8 July 2009

Filling of large reservoir at Stockton Mine a milestone in Ngakawau River water quality improvement

Mine water from Solid Energy’s Stockton Mine in Buller has begun flowing into a huge new reservoir, a significant milestone in a long-term project to improve the quality of water in the nearby Ngakawau River.

The $14 million Mangatini Sump, built at the mine, will hold up to a million cubic metres of water and sediment. It is the largest in a series of structures designed to ensure runoff from the mine’s main working areas is treated prior to discharge to the Mangatini Stream and Ngakawau River. The project is designed to meet water quality targets agreed with the local community and West Coast Regional Council in 2005. The works, begun in 2006, have to date cost $40 million.

Future work includes relocating a system which adds finely powdered limestone to the sump water to lower acidity levels and remove metals. This will be done by Christmas and will lead to a marked improvement in the appearance of the downstream water. Work is due to begin to enlarge and improve the performance of a settlement dam in the south of the mine and additional treatment measures in the Fly Creek catchment are being planned. These improvements should be completed in 2010. Fly Creek runs into St Patrick Stream, another tributary of the Ngakawau River.

Solid Energy’s Chief Executive Officer, Dr Don Elder, says the ongoing improvement in river water quality since about mid-2007 is a major achievement and is now a source of significant pride for the energy company, even though environmental work and changes are not yet complete at Stockton. Filling Mangatini Sump means Solid Energy is now even closer to fulfilling its commitments to the local community.

“There’s been coal mining on the Stockton Plateau since the 1880s and it is an understatement to say that, for most of that time, it wasn’t being done with the environment as a high priority,” Dr Elder says. “In March, 2004 we publicly acknowledged that in the past some of our mining activities had fallen short of environmental best practice.

“We made a commitment to improve our environmental performance and asked people in our local communities and other stakeholders to work with us to investigate and develop workable solutions. What was clear from the beginning was that the situation could not be turned around overnight. We asked for people to be patient, recognising that it would be our actions and the results, rather than our words, that people would look to in finally judging us.
“The agreed solution involves prevention as well as treatment. There are physical works like the reservoirs, pipelines and the water treatment plant, and it’s also about the way we plan and carry out our mining, about how we deal with waste rock and the techniques we’re using to re-establish plant cover once work has finished. To get long-lasting improvement, we’ve changed the way we do things at Stockton,” Dr Elder says.

“The effort from our people and our contractors has been substantial but a lot of credit needs to go to the local people who have given so much of their own time over the years, learning about the challenges and potential solutions, discussing them with us and the Regional Council and keeping an eye on developments all the way through. I think it’s as a result of their commitment that other people in the wider community have had confidence that things are on the right track.”

The Stockton water management programme in the Mangatini catchment

Measures to improve water quality at Stockton form one part of the mine’s wider environmental management programme. Water management in the Mangatini catchment (which collects water from the main working areas of Stockton Mine) is designed to ensure the Ngakawau River downstream is consistently of a quality which is suitable for native fish. The targets and timetable were agreed with the local community and progress is monitored by independent audit and reported to interested local people and the Regional Council. Last year, whitebaiters on the Ngakawau reported some good runs of fish and there will be strong interest in seeing if the river again proves to be attractive to whitebait when the season opens in September.

Alongside the approximately $40 million capital cost, the ongoing cost of maintaining and operating the system is estimated to be $8 million a year. It has three elements – minimisation, prevention and treatment.

Minimisation and prevention: Mining pits and other work areas such as roads are engineered to minimise the amount of clean water which can enter these disturbed areas. Mine planning also aims to reduce the amount of disturbed area, while a strong rehabilitation effort has resulted in up to 30 hectares of former mine land being rehabilitated each year.

Advances in the way overburden rock is handled at Stockton also assist the prevention regime. Low-permeability capping layers isolate potentially acid-forming rock from air and rain, significantly reducing the impact on downstream waterways.

Treatment: Before being returned to the area’s natural waterways, water affected by mining is collected and passed through a series of passive and active systems designed to both remove impurities and ensure the overall water chemistry is suitable for sensitive native species. Associated scientific work has resulted in solutions to remove suspended metals from mine water. The treatment system includes a $10 million water treatment plant, commissioned in late 2007, which can clean up to 1.4 million litres of water an hour.
Building the Mangatini Sump involved removing 1.8 million cubic metres of rock. It is the largest of a series of reservoirs which retain mine water, allowing time for coal fines and sediment to settle to the bottom. It is estimated it will take 50 years for the trapped sediments to fill half of Mangatini Sump.
ends

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