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Slide Background

CAPACITY OF CLAY SEALS TO RETAIN RESIDUE LEACHATE

Thomas, G.A., Allen, D.G., Wyrwoll, K-H., Cooling, D. and Glenister, D.

Historically, tailings and process residues have been placed in impoundments either constructed with engineered clay liners or excavated from clay basement materials, collectively considered here as ‘clay seals’. The capacity of these clay seals to retain the leachates is an important consideration in the design of such impoundments. This is especially important when considering the potential environmental impacts that would result from any ensuing contamination of the underlying groundwater system.

Over the last several years, Alcoa World Alumina Australia has carried out a number of investigations on clay seals in such impoundments (residue areas). The investigative work has focussed on its Kwinana refinery where permeable sandy soils increase the dependence on engineered clay liners to contain residue leachate well beyond the operating life of the residue areas. These investigations included detailed examination of cores of clay liners and clay-leachate reactivity experiments.

These investigations reveal that clay seals, designed to have low permeability to residue leachate, can experience enhanced permeability from alteration of clay minerals and the formation of new compounds due to clay-leachate interactions. These investigations indicate that the principal mechanisms involved in clay-leachate interactions are dissolution, precipitation and ion-exchange reactions. It has also been found that the residue leachate can move through the clay seal more in response to its structural features than to alteration of clay minerals as a result of clay-leachate interactions.

These findings have implications in terms of the long-term management of impacts on groundwater quality resulting from enhanced seepage of alkaline leachate from residue areas, which rely solely on clay materials as a base seal. Alcoa has responded to these implications through a number of measures to both minimise and ensure effective management of any groundwater contamination.