New Zealand Journal of Geology & Geophysics abstracts
Temporal and spatial variability of acid rock drainage in a
rehabilitated coal mine, Wangaloa, South Otago, New Zealand
Michelle Begbie
Dave Craw
Cathy Rufaut
Candace Martin
Geology Department
University of Otago
PO Box 56
Dunedin 9054, New Zealand
Corresponding author: dave.craw@stonebow.otago.ac.nz
Abstract The Wangaloa open cast coal mine ceased operations
in 1989, with no restoration of the 252 ha site, and moderate acid rock
drainage developed. A major rehabilitation programme was initiated in
2002 with removal of exotic vegetation, and extensive planting
(>60 000) of native seedlings was begun in 2003. By 2006, most
seedlings were thriving, and, combined with adventive exotic weeds, a
70% vegetation cover had been achieved. The site substrates were highly
variable on the 10–100 m2 scale, and have been characterised
by paste pH (>700 measurements). In 2003, substrates had moderate
acidity (pH = 4.5 ± 0.9) with distinctly acid patches (pH down
to <2). By 2006, the average substrate pH was essentially unchanged.
Some distinctly acid patches had higher pH, and one patch had
apparently become more acid. Water compositions (>100 samples from
15 sites) were also highly variable spatially and temporally. Incoming
stream and rainwater (pH 5–6) chemically interacted with acid
substrates, especially waste rock piles that contain pyrite-bearing
material, and evolved to lower pH (pH down to 3.4), sulfate-rich
waters. A pit lake on the site receives most surface and groundwater
runoff, and this lake, with a water residence time of 1–2 yr, controls
the site discharge water quality. The lake pH varies on a monthly
time-scale from 4.5 to 6.5, synchronised with pH variations in
groundwater boreholes in waste rock. In addition, there has been a
general increase in pH of the lake during rehabilitation from
consistent pH 4.6–4.8 before rehabilitation to near pH 6 during
rehabilitation. The sulfate/chloride ratio of lake water has decreased
from 20 to <10 during rehabilitation as well. These changes in lake
water composition from year to year may be a result of increased input
of rainwater that has had less interaction with substrate than runoff
water had before rehabilitation began.
Keywords acid mine drainage; lignite;
sulfate; pH; boron;
water; rehabilitation
G07003; Online publication date 25 July 2007; Received 21 February
2007; accepted 5 June 2007
New Zealand Journal of Geology & Geophysics, 2007, Vol. 50:
227–238
0028–8306/07/5003–0227 © The Royal Society of New Zealand 2007
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