Effect of accelerated weathering and leaching on the chemistry and phytotoxicity of coal-mine overburden

Overburden weathering may influence its effects on soil development and plant growth. Here, an accelerated-weathering experiment was used to determine how weathering affects the chemistry and toxicity of coal-mine overburden. Seven samples of overburden were collected; two samples were from a heap of partly weathered material, and five from mining pits. In both cases, the most weathered top 10 cm was removed. Samples included coal-rich clays (with > 1% coal) and coal-free clays. The samples were artificially weathered by subjecting them to 20 cycles of drying-rewetting-freezing-thawing. Coal-rich clays that have not been weathered were acidic and toxic to Sinapis alba seedlings, but weathering increased their pH, reduced their conductivity and tended to reduce their phytotoxicity. In contrast, weathering tended to reduce the pH of coal-free alkaline clays. S. alba grew poorly in weathered coal-free clay, but when grown in coal-rich clays it grew better in some substrates whereas others were phytotoxic due to their high As content. Weathering decreased Al, As and Na contents and decreased substrate mass by 1–37%. Overall, these results indicate that weathering changes the properties of overburden in ways that are important for soil development and plant growth but the nature and extent of the changes depend on the initial composition of the substrate.