When the rain comes

The Stockton open pit mining operation is not only the largest in New Zealand but it has a unique problem – coping with a whopping seven metres of rainfall most years and a neighbouring native reserve below its slopes. By ALAN TITCHALL.

Opencast mining at Stockton takes place on an undulating plateau 100 metres above sea level on the West Coast of the South Island, 35 kilometres north of Westport. The ecology and environmental considerations are enormous as the operation is right on the steep ridge boundary of Department of Conservation land and, without suitable treatment systems, the region’s high rainfall leaches minerals from the mine’s disturbed soils and washes material into the local Ngakawau River.

As the coal measures at Stockton were formed in a marine environment they are rich in the mineral pyrite (iron sulfide) and aluminum. A combination of excavation of the overburden and heavy rain creates acid rock drainage where the pyrite is exposed to air and water, which causes it to break down into a mild sulfuric acid.

The solution is a number of reservoirs, sumps and dams to contain the runoff and drop out the suspended solids before the water is treated for recycling and before being returned to the area’s natural waterways.

The works, begun in late 2005, have to date cost $40 million, and involved large-scale water treatment facilities built around the Mangatini Stream and its tributary, Ford Creek, which drain the area of the Stockton Plateau most impacted by coal fines from mining operations. Work also involved URS, an environmental and engineering consultancy, who designed  several dam structures in close association with the Stockton water management project (SWaMP) team. In addition to the $40 million capital cost, the ongoing cost of maintaining and operating the system is estimated to be $8 million a year.

The treatment facilities involve a large network of engineered dams and deep sumps into the sandstone, sealed with impermeable granite, that provide primary settlement of heavy sediments. Pipelines then take the water to a water treatment plant for secondary treatment, before it is discharged into the natural waterways.

The system is made up of three earth-fill dams constructed from sandstone, granite and specialised drainage gravels, and a number of sumps. Duncan’s Sump, for instance, involved the excavation of 200,000 cubic metres of rock to form an 80,000 three cubic metre reservoir. A twin pipeline, decant system and a pump station decants water off the top of the reservoir and pipes it to a water treatment plant.

Duncan’s sump was excavated with sequential drill and blast. Each of the 62 blasts below the coal floor, ranging in size from 15 to 27,000 cubic metres, was monitored with geophones for ground vibration, which needed to be kept within a tolerable level to prevent damage to the sumps’ dam wall.

The most recent treatment facility in this complex system to be completed is the $14 million Mangatini Sump that holds up to a million cubic metres of water and sediment, and is the largest in the series of structures designed to retain run-off from the mine’s main working areas so the coal fines and sediment can settle to the bottom before the water is discharged to the Mangatini Stream and Ngakawau River. Almost two million cubic metres of rock was removed to build the Mangatini sump and it is estimated it will take 50 years for the trapped sediments to fill just half of it.

Future work includes relocating a system that adds finely powdered limestone to the sump water to lower acidity levels and remove metals. Solid Energy says 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 planned (Fly Creek runs into St Patrick Stream, another tributary of the Ngakawau River).  These additional improvements are expected to be completed next year.

Solid Energy’s chief executive officer, Dr Don Elder, says the ongoing improvement in river water quality at Stockton since about mid-2007 has been 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.

“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,” he says. 

“In March, 2004 we publicly acknowledged that in the past some of our mining activities had fallen short of environmental best practice.

“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.”

Elder says a lot of the 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 locals will also be the final judge of the water management in the Mangatini catchment (which collects water from the main working areas of Stockton Mine).

The quality of the Ngakawau River downstream will be self evident and 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,” says Elder.  

Covering footprints

Vegetation and sandstone overburden, up to 25 metres thick in places, has to be removed before the coal can be recovered. The hard sandstone is first blasted before being removed by excavator and truck and placed in an adjacent mined-out area ready for rehabilitation and replanting. Specialised blasting techniques engineered to control vibration and minimise ground movement are employed, but a system of wire barriers are needed to protect the DoC reserve from overburden falling down the slopes.

When any overburden is removed, great care is taken to minimise the surrounding area of land disturbance and it is placed in specially formed disposal areas waiting to be used to  rehabilitate the area following mining.

And the rehabilitation of the opencast mine has its own unique problems. The very high rainfall has eroded, over time, most of the top soil from the plateau, so the vegetation that has evolved in the area is endemic to Stockton’s sub-alpine environment. The replanting programme needs to ensure that plants are hardy and are resistant to low soil fertility, high rainfall and very cold conditions.

Solid Energy says the rehabilitation of disturbed areas has been designed as a result of extensive trials carried out to prove the most suitable vegetation and best planting techniques in this sub-alpine environment.