The Weaver's Park lesson

The development of Weaver’s Park, a recreational amenity in the township of Huntly, is a classic example of the environmentally friendly face of today’s extraction industry and creating community value after mining.

The original Weaver’s Crossing Opencast Mine, part of the Rotowaro coalfield near Huntly, was mined by both underground and opencast operations between 1958 and 1993, and was about 60 metres deep when closed.

The ongoing rehabilitation of the former Weaver’s mine began in 1995, moving up a gear in 2000 by turning the pit into a lake through natural run-off from the surrounding catchments – a process that took 11 years.

The resulting lake, now surrounded by a 90-hectare park featuring over 30,000 trees, shrubs and grasses, as well as wetland areas, circumnavigated by walking and mountain bike tracks, was handed over to the Waikato District Council three years ago.

Children from Huntly schools and pre-schools, along with sporting clubs and other members of the community, got involved with the plantings at the park, including those of the Huntly Environmental Awareness for the Future – an initiative supported by Solid Energy.  

The finished park, which cost Solid Energy $2 million and is to be renamed Puketirini by the council, was given the thumbs  up by Dr David Bellamy during his visit there in 2006. It is also the site of the new Waikato Coalfields Museum that is transferring from what used to be the Huntly Mine manager’s residence to set up (as stage one), in the old Huntly railway station, situated in the park. The plan is to build up the attraction into a national coal museum covering all of the country’s coal mining operational and social history.

Over the summer, the lake is well used by the public (particularly the diving platform built about 20 metres out into the lake), while throughout the year it is the home of the New Zealand School of Commercial Dive Training, the country’s only organisation with training programmes in underwater construction trades.

New Zealand has gained a reputation as the world’s best training ground for underwater construction, despite our lack of deep lakes and, with its surface area of 50 hectares and at over 50 metres deep, is ideal for the three month diving course that attracts around 50 people a year – mostly Kiwis, but also a number from around the Pacific and Asian regions.

The company has set up a permanent platform in the middle of the lake (right) that features a decompression chamber and diving bell to travel to the underwater worksite. A lot of the graduates end up working around the world’s ocean oil and gas drilling sites.

Fishy success

Not all the lake’s visitors are welcomed – some are shot and turned into fishmeal at the annual, two-day New Zealand Bowhunters Society World Koi Carp Classic, supported by Solid Energy and the Department of Conservation, and now in its 18th year.

The huge carp were brought to New Zealand as pets and escaped into the Waikato’s waterways around three decades ago. It is a good traveller and has become an aquatic pest in the region where it is known as the ‘possum of the waterways’. Its destructive lifestyle is a major cause of the muddiness of North Waikato lake waters.

On the other hand, the presence of native fish in the waterways of Rotowaro’s working pits signals Solid Energy is doing the right thing environmentally. 

Over the past15 years, as the Rotowaro Opencast Mine expanded, Solid Energy had to divert about three kilometres of streams and waterways from its operations. These included the Awaroa Stream and the Mangakotukutuku Stream to access the Awaroa 4 pit, that has been diverted a couple of times during operations. The stone and gravel lined diversion is designed and excavated so it meanders and properly aerates the water for aquatic life.

A few years ago, Solid Energy commissioned an independent environmental study of these diversions to assess its general practices and procedures for diverting streams. The resulting survey of aquatic wildlife proved that the company’s aquatic mitigation techniques have been very successful. Five species of fish were found, including the giant kokopu – a rare find in the Waikato region and considered be very sensitive to the quality of its environs.

Top soil trials

Another environmental project at Rotowaro is a small-scale soil improvement research trial on rehabilitated mine land using organic biosolids material from local council wastewater treatment plants.

The improvement properties of the biosolid by-product are being assessed for possible use as topsoil in further mine rehabilitation projects, as is practiced in rehabilitating some mining areas overseas. Rotowaro is not blessed with good topsoil. The biologically-treated solids from council wastewater will potentially help to build up organic matter in solids and replace nutrients, reducing the need for artificial fertilisers. 

In these trials, different quantities of biosoils are applied to equally-sized plot areas to measure the effect on surface water-run off and ground water. Water and soil samples are collected and analysed across the plots.

Also involved in the project is Environment Waikato and so far the results look good, says Solid Energy, who is conducting a similar trial at its Stockton Opencast Mine on the West Coast. Acid rock drainage is an ongoing issue for Solid Energy to manage at Stockton the enriched topsoil can be used as a capping layer to discourage the production of acid rock drainage by trapping water and creating an oxygen barrier,

Presuming the trials are successful, the company says it will begin a full scale biosolids remediation programme this year.

Coal ash comes full circle

The burning of coal at the Huntly thermal power station, much of it from Rotowaro, generates a lot of ash which operator Genesis Energy has to dispose of.

An initiative where ash is returned and placed in the backfill of old pits at the Rotowaro Opencast provides an environmental solution to its management and disposal.

In 2007 alone, some 147,000 tonnes of coal ash was returned to Rotowaro following successful trials in 2006. Now around 150,000 tonnes of ash is transported annually in covered trucks from ash ponds near the power station along a private road and then deposited and sealed in specially-constructed pods in the old Awaroa 3 pit, just northwest of the Awaroa 4 (the last operational pit).

Over the next eight to 10 years, Solid Energy plans to dispose of 1.2 million tonnes of ash as backfill in this old pit as part of its rehabilitation.

The ash self-compacts down like cement and is eventually covered with overburden and contoured. Water samples are taken weekly to monitor leachate.