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P.J. Ritchie and N.A. Currey

Kidston Gold Mines commenced operations in 1984 and built a dam to safely store the tailing waste from the ore processing. The dam was progressively raised 5 times (3 downstream and 2 centreline lifts) and has an active surface area of 310 hectares; stores 66 Mt of tailing and is 32 metres high at its maximum height. The dam was decommissioned in September 1997.
Rehabilitation planning for the tailing dam commenced in 1994 with an 11 hectare direct revegetation trial established in March of that year. A 40 ha trial was established in 1998. Both sites have been the subject of intensive scientific research by the (University of Queensland) Centre for Mined Lands Research group. This research assisted in understanding the issues of revegetation stability and sustainability, biological cycling, soil chemistry and surface erosion.

The aims of rehabilitation is to meet the Queensland Department of Mines and Energy (DME) key closure criteria. These include; creating a stable landform, not only for the dam wall structure but also of low surface erodibility, maintenance of acceptable downstream water quality by controlling poor quality seepage and runoff and by meeting an acceptable final end land use criteria for the structure.

Ongoing research is addressing the long term hydrology of the tailing dam with an aim towards understanding the overall water balance. Three consulting groups are involved in what is considered to be a novel approach. Evapotranspiration rates from pasture and tree species have been measured during the 1999 wet and dry season. This information, along with climatic and soil suction data is then used as one of the key parameters for the unsaturated zone modeling. One output from the “Soilcover” model is seepage into the saturated zone in the tailing dam. Water movements in the saturated zone are being modelled using Modflow. The acid oxidation potential for the dam is also being evaluated in light of the long term water movements in the saturated and unsaturated zones of the dam. This process will allow short and long term prediction of dam seepage quality and quantities.

The geotechnical stability of the final dam wall structure as defined by the Factor of Safety, ranged from 2.0 to 2.3, which meets the long term DME recommended stability target FOS of 1.5 for slopes.

In order to evaluate the impact of metal toxicities in grazing cattle, a grazing trial has been established on the pasture covering the surface sediments of the tailing dam. This work is being supported by the Qld EPA, Qld DME, Qld Health and the NRCET, and will assist in understanding metal uptake in grazing animals on rehabilitated mined lands.

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