Determining the capability of a vegetation cover to limit effluent leaching from a waste impoundment.

Abstract

A final cover on a waste impoundment is the main physical barrier between the waste impoundment and the environment designed to protect against physical, chemical and biological factors isolating the waste from the atmospheric environment. Since the early 1990‟s regulators in the United States have started accepting vegetation covers in lieu of the prescriptive covers. Currently in South Africa, data that provide field performance comparisons of alternative vegetation covers are few or non-existent; hence a research program was undertaken by an industrial corporation in South Africa to determine the potential use of vegetation covers. In proposing a practical way forward, the Company (AECI Limited) reached an understanding with the Regulators that a vegetated evapotranspiration (ET) cover, would be acceptable provided that its performance in limiting surface water infiltration (and subsequent leaching) could be quantitatively demonstrated. The overall object of this research was to determine the capability of vegetation cover to limit effluent leaching from a waste impoundment. Analysis of the following sub-objectives were required to address and give answers to this study (1) determine, as accurately as possible a climatic water balance on the vegetation covers, (2) determine the geohydrological properties of the material of the waste impoundment, (3) determine the fate of the water i.e. proportion reused via evapotranspiration as opposed to the proportion infiltrating the waste body beneath the root zone and (4) determine the leaching potential below the waste. The study identifies and evaluates the climatic (above ground) and geohydrological (sub-surface) parameters used to estimate the water balance of the materials for a waste impoundment. The study then utilizes these parameters at the respective sites in a finite-element model, called the HYDRUS-2D model, to simulate the water balance of the material. The simulated water balance results were then compared against collected field data, which provide the evidence of the efficiency of a vegetation cover to limit effluent from the impoundment.