Antimony and arsenic dispersion in the Macleay River catchment, New South Wales: A study of the environmental geochemical consequences

Abstract

A baseline geochemical study of stream sediments and waters of the Macleay River catchment in northeastern New South Wales indicates that although most of the catchment is unaffected by anthropogenic or natural inputs of heavy metals and metalloids, the Bakers Creek - trunk Macleay-floodplain system has been strongly affected by mining-derived Sb and As. The dispersion train from the Hillgrove Sb - Au mining area to the Pacific Ocean is over 300 km in length. Ore and mineralised altered rock from Hillgrove contains vein, breccia-hosted and disseminated stibnite, arsenopyrite, pyrite and traces of gold. Historic (pre-1970) mine-waste disposal practices have resulted in high to extreme contamination of stream sediments and waters by Sb and As for ~50 km downstream, with high Au values in the sediments. In this high-energy fluvial interval, average values of Sb in stream sediment exceed the catchment background (1.1 ppm Sb) by a factor exceeding 200. Similarly, average As values exceed catchment background (7.9 ppm As) by a factor of over 20. Drinking-water guideline values for Sb and As are exceeded by one to three orders of magnitude. Further downstream, stream sediments and floodplain sediments have lower Sb and As contents, although Sb values consistently exceed current interim sediment quality guideline values. Water quality in the trunk Macleay is not strongly affected and drinking-water guideline values for Sb and As are only exceeded locally. Although small volumes of near-neutral mine-water seepages at Hillgrove contain extreme values of dissolved Sb and As, it is likely that much of the stream-water loadings of Sb and As have been liberated from reaction with contaminated stream sediments under near-neutral oxidising conditions. It is demonstrated that Sb and, to a lesser extent As, are bioavailable, with uptake into common riparian vegetation and aquatic alga species. Flood dispersion of contaminated sediment over the Macleay floodplain has formed a layer up to 60 cm thick. In this environment, mobility and bioavailability of Sb and As occur in reducing organic-rich conditions in backswamps, oxidising acidic conditions attending acid sulfate soils and in contemporary pasture and crop growth. Estimates of sediment migration rates and amounts of Sb and As transported in suspension and solution imply that the catchment contamination will be long-term (centuries to millennia) such that environmental effects need to be ascertained and management strategies implemented.