Seasonal Changes in Arsenic Concentrations and Hydrogeochemistry of Canadian Creek, Ballarat (Victoria, Australia)

A 10-month study of surface waters in Canadian Creek (Ballarat, Victoria, Australia) showed the significant influence of historic gold mining waste material. The investigation focussed on the hydrogeochemistry of the surface waters and soils in order to: (1) document the levels and seasonal trends in major, minor and trace elements in the creek, (2) identify the process by which As is released from the soil/waste mining material to surface waters. For most dissolved major and trace elements (Na, Ca, Mg, K, and As) in surface waters, the concentrations decreased with the increasing rainfall and flow conditions except for Al and Fe. Two sites selected along the creek (< 1 km apart) allowed evaluation of the possibility that mining waste material is contributing to the elevated As concentrations (up to 145 μg/l) in downstream surface water. Arsenic concentration varied more than 28 fold seasonally and was highest in autumn and lowest in spring. Elevated concentrations of As (up to 1946 mg/kg) at the downstream site indicated the presence of a source of As concentration in both surface and subsurface soils. Oxidation of arsenic sulphides under aerobic conditions with redox fluctuations (7 to 201 mV) could cause elevated As levels in the creek. Significant statistical correlations among the major cations (Ca, Na and Mg) point to a common source(s) resulting in neutral to slightly alkaline (pH ～ 6.5 to 7.8) surface water. Fe and Al secondary phases under oxidising conditions are a significant controlling mechanism for the mobilization of As in highly contaminated soils (> 1500 mg/kg) in the study area. The large As adsorption capacity of Fe and Al could be limiting extreme mobilization into the water. Rainfall with relatively low pH is possibly causing mobilisation of Al, Fe and As from highly alkaline soils (pH ≈ 9.0) into the nearby creek.