Mitigation of Acid Mine Drainage via a Revegetation Programme in a Closed Coal Mine in Southern New Zealand

Abstract

Acid mine waters (pH 3–5) at the closed Wangaloa coal mine have resulted from surface runoff and groundwater seepage in contact with pyrite-bearing waste rock piles. The low nutrient content, physical factors, and elevated boron levels, all combined with the low pH (down to pH 2) of most waste rocks have limited early planned revegetation and natural plant colonisation. A renewed programme of site rehabilitation, started in 2000, focussed on establishment of near-complete ground cover and functioning ecosystems. Small patches (tens of m2) of low-pH, boron-rich, unvegetated substrates persist after more than 10 years of rehabilitation and introduction of a wide variety of species, but natural colonisation is slowly advancing on these remnants. The rehabilitation has coincided with and contributed to a rise in runoff water pH from ≈4.5 to ≈5.6. This decrease in severity of acid mine drainage (AMD) has apparently become sustainable without further intervention. Near-complete vegetative cover occurred by a combination of a major planting programme and natural colonisation from nearby islands of established native species. Both processes were accompanied by development of functioning ecosystems, and supported by increasing invertebrate diversity and abundance, which are ensuring the persistence of the ameliorative effects on AMD at the site. Vegetation established via natural processes can function ecologically at a higher level than comparable planted vegetation but may not lead to the desired plant cover on some substrates.