Chemical impacts of acid mine drainage in the Dee River, downstream of the Mt Morgan Mine, Central Queensland, Australia

Inadequate rehabilitation over the life of the historic Mount Morgan mine (1882-1981), and since tailings retreatment ceased in 1990, has led to the continued impact of acid mine drainage on the Dee River.

The water quality along the Dee River, for 18 km downstream of the mine site to its junction with Fletcher Creek, is characterised by low pH, consistently below 3.5 (Howse, 2003). The concentration of metals in surface water is generally elevated adjacent to the mine (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) and is acceptable for irrigation/stock water downstream of Fletcher Creek (Al <0.5 mg/L, Cd <0.005 mg/L, Cu <0.05 mg/L, Pb <0.05 mg/L and Zn <0.05 mg/L).

This study investigated the chemical impacts of acid mine drainage from the mine; in particular the concentration of metals was measured in surface water, groundwater, sediment, soil and agricultural produce.

The Dee River Research Project was initiated by the Wowan Dululu Landcare Group (Central Queensland) - a community group with common interests in sustainable farming, grazing and other environmental and community issues. Determining whether acid mine drainage had impacted on agricultural produce was of particular interest to the rural community of Wowan and Dululu.

The metal concentrations in lucerne, lucerne hay and citrus suggested that acid mine drainage from the Mount Morgan mine did not significantly impact on the quality of these produce. The concentrations of Cu, Fe, Mn, Pb, Zn and S in lucerne hay were acceptable for dietary intake by cattle. The concentrations of Cd, Cu, Pb and Zn in all mandarin samples were below the maximum permitted concentration set in the Australian Food Standards Code.

Four acid flow events and fish kills occurred in the Dee River between November 2000 and February 2001 as a result of acid mine drainage from the historic Mount Morgan mine. Measurements of pH along the Dee River clearly demonstrated the movement of a slug of acidic water down the river during each of the four flow events (pH 3.0 at 3 km (Kenbula weir), pH 4.8 at 60 km and pH 6.1 at 80 km downstream of the mine site). Laboratory analysis of water samples indicated Al, Cu and Zn concentration orders of magnitude above ANZECC guidelines. Fish kills occurred with each flow event and killed an estimated total of 26,000 fish.

From November 2000 to February 2001 over 33,500 ML of acidic water (pH <5) flowed along the Dee River and transported over 50 tonnes of Al, 5 tonnes of Cu, 3.5 tonnes of Zn and over 1,500 tonnes of SO4.

Sediments downstream of the Mount Morgan mine site were contaminated with Cd, and Zn and particularly Cu where concentrations of 1,700 mg/kg were 25 times greater than the ANZECC (2000a) interim sediment quality guidelines. Soil on the Dee River floodplain (60 km downstream of the mine) was contaminated with Cu; the maximum concentration of 2,510 mg/kg exceeded the NEPC (1999) Ecological Investigation Level (100 mg/kg) and Health Investigation Level (1,000 mg/kg).

A large proportion (> 50%) of Cu, Cd, Mn and Zn in sediment and soil was found to be associated with the most mobile fractions; metal concentrations in these fractions alone mostly exceeded the ANZECC (2000a) interim sediment quality guidelines/ NEPC (1999) guidelines. The significance of these results is discussed with respect to potential bioavailability of metals to cattle.

Peak metal concentration in groundwater coincided with high metal levels of acid surface water of the Dee River at two sites (6 km and 38 km downstream of the mine site). Results presented herein support the premise that metal -rich water originating from the Mt Morgan mine site could infiltrate groundwater, particularly during acid flow events in the Dee River when preceding groundwater levels are low.

The fate of the metals from the Mount Morgan mine site was inferred from the results and the contaminants pathway depicted in a conceptual model.