Landfill of Aluminium Smelter Waste at Wallaroo, NSW, Australia

Geotechnical investigations to assess the physical nature of the subsurface conditions, the gecchemical
properties of the soils and groundwater and the hydrogeological regime existing in an area proposed for the landfill disposal of Aluminitnn Smelter waste were undertaken to ensure safe and responsible managenent of this waste material was carried out.

The studies of the Wallaroo site have indicated the deeply weathered soil profile to have serne useful
characteristics for landfill disposal. The Pennian rocks have been strongly and deeply 1tIeathered to a heavy clay that has a low permeability and the clay mineralogy has been established to have a significant adsorptive capacity for the waste chemistry.

A hydrogeologic model of the site was prepared using finite elerrent canputer techniques and correlated using
rronitored response fram piezerneters installed within serne 90 boreholes drilled to evaluate the site. The groundwater level response and its sensitivity to variable infiltration rates and permeability parameters was then predicted using various model rainfall events.

Based upon the investigation data and the rn:xlelling results, a proposed “Waste Disposal Area” was selected. Two trial pits were constructed in this area, using sand to simulate the waste material. The area of these trial pits was extensively instrtnnented to assess rainfall infiltration to the pits and surrounding ground. The area was then subjected to extrerrely high simula,ted rainfall, using a spray irrigation syst6TI, and the performance assessed.

The effects of leachable fluoride and soditnn and the high pH on the acidic clay have been studied in batch tests and distribution coefficients and maximum adsorption capacities measured. High concentrations of soditnn and fluoride have also been passed through canpacted clay core to study the effects on permeability and a decrease in permeability has been observed.

Following these extensive studies approval was granted for the disposal of wastes in the landfill on the basis of the inherent favourable geotechnical properties of the site together with the various safeguards provided by ongoing monitoring and the favourable chemical interaction of the leachate with the surrounding natural clay. Initial disposal canpaigns have now been conducted successfully.