Dynamics of arsenic in the mining sites of Pine Creek Geosyncline, Northern Australia

Abstract

The transportation and fixation of arsenic (As) in soil and sediments from five mine sites within the Pine Creek Geosyncline, Northern Territory, were examined based on measurements of operationally-defined fractions of As in soils, sediment and evaporates. Arsenic was mainly retained in sediments in the form iron arsenate (Fe-As). In wetland systems, As was retained as Fe-As together with calcium arsenate (Ca-As) from alkaline groundwater and organic-bound As from detrital material. In retention ponds As was retained as Fe-As, Ca-As and residual As (Res-As) up to 1700 mg/kg. Sediment traps can retain As from alkaline and acidic source seepages. The retention of Res-As and other mineral particulates during erosional or controlled process water discharges was associated with high Fe-As and organic-bound As in sediment. Arsenic was retained as Fe-As, Ca-As and residual As in 100 year old tailings at Millar's Battery, Union Reefs mine nearby McKinlay River and the small copper mine lease MLN 95 adjacent Copperfield Creek nearby Pine Creek. Natural geo-mobilisation of As was observed in upstream sediments at Copperfield Creek (5-8 mg/kg), Mt. Bundey Creek (10-12 mg/kg), upstream Ryan's Creek (10-12 mg/kg) and downstream East branch Ryan's Creek (7 mg/kg). Erosion of As-containing mineralisation was observed in the McKinlay River upstream and downstream (23-26 mg/kg) and upstream Ryan's Creek boundary of the Goodall mine lease MLN 1049 (24-40 mg/kg). Overall, As was mainly retained in sediments in the form Fe-As. The concentration data for As were used to propose mechanisms of As dispersion and retention occurring at the various mine sites that can be utilised for future mine water management design to minimise As dispersion.