Mobilisation and attenuation of arsenic around gold mines, east Otago, New Zealand

Abstract

Arsenic is released into the environment by decomposition of natural arsenopyrite in gold‐bearing veins in east Otago. Natural release is slow, but is accelerated by mining activity. Waters from old mine adits have pH of 6–7, compared to normal groundwater pH of 7–8, and adit waters have arsenic concentrations up to 4 ppm compared to background levels of c. 0.01 ppm. Tailings disposed about 80–90 years ago into what is now a wetland at Barewood have arsenic concentrations of up to 33 000 ppm, principally in the form of the secondary arsenic mineral scorodite. The maximum dissolved arsenic concentration in the Barewood tailings is 0.75 ppm at pH of c. 5.5. Dissolved arsenic release at that pH may be restricted to levels near 1 ppm by the formation of scorodite with solubility and/or kinetic controls on scorodite redissolution. Modern mining activity in the Macraes area has resulted in localised dissolved arsenic concentrations in excess of 200 ppm at pH near 10. Evaporation of this water produces scorodite precipitates. Dissolved arsenic released from arsenopyrite decomposition is attenuated by fine‐grained minerals, predominantly phyllosilicates, in soils around weathered veins, and in wetlands downstream from mine workings. Lithic soils can hold at least 50–100 ppm As on the fine fraction (c. 1 μ,m), and wetland soils can hold hundreds to thousands of parts per million on their fine fractions. Both soil adsorption and scorodite mediation slow the release of arsenic into the environment, allowing more effective dilution by downstream waters to safe arsenic levels.