rate from mine pit walls in Brunner Coal Measures

Wall wash samples were collected from outcrops of Brunner Coal Measures aged between 3days and 138years in the Stockton, Denniston and Garvey Creek coalfields of the west coast of the South Island. The purpose of these wall wash samples is to measure differences in the amount of acid and trace elements released as the rocks weather to predict the longevity of acid mine drainage (AMD) from pit wall run-off. The concentration of acid released in wall wash is relatively high and constant for the first year, followed by a rapid decrease that can be fitted with a power function. Data analysis indicates that AMD from pit walls is likely to have an impact on water quality for 64-136years. The most abundant component of acidity in the wall wash samples from fresh pit walls (c. <1year) is Fe. At moderate age sites (c. 3-20years), Al is the most abundant component of acidity in the wall wash, and at the oldest sites (c. 62-138years), H+ is the most abundant component.