Sulphur-rich minerals such as authigenic pyrite
(FeS2) are commonly associated with coal mine workings. Metallic
elements such as arsenic (As), copper (Cu) and zinc (Zn) are enriched in the
pyrite relative to their concentrations in the surrounding environment. Exposure
of pyrite and coal to air and water results in a series of oxidation and
hydrolysis reactions that are commonly catalysed by species of Thiobacillus
bacteria. These reactions lead to a lowering of the pH in the drainage
waters (acid mine drainage, AMD) and subsequent mobilisation of trace metals in
environmentally significant concentrations.
This study assesses the spatial distribution of As, Zn
and Cu concentration in water in relation to pH and Eh (redox potential)
downstream from the tailings at the Wangaloa opencast lignite mine in southeast
Otago. This mine was abandoned in the late 1980s due to a decrease in the
economic viability of mining the lignite. The area is showing the characteristic
symptoms of AMD, which is detrimental to the health of several Pinus radiate
growing on some of the tailings.
The pH of the drainage waters can get as low as 2, and
in some instances, lower, but it returns to near neutral (pH 6.5) up to 200
metres down stream from the base of mine waste piles. It is well documented that
the atmospheric oxidation of pyrite is catalysed microbially, causing a
substantial decrease in the pH of the drainage waters. Acidification is then
counteracted predominantly by natural abiotic remediation processes occurring
downstream.