Acidic receiving waters impact on metal solubility from conventionally treated mine drainage

Research, management and regulations surrounding acid mine drainage typically focuses on the composition of the drainage and treated discharge. Absent from applied and site based research, and consequently regulation, is how lime neutralised alkaline discharge behaves once discharged into naturally acidic streams. In effect , precipitated metals in the alkaline discharge and sediment, the result of lime treatment, are re - acidified as water returns to its naturally acidic state, causing the dissolution of metals into the environment. This is the case at Clarence Colliery, an underground coal mine located in a World Heritage listed area 100 km from Sydney. Total and dissolved metals in water and sediment were measure d longitudinally downstream and sediment from the mine discharge was ex posed to different pH (4 - 8) in extraction tests. Results revealed that remobilisation of metals commenced as soon as 200m down stream (pH change 8.05 to 7.75), with contamination extending 21km downstream. Metal solubility increased in sediments as pH retuned to its natural acidic condition (pH 5). The study highlights the need for environmental regulators to assess metal remobilisation from conventional lime treatment systems and points to legacy contamination issues associated with metal rich sediments alr eady located in stream