Dilution Processes of Rainfall-Enhanced Acid Mine Drainage Discharges from Historic Underground Coal Mines, New Zealand

In much of the world, seasonal weather patterns cause a flush of dissolved contaminants from mined areas at the beginning of wet seasons. However, on parts of New Zealand's west coast, the annual rainfall exceeds 6000 mm/year, with frequent rain days regardless of season; in these conditions, chemically rich flushes are short-lived and subsequently diluted. We studied two historic underground mine sites on the Denniston Plateau. Historic drainages from the Coalbrookdale workings into the Cascade Mine area discharge between approximate to 100 and approximate to 1000 L/s, depending on rainfall volume and frequency. The frequent rain on the plateau dilutes the Coalbrookdale discharge waters, increasing its pH and decreasing dissolved AMD constituents. During short-term high rainfall events, the increased flow causes a flush of stored AMD for less than 10 h, with decreased pH and increased dissolved concentrations of AMD constituents. In contrast, historic drainage at the Sullivan Mine showed little response to rainfall and there was virtually no changes in chemistry during or after high rainfall events. While negative downstream impacts occur at both sites, this study showed that quantification of the variability within a site's flow regime is critical in understanding the impact of a mine site's AMD in high rainfall areas.