Since 1950 Stockton Open Cast Mine has excavated over 60 million tonnes of
Brunner Coal Measures overburden sandstones and mudstones. The current operator,
Solid Energy New Zealand, recognises that acidic drainage from this overburden
and from historic underground mines has led to depressed pH levels and elevated
levels of metals in streams draining the Stockton Plateau. These streams report
to the Ngakawau River. To quantify the effects of mine site acid drainage
remediation/mitigation on the lower Ngakawau River water quality, a catchment
hydrogeochemical model has been field calibrated and ecotoxicology tests have
been undertaken. The hydrogeochemical model indicates that pre-mining and
present pH values in the lower Ngakawau River are similar but present aluminium
and iron concentrations are elevated. Ecotoxicology tests have semi-quantified
the critical metal concentrations and interim water quality targets to achieve
ecological recovery are presented. Analyses of long-term total suspended solids
(TSS) and turbidity monitoring in the lower Ngakawau River and St Patrick's
Stream indicate that TSS of 120 mg/L and turbidity of 20 NTU are exceeded for
<20% per cent of the time. hence, specific criteria to protect migratory>
native fish from turbid water are currently met in the Ngakawau River and St
Patrick's Stream.