Chapter 9 Biogeochemistry of Copper in the Fly River

Abstract

Abstract The factors affecting the distribution and mobility of copper in the Fly River system are described. Once immersed in river water, mine-derived particulates are subject to a number of processes which may result in release of copper into solution. Copper sulfides present in the mine-derived sediments are highly insoluble in river water, but undergo chemical oxidation during processing at the mill and bacterially mediated oxidation in the water column which increases the reactivity of the particulate copper. Solution chemistry is also important with the highest release of copper being found at low pH. Over the pH range encountered in the river system (typically 7.0–8.2), complexation of copper by natural organic matter resulting in the formation of soluble copper–organic matter complexes is an important factor governing the dissolved copper concentrations in the river system. When the river floods, a substantial proportion of mine-derived sediments are transported laterally onto the floodplain and into off-river water bodies. Under wet conditions, these environments tend to act as sinks for copper with very little release of copper into solution. However, drying of deposited mine-derived sediments, as occurs under dry climatic conditions (e.g., El Nino), followed by wetting is likely to cause widespread release of copper into solution. In the Fly River Estuary, particulate copper concentrations in suspended sediments rapidly drop to background concentrations (ca. 42 μg/g) owing to dilution and mixing with the large pool of natural suspended sediments. Dissolved copper concentrations decrease by ca. 50% in the low-salinity zone of the estuary owing to a combination of precipitation and adsorption reactions which remove copper from solution. Based on the processes identified, a process-based geochemical fate/transport model was developed to predict dissolved copper concentrations in the river system over mine life and beyond. The model predicts that it will take at least 40 years post mining for the river system to return to pre-mine copper concentrations. This is because of the large amount of copper-contaminated sediments stored in the river system and on the floodplain which will continue to be a source of soluble copper to the system for decades.