Underground and
opencast coal mining has occurred 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 from historic and current mine workings are the
Mangatini and St Patrick Stream catchments. Solid Energy New Zealand Ltd (SENZ)
is using active (e.g. alkali dosing) and passive (e.g. engineered cover)
technologies to mitigate the adverse environmental effects associated with ARD
contaminated waterways. This paper discusses the passive technologies. Principal
strategies include the capping of high capacity potentially acid-forming volumes
of overburden. The mudstones were found to generally have the highest sulphur
levels compared to other overburden lithologies. Areas with high mudstone
volumes are prioritised to be encapsulated and 390 Ha is planned to be capped in
the Mangatini catchment in the near future.

Monitoring beneath
granite, cement kiln dust and coal combustion product covers show oxygen can be
reduced to less than 10% of atmospheric levels. Laboratory column leach tests
that were subjected to oxygen flux rates of 10 % showed reducing oxygen ingress
into acid-forming material can result in significant acid load
reductions.

The
oxygen content below the cement kiln dust cover was generally lower than 5% and
water quality measured in lysimeters beneath this cover improved after 50 weeks
of emplacement. Column leach tests containing saturated samples were undertaken
to determine the most appropriate water medium in preventing pyrite oxidation.
The sample saturated with ARD (pH ~ 3) had a negligible acid production rate and
leached very minor Al concentrations although Al is soluble at low pH. This
indicated saturating waste rock dumps with ARD can be an effective method in
mitigating ARD formation.