Park expects further ecological improvements will occur after plans are put in place this year by the Tasman pulp and paper mill to close its bleaching plant. The mill complex has been discharging wastewater into the river since 1955.
He says the mill will instead focus on unbleached pulp products, a step which will result in reductions in chemical use and the amount of colour waste entering the water.
The council has been responsible for managing and monitoring the water quality of the river since the Resource Management Act (RMA) was introduced in 1991.
At the time Tasman was pumping over 160 million litres of industrial waste into the river every day, oxygen and toxicity levels were at a point where fish were barely surviving and tannins and lignin waste from the wood products manufactured at the mill had turned the water black.
"Colour was a particular focus for the community," says the council's regulatory services general manager Sarah Omundsen. "Although lignin causes water to look unappealing, there is little or no ecological impact and it can occur naturally at high levels in rivers and streams."
But she says many believed the river was being poisoned and it was commonly referred to as the "black drain."
Most of the pollution has been caused by discharges from the mill, though sewage, geothermal and stormwater run-off have contributed along with nitrogen from a small number of dairy farms.
In the years since the 1990s, a series of process and operational changes at the mill have reduced the amount of contaminants entering the water with the result, Park says, the amount of colour discharge has fallen from more than 100 tonnes a day to about 15 tonnes now.
Park says the mill is required under a seven-year research plan to identify ways in which this can be cut to less than 10 tonnes, the amount determined to not cause a conspicuous change in the appearance of the river water (a requirement of the Resource Management Act).
The river flows for 65km from Lake Tarawera, past Kawerau before turning north where it reaches the Bay of Plenty coast about 6km west of Edgecumbe.
But Omundsen says the mill's ability to meet consent requirements is regularly monitored through self-reporting and compliance checks by council staff.
"The health of the water is also monitored by the council and includes regular site visits, audits, water quality sampling, water temperature, flow and pH (acidity) levels," she says.
"Indications from recent monitoring tells us that the myriad of changes introduced over the last 20 years (technology changes as well as closure of two paper machines and a mechanical pulp mill) have significantly reduced the level of contaminants in the river and improved its appearance.
"When development of the Tarawera Plan (following the introduction of the RMA) started the colour levels in the river were at least three times higher than what they are now."
Omundsen says while the ecology in the lower river is still affected to varying degrees, it has improved markedly over the last three decades.
