The Evaluation of Soil Covers used in the Rehabilitation of Coal Mines
Report No. 1002/1/04
March 2004

EXECUTIVE SUMMARY

1. INTRODUCTION

Environmental impacts related to coal mining have long been recognised. Of particular concern is the impact on water resources where long-term deterioration of water quality may threaten water resources and aquatic ecosystems in particular catchment areas.

Huge surface areas are disturbed during opencast and high extraction mining and large areas are covered by coal discard dumps. The high rainfall infiltration rates into these facilities mean that large leachate volumes are generated. In addition, the high oxygen ingress rate into the spoils and discard results in increased oxidation of sulphide minerals and increased growth of iron-oxidising bacteria populations. For spoils and discards prone to acidification, this accelerates Acid Coal Mine Drainage processes, resulting in reduced pH and increased soluble heavy metal concentrations and salinity loads.

One of the obvious and practical sustainable methods in limiting the generation of leachate from coal discard dumps and opencast mines is the provision of vegetated soil covers. Soil covers are probably the most commonly used mitigation measures to limit the generation of leachate from landfills, tailings facilities and waste rock piles.

The main objectives of soil covers should be to limit water infiltration and impede oxygen ingress into the coal discard or spoils and to inhibit the growth of large iron-oxidising bacteria populations. In this way, the leachate volumes are significantly reduced while the quality of leachate is improved because of lower sulphide oxidation rates.

International literature contains many studies where soil covers have been researched and used for the mitigation of many kinds of mine wastes including tailings, rock dumps and coal discard. The research conducted at the Kilbarchan experimental site is, to the authors' knowledge, the first detailed research on soil covers designed to protect water resources.

This report incorporates the results of an extensive ongoing research project, which was initiated in 1993 with the construction of test cells, simulating cover configurations of coal discard dumps, at the experimental terrain near the old Kilbarchan Collieries. The construction of the test cells formed part of Phase I of the research project, which aimed to evaluate the performance of the different test cells in limiting leachate generation into coal discard.

The main objective of this research project (Phase II) was to understand better the factors that result in the generation of leachate. From this understanding, existing unsaturated flow models could be calibrated and the calibrations used to optimise cover design, given the soils available for cover.

The objectives for this research project according to agreement are:

• To develop a clear understanding of the flow mechanisms in the top unsaturated soil layers of a natural soil cover on a microscale, in order to model recharge accurately. (Existing mathematical models to be used as a basis for the study).
• To obtain continuous soil moisture profile data through the top soil layers in order to be able to calibrate the mathematical model developed in the first objective.

In view of the obvious relationship between leachate quality and oxygen ingress through soil covers, the research team investigated this aspect further. It is believed that the use of soil covers may result in better quality leachate because of the covers' ability to impede oxygen ingress, thereby reducing sulphide oxidation rates.