The environment effects of coal mining in Otago, New Zealand. A comparitive study of environmental processes at three historic coal mines in Southeast, Otago.

Black, A

Waste piles created from coal mining activities at three historic mines in southeast Otago have exposed associated pyrite (FeSz) to atmospheric conditions and microbial activity. Subsequent acidification of the surface tailings and drainage waters (acid mine drainage, AMD) has occurred at the Wangaloa opencast, Elliotvale opencast and Brighton adit mines. Mobilisation of arsenic (As) copper (Cu) and zinc (Zn) has occurred, partly as a result of the decrease of pH levels and seasonal changes in hydrological factors, leading to elevated concentrations of these metals in the receiving waters. Authigenic pyrite is enriched in arsenic, to levels reaching 100 ppm. The pyrite is found within the coal measures, which have been deposited in a marginal marine environment and are contained within the Taratu Formation. Copper and zinc are not elevated in either the coal measures or pyrite, and levels generally fall within background fields as determined by geochemical analyses of basement schist in southeast Otago. Discharge and receiving waters, associated sediments, waste rock and designated background samples were collected for analyses during the driest (summer) and wettest (winter) seasons of 1999/2000, with a pilot study undertaken at Wangaloa during winter 1998. During the wettest season of 1998 and 1999, water discharges from the waste piles contained up to 0.7 ppm (mg/kg) arsenic (1998 pilot study). During the wettest period of 1999, no such levels of arsenic were observed, with the highest level attaining 0.07 ppm arsenic. Copper and zinc levels at all three sites were similar, with similar ranges to those of the 1998 pilot study. Copper and zinc were locally elevated in waters with zinc concentrations reaching 1 ppm. During the driest season of 1999, only a few water samples had anomalous metal concentrations. The Wangaloa and Brighton mine drainage, on average generally attained higher metal concentrations than Elliotvale and background levels. The absence of strongly elevated metal concentrations during the drier season is a result of strongly depressed water levels within the waste piles. Flushing of acid and metals occurs when the water levels increase in the waste piles with the onset of the wettest season. During the drier season, remnant pyrite has been readily oxidising as a result of the increase in available atmospheric oxygen variously transported through the waste piles. Adverse effects from remnant waste piles appear to be highly localised and transient due to a combination of geochemical, hydrological and climatic factors. The presence of wetland areas at each site is advantageous, as these sites are sources of passive treatment for AMD discharges. International studies have shown that microbial activity can increase the rates of pyrite oxidation. The sulphur biological potential was assessed for each site and it was found that most substrates contain varying abundances of sulphur oxidising bacteria (SOB). It is suggested that the biological influences of these bacteria are limited in southeast Otago coal mines as a result of several environmental conditions. In particular, low annual temperature (12°C-15°C), is known to have a substantial limiting effect on rates of microbial growth and metabolism.

ix., 165 p ., ill., Maps in text., 30cm.

2000Black_A