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Vegetation Dieback on Clay-Capped Pyritic Mine Waste

School of Land and Food, University of Queensland, St. Lucia 4072, Australia;

D. R. Mulligan

Centre for Mined Land Rehabilitation, University of Queensland, St. Lucia 4072, Australia.

^* Corresponding author (n.menzies{at}uq.edu.au).

ABSTRACT

Waste rock dumps at the abandoned Rum Jungle uranium mine were capped with clay covers in order to limit entrance of water and oxygen into the dump, and hence to reduce generation of acid drainage. The soil covers consisted of a clay seal layer overlain by moisture retention and erosion resistant layers and stabilized with vegetation. An investigation of this cover system, primarily at one of the dumps, was undertaken in response to the concern that the vegetation dieback apparent on some areas was spreading. Reduced vegetative cover could potentially threaten the integrity and effectiveness of the rehabilitation. Areas of vegetation dieback on soil covers were shown to be the result of Cu toxicity. The soil covers had been acidified and contaminated with Cu by capillary rise of water from pyritic waste immediately underlying the soil capping. Over time, the surface soil on areas with progressively deeper capping is being contaminated, resulting in a gradual expansion of the bare area. The activity of Al³⁺ in the soil solution appears to be controlled in these contaminated soils by the solubility of an Al-SO⁴ mineral, resulting in nonphytotoxic Al levels in the surface soil, despite the low pH. The lower portion of the clay seal layer was sufficiently acid that Al toxicity would restrict root growth.