Addressing the environmental effects of mining on the Ngakawau river

On 17 May 2005 Solid Energy committed to a five-year programme to improve the water quality of the Ngakawau River. Following extensive stakeholder consultation, Solid Energy committed to agreed waterquality targets of pH ? 4.7 (99% of the time); Al < 1 mg/L (99% of the time); NTU to be no greater than 25 NTU (based on a 30-day rolling median); and clarity to be >54 cm (NIWA Clarity Tube) > 90% of the time during base flow conditions (base flow is typically observed 70 to 75% of the time).

Significant management systems were developed and infrastructure constructed to achieve these targets including site-wide sedimentation ponds, the Stockton Black Water Treatment Plant; the Mangatini Limestone Dosing Plant; and the Mangatini Sump (900,000 m3 ).

For the period January 2011 to September 2011 the target for pH was achieved 97.3% of the time, Al 98.3% of the time, NTU 100% of the time and clarity 89.8%, assuming base flow occurs for 70% of the time. Significant progress has been achieved compare to pre 2005 standards and noteworthy results include the return of galaxids inanga (whitebait) in the Ngakawau River. Further work is required to achieve the agreed targets and Solid Energy has plans to mitigate other sources of acid mine drainage and turbidity to the Ngakawau River.