Solid said the sump will hold up to a million cubic metres of water and sediment.

“It is the largest in a series of structures designed to ensure run-off from the mine’s main working areas is treated prior to discharge to the Mangatini Stream and Ngakawau River,” the major Kiwi coal producer said.

“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.”

By Christmas, Solid plans to relocate a system that adds finely powdered limestone to the sump water to lower acidity levels, remove metals and improve the appearance of the downstream water.

Next year, work will be done to enlarge a settlement dam south of the mine and more treatment measures will be added in the Fly Creek catchment area, with the creek running into St Patrick Stream, another tributary of Ngakawau River.

While all the environmental work was not yet complete, Solid chief executive officer Don Elder said the ongoing improvement in river water quality since around mid-2007 was a major achievement and now a source of significant pride for the company.

“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,” Elder said.

“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.”

Solid has spent about $40 million in setting up its water management systems, including a $10 million water treatment plant in late 2007. Operating the entire system is estimated to cost $8 million a year.

The company has also reached a key goal of ensuring the Ngakawau River has water suitable for native fish.

Solid’s ongoing efforts see up to 30 hectares of formerly mined land being rehabilitated each year.

For mine development, Solid is working on its two Cypress pits with first coal expected from the northern one in late 2011.

The Cypress pits are expected to contribute about 5 million tonnes of coking coal to Stockton’s production over about 10 years.