5.7 Case study 2:Remediation of former timber treatment plant


The site, which is approximately two hectares in size and zoned industrial, is located four kilometres west of Paihia and is adjacent to the intersection of the Puketona and Haruru Falls roads. A small stream, which flows through the site, is an unnamed tributary of the Waitangi River.

Industrial site after remediation work has been completed.

Site after remediation work has been completed.

Site history

From the early 1960s the site was used for sawmilling and timber preservation. In the early 1980s the sawmill and timber treatment plant was burnt to the ground. A building permit was issued by the then Bay of Islands County Council to rebuild the plant upstream from the Paihia water supply intake. In 1990 the then owner of the site withdrew his application for resource consents for the site and the entire operation was closed down.

During the time that the timber treatment plant operated a number of different methods were used to preserve timber. These included tanalith treatment using copper, chromium and arsenic, a boric treatment using boron salts and immersion treatment using the now banned chemical, pentacholorophenol (PCP).

Initial environmental assessment

Aside from the routine discharge consent compliance monitoring that was carried out at the site from 1984 onward, in June 1994 an initial on-site contamination assessment was undertaken by Council staff.

A number of water and sediment samples were collected from and adjacent to the site. This included areas where it was likely to be contaminated and areas where, due to the distance from the timber preservation plant, there should not have been contamination.

Elevated levels of copper, chromium and arsenic were detected at a number of the sample sites, but no pentachlorophenol. It was concluded that remediation was needed if the site was to be used for other purposes.

Further assessment

In early 2005 a more detailed site assessment was undertaken by Council staff. The aim of the investigation was to more accurately determine the extent of the contamination identified earlier and to collect samples from areas of the site that had not been sampled previously.

This second investigation of the site showed that residual contamination from timber treatment chemicals was confined to an area close to the timber treatment plant and drip pad. The sampling across other areas of the site showed little or no contamination from timber treatment chemicals.

Site remediation

During August 2005 the concrete drip pad was removed. The removal of the pad exposed an area of heavily contaminated soil (approximately eight cubic metres) which was subsequently removed, treated and disposed of in a secure landfill outside the Northland region.

Validation sampling

Following the removal of the soil from below the drip pad, validation sampling was undertaken in the area of excavation. The results of analysis showed that the remaining soils had only slightly elevated levels of contamination from timber treatment chemicals.

Following this round of sampling, fresh uncontaminated clay and soil was utilised to back fill the excavated trench. Further sampling confirmed that the site now complied with the Guidelines for selected timber treatment chemicals (MFE 1997).