Legacy contamination of river sediments from four decades of coal mine effluent inhibits ecological recovery of a polluted world heritage area river

A revised environmental licence that authorises the disposal of coal mine effluent has reduced the severity and spatial extent of water pollution and associated ecological impairment of a high conservation-value river flowing into and within the Greater Blue Mountains World Heritage Area. This study investigated water quality and the ecological condition of the Wollangambe River above and below a colliery effluent outfall and assessed the longitudinal impact 22 km downstream. Results are compared to a previous study conducted in 2012/2013 when the environmental licence allowed hazardous pollutant discharges (zinc, nickel) from the colliery. The current study revealed that water quality and river sediment at sampling sites in close proximity (< 2 km) to the effluent outfall continues to contribute ecologically hazardous concentrations of metals and river macroinvertebrates reflect diminished ecological health. However, further downstream monitoring has revealed a significant improvement in ecological condition that can be directly attributed to the revised pollution licence. We hypothesise that the ecological recovery of the most contaminated reaches of the river that lies proximate to the discharge point is constrained by four decades of accumulated zinc and nickel within river sediments. Nickel (978 mg/kg) and zinc (2080 mg/kg) exceeded ecosystem protection guidelines by 45 and 10 times, respectively. The study highlights the importance of appropriate and site-specific environmental licencing to protect riverine ecosystems of conservation significance from long-term contamination.