Underground and
opencast coal mining has been ongoing since 1896 on the Stockton Plateau,
West Coast,
New Zealand. Acid
Rock Drainage (ARD) from historic and current mine workings has had an adverse
environmental impact on the surrounding aquatic environment. The two main
catchments affected by ARD are those of the Mangatini and St Patrick streams.
These catchments flow into the Lower Ngakawau River, which holds high amenity value for
the local community. Water quality targets set by Solid Energy for 2010 in the
Ngakawau
River include increased pH
and clarity and a decrease in certain dissolved metals.

At-source ARD
minimisation options are the first stage in mitigating the effects of mining
acid forming rocks and reducing acid loads in the Mangatini and St Patrick
stream catchments. ARD minimisation techniques reduce the quantity of acidity
generated and thus reduces the treatment required. Treatment will be required
for many years at Stockton due to current and historical acid
formation. This paper looks at the treatment options that Solid Energy has been
investigating including limestone leach beds, engineered wetlands, oxic
limestone drains, and limestone dosing of catchment drainage. Particular focus
is given to fine limestone dosing of the Mangatini stream and subsequent water
quality improvements.