A laboratory simulation of the mixing of coal mine-affected water with Fitzroy Basin river water

Coal mine-affected water (CMAW) releases contribute to the total wet season stream-flow in the Fitzroy River Basin. Using data from the QLD government ‘Fitzroy River’ website, the flow attributable to CMAW releases in the Mackenzie River sub-basin were calculated as 3.75% and 2.1% in the 2012-13 and 2013-14 wet seasons, respectively. This study used laboratory simulation to investigate the changes in copper (Cu) composition in river water when CMAW is released in the Mackenzie sub-basin. CMAW was collected directly from a mine’s on-site holding dam. CMAW from this dam can be released into a Mackenzie River tributary when certain water quality and flow conditions are met. In April 2015 this tributary was dry, so river water was collected further downstream in the main trunk of the Mackenzie River. Changes in water quality were monitored in eight different quantities of CMAW mixed with the river water (range = 0.2-10% CMAW). Electrical conductivity increased linearly with increasing % CMAW, whereas non-linear responses and possible chemical reaction (indicated by curve inflections) were observed for pH, TSS and alkalinity in the 0.4% and 2% mixtures. Two 5L mixtures of 0.4% and 2% CMAW in river water were continually mixed at a constant speed for three days. Nine DGT samplers were placed in each mixture; temperature, EC and pH were monitored daily. A 30-mL aliquot for total dissolved Cu, a 50-mL aliquot for total alkalinity, and three DGT samplers, were removed from each mixture at three time points (24, 48 and 72 h). The suspended particulate matter (SPM) in the initial samples and final mixtures were collected and are being characterised. Total dissolved Cu concentration in the mixtures was compared to the original waters (river water = 2.2 µg/L; CMAW = 0.97 µg/L). At t=0 h the 0.4% mixture had 1.7 µg/L of Cu, while the 2% mixture had 1.9 µg/L. After 24 h both mixtures had 2.1 µg/L. Cu concentration in the 0.4% mixture was 2.0 µg/L at 48 and 72 h. Cu concentration in the 2% mixture at t=48 h dropped to 1.6 µg/L. These changes over time may be significant for compliance monitoring during CMAW releases. Cu concentration in the SPM were: river water = 34 mg/kg, CMAW = 22 mg/kg, 0.4% CMAW = 34 mg/kg, and 2% CMAW = 30 mg/kg. Analyses of the DGT eluents are currently being verified. It is noted that the composition of the original water samples could vary from day-to-day, site-to-site and mine-to-mine, depending on different climatic and operational conditions. Hence, it is recommended that this investigation be repeated numerous times, controlling different variables. The current method could be improved to more closely represent in-situ river mixing conditions. Overall, the study provided a unique approach to determine the potential effects of releasing CMAW into the Fitzroy Basin river system. The preliminary findings were that Cu concentrations and other water quality parameters varied over the 72 hour mixing period. Cu concentrations were more variable in the 2% mixture. This indicates that the time of sampling and volume of release are important compliance monitoring considerations. This methodology could be applied or modified to study other toxicants of interest