Environment Waikato decided not to prosecute Carter Holt Harvey for a discharge of sludge and sediment following a dam failure at Kinleith Mill in December last year.
This week’s Regulatory Committee meeting heard that de-sludging of the mill’s treatment system began in January 2002 to improve its performance. One of five treatment ponds was used to provide temporary storage and de-watering of the sludge.
The sludge was made up of dead organisms that digested the effluent. Due to the age of the sludge it would include contaminants from historical operations including PCP, an old wood preservative.
On December 10 last year the area was hit by a high intensity storm and the site engineer undertook an emergency action plan to reinforce the pond. Despite this, the dam failed at 1 a.m. and the contents were lost downstream.
It appeared that harvesting in the nearby catchment had caused water to rapidly reach the dam and exacerbated the failure. The dam had been in operation for 50 years with no previous problems. Processing Industry programme manager David Stagg said the company’s response was commendable in the circumstances and it had co-operated with staff.
Environment Waikato’s investigation of the failure found that the total capacity of the modified pond for de-sludging was about half of its normal design flow capacity. Mr Stagg said the investigation had indicated that the dam should be designed for a one in a hundred rainstorm event and that the company had made changes to the design of the dam for this.
Effects of the discharge were investigated by Environment Waikato. Mr Stagg said that no adverse short-term effects were measured in receiving waterways and it was expected that long term effects were likely to be minor.
Cr Jenni Vernon said the company was fortunate not to be prosecuted and it was only due to their co-operation and the low level of effect that it escaped prosecution. The Committee warned that any similar future events were likely to be prosecuted.