Bioaccessibility of metals in sediments and soils impacted by acid mine drainage in Mount Morgan (Queensland, Australia)

Acid mine drainage (AMD) into the Dee River from the historic gold and copper mine in Mount Morgan, Queensland (Australia) is a continuing problem. The pH of the river water is consistently below 3.5 for 18 km downstream of the mine site to its junction with the unimpacted Fletcher Creek. Metal levels in the river water adjacent to the mine were generally elevated (e.g. Al 191 mg/L, Cd 0.06 mg/L, Cu 16.7 mg/L, Pb 0.10 mg/L and Zn 6.4 mg/L). Sediments downstream of the mine site contained elevated Cd, Zn and especially Cu (up to 1700 mg/kg). Soils on the Dee River floodplain (60 km downstream of the mine) were also contaminated with Cu (up to 2510 mg/kg). The bioaccesibility (or potential bioavailability) of metals in sediments (sampled during low and high flows) and in soils were assessed using the BCR sequential extraction (into four fractions) scheme. The fractionation scheme provides a helpful tool in environmental risk assessment by predicting metal contaminant availability. More than 50% of Cu, Cd, Mn and Zn in sediments and soils were associated with the two most mobile fractions (acid-soluble, F1 and reducible, F2), with metal concentrations in these fractions alone mostly exceeding the Australian sediment or/soil quality guidelines. Analyses of lucerne and mandarin oranges grown in the area showed no significant metal contamination. Cu and Zn plant contents in pasture grass in the AMD-contaminated floodplain showed good correlation (0.81 and 0.70, respectively) with their respective soil contents, consistent with predicted bioaccessibility (approximately 60% for F1+F2) of the soil-bound metals. More studies are needed to confirm bioaccessibility of Cd, Mn and Pb, which showed high F1+F2+ reducible fraction, F3.