Arsenic contamination at the Mole River Mine, northern New South Wales

Mining and processing of arsenopyrite ore at the Mole River mine in the 1920-1930s resulted in abandoned mine workings, waste dumps and an arsenic oxide treatment plant. Weathering of waste material (2.6-26.6 wt% As) leads to the formation of water soluble, As-bearing mineral salts (pharmacolite, arsenolite, krautite) and sulfates which affect surface waters after rainfall events. Highly contaminated soils, covering about 12 ha at the mine, have extreme As (mean 0.93 wt%) and elevated Fe, Ag, Cu, Pb, Sb and Zn values compared with background soils (mean 8 ppm As). Regionally contaminated soils have a mean As content of 55 ppm and the contaminated area is estimated to be 60 km(2). The soils have acquired their metal enrichments by hydromorphic dispersion from the dissolution of As-rich particulates, erosion of As-rich particulates from the dumps, and atmospheric fall-out from processing plant emissions. Stream sediments within a radius of 2 km of the mine display metal enrichments (62 ppm to 27.5 wt% As) compared with the mean background of 23 ppm As, This enrichment has been caused by erosion and collapse of waste-dump material into local creeks, seepages and ephemeral surface runoff, and erosion and transportation of contaminated soil into the local drainage system. Water samples from a mine shaft and waste-dump seepages have the lowest pH (4.1) and highest As values (up to 13.9 mg/L), and contain algal blooms of Klebsormidium sp. The variable flow regime of the Mole River causes dilution of As-rich drainage waters to background values (mean 0.0086 mg/L As) within 2.5 km downstream. Bioaccumulation of As and phytotoxicity to lower plants has been observed in the mine area, but several metal-tolerant plant species (Angophora floribunda, Cassinia laevis, Chrysocephalum apiculatum, Cymbopogon refractus, Cynodon dactylon, Juncus subsecundus and Poa sieberiana) colonise the periphery of the contaminated site.