Water quality impact from the discharge of coal mine wastes to receiving streams : comparison of impacts from an active mine with a closed mine

This study examined two underground coal mines in the Sydney basin and investigated the water chemistry impact from their wastewater discharges to surface receiving waters. One mine closed 17 years prior to the study, and the other was still active. The geology of both mine locations shared many similarities and some important differences that influenced their wastewater chemistry and its subsequent impact on receiving waterways. Water quality of wastewater discharges from the two mines and their receiving waterways was investigated over a 6-month period. Both mine discharges caused comprehensive modification to receiving water chemistry. The closed mine increased electrical conductivity (EC) 3.3 times from upstream (33 μS/cm) compared to downstream (108 μS/cm). In comparison, the active mine increased EC by 9.4 times (173 μS/cm) upstream to 1628 μS/cm downstream. Both coal mine wastes increased the concentration of different contaminants to levels that are potentially hazardous for receiving water ecosystems. The active mine increased bicarbonate concentration in the receiving water by more than 60 times to 743 mg/L. The closed mine increased zinc and nickel concentrations in its receiving stream by 70 and 20 times to 318 and 360 μg/L. The active coal mine discharge was dominated by sodium and bicarbonate ions compared to magnesium and sulphate ions in the closed mine drainage. Although both receiving waters were sodium and chloride dominated upstream of the mine waste, their ionic composition was strongly modified due to the inflow of coal mine wastes. Results from this study are a reminder that water pollution from coal mines is important for both active mines and for closed mines decades after mining activity ceases