Metal speciation in sediments and soils associated with acid-mine drainage in Mount Morgan (Queensland, Australia)

ABSTRACT

This study investigated the influence of acid-mine drainage (AMD) from the historic gold and copper mine in Mount Morgan, Queensland (Australia) on Dee River sediments and adjacent agricultural soils during pre- and post-acid flows and the speciation of metals in these sediments and soils. This was done to assess the potential bioavailability of the metals and to provide baseline information for parallel research on the potential of the metals to enter agricultural produce. Sediments at site 5 (20 km downstream of the mine site) contained Cd, Cu, and Zn levels of 4.8, 2631, and 602 mg kg⁻¹, respectively, during low flow, and 2.6, 1835, and 756 mg kg⁻¹, respectively, post high or acid flow. Sediments at site 8A in the floodplain area (about 60 km downstream of the mine), showed Cd, Cu, and Zn levels of 0.6, 294, and 159 mg kg⁻¹, respectively, during low flow and 1.5, 689, and 295 mg kg⁻¹, respectively, post high (acid) flow. Sediment cores (18-cm long) from site 8A gave ratios of Cd, Cu and S concentration in the top slice (0–6 cm from riverbed surface) to their concentration in the deeper slice (12–18 cm) of 36, 9 and 40, respectively, indicating historical AMD contamination. Lead was not a serious AMD contaminant as its level varied less than two-fold within the core in all cases. Sulfur levels were consistently high (mean = 10267 mg kg⁻¹) in the entire 12-cm core from site 7B and were about 10 times the surface (0–6 cm) levels from the other sites, suggesting possible geogenic S sources at this site. Mean Cu level was 1783 mg kg⁻¹ (range = 368–2510) in soils from the Dee River floodplain. BCR sequential extraction showed very similar average distribution of metals in the four fractions (F1 to F4) in sediments (from sites 5, 7B and 8A) and in the floodplain soils (site Dee P2). The sums of the acid-soluble (F1) and reducible (F2) fractions relative to the total metal content were Cd = 58%, Cu = 58%, Pb = 47% and Zn = 67% in sediments, and Cd = 67%, Cu = 63%, Pb = 47% and Zn = 58% in soils. Cu levels in these two most labile fractions alone exceeded the Australian sediment trigger value by an average factor of 9 (range = 3–23) in sediments, and the ecological investigation level for soils by an average factor of 12. The study demonstrated that AMD contamination spanned at least 80 km of the Dee River downstream of the mine and 30-cm depth of the riverbed and floodplains, with at least 50% of the total metal (particularly Cd, Cu and Zn) in the sediments and floodplain soils being potentially bioavailable to aquatic life and to the agricultural produce in the area.

KEYWORDS:

• Acid-mine drainage
• bioavailability
• metals
• Mount Morgan
• sediments
• soils
• speciation

Acknowledgments

The authors gratefully acknowledge the kind assistance of Ms. Roslyn Howse during the sampling exercises.

Funding

This project was funded by the Queensland Department of Natural Resources & Mines and the Wowan Dululu Landcare Group (Queensland).