Southland contains over
70 per cent of New Zealand's recoverable coal resources, and an investigation
into the potential effects of coal mining on Southland's water resources is
therefore of interest. This paper investigates pit-lake water quality in
relation to current mining activities within the sub-bituminous coal and lignite
deposits of Southland.

Water quality data was
collected from pit-lakes in sub-bituminous and lignite deposits from October
2005 to March 2006. After periods of settled weather, turbidity increased with
depth at the sub-bituminous pit-lake (10 - 21 NTUs from 5 - 30 m). In the
stratified sub-bituminous pit-lake in March 2006, heavy rainfall caused higher
turbidity in the epilimnion than the top of the hypolimnion (20 NTU vs 13 NTU)
reflecting surface water runoff (up to 1993 NTUs) and epilimnion mixing. In
contrast, turbidity at the lignite sites was variable due to the shallowness of
the lakes, but noticeable increases following disturbance and adverse weather
were recorded. High turbidity at the sub-bituminous deposits was attributed to
the presence of clay minerals such as kaolinite whereas turbidity at the lignite
sites was more likely short term and a result of heavier minerals such as
quartz.

Trace element
concentrations at both sites were low when compared to mine waters in other coal
areas in New Zealand and
no samples exceeded ANZECC water quality trigger values for 80 per cent species
protection. Pit-lake water pH levels were lower and more variable in the lignite
deposit (4.18 - 7.30 vs 7.24 - 8.98 in the sub-bituminous deposits). In the
lignite pit-lakes, higher concentrations of trace elements (eg Dissolved As -
0.034 ppm and Dissolved Mn - 1.09 ppm) and lower pH (4) were observed at the
bottom of active pits compared to inactive pits. It is possible that quartz
pebble conglomerates containing sulphide minerals within the lignite seams are
responsible for this.

Further investigation
into the mine waters, waste rock and lake sediments will help get a better
understanding of the results observed and long term processes that
occur.