7 Conclusions

On a site-by-site basis, risks to human health and the environment from soil contaminated by PCP and dioxin are associated predominantly with the sites (approximately 35) that were relatively large users of PCP. Investigations of some of these sites have shown (although this is not reported in this study) that PCP and dioxin have migrated off site and contaminated, respectively, water bodies and sediment (dioxin is tightly bound to particulate matter in the absence of oil), including two sites where PCP in oil was used as a preservative (Waipa and Hanmer Springs).

Attention to health risk assessment is strongly indicated for such sites if a change from an industrial use to residential use or a life-style block were proposed. Given the scale of operations on these sites and their locations, such changes in land use are unlikely.

Unless the site was a large user of PCP, investigations of PCP contamination by the timber industry and regional councils indicate that little off-site migration of PCP (and by implication dioxins) is likely in either groundwater or surface water. Thus on a site-by-site basis, if little potential for dioxin entry to the food chain or the aquatic environment is apparent, the potential risk to human health is confined to direct exposure to the soil where PCP has been used.

Small-scale PCP use sites

The majority of the study results representative of these 220 sites are below the criterion for an unpaved industrial site, indicating that they pose little potential risk for a continuing industrial use. If there was a change in use to residential, only a small proportion of the affected areas would have dioxin concentrations above the residential criterion. In these areas there could be some increase in risk of exposure if they were not remediated. If there was a change in site use to agriculture, a considerable portion of the affected areas would have dioxin concentrations above the relevant criterion, and there could be significant potential risk if the areas were not remediated.

Medium-scale PCP use sites

The majority of the study results representative of these 28 sites are below the criteria for a paved or an unpaved industrial site. Only where the dioxin concentrations exceed the unpaved industrial site criteria is there a significant risk for ongoing industrial use. In these situations the most significant contamination should be removed or the affected areas paved and a management plan developed to ensure the integrity of the paving, and for appropriate procedures to be followed should the paving need to be broken through (eg, for maintenance).

The criteria for residential or agricultural use are exceeded for most locations and there would be significant potential for risk if a change to either of these land uses were permitted. Provisions should be made by territorial authorities to ensure that such changes in use are not permitted, unless there is a validated clean-up to the relevant criterion.

Large-scale PCP use sites

The majority of the study results representative of these six sites are below the criterion for a paved industrial site. There is the potential for significant risk for ongoing industrial use in the few locations where the unpaved criterion is exceeded unless this contamination is removed or the affected areas are paved and a management plan developed to ensure the integrity of the paving, and for appropriate procedures to be followed should the paving need to be broken through (eg, for maintenance).

The criteria for residential or agricultural use are exceeded for most locations, and there would be significant potential for risk if a change to either of these land uses were permitted. Provisions should be made by territorial authorities to ensure that such changes in use are not permitted, unless there is a validated clean-up to the relevant criterion.

Limitations on the analysis

A more comprehensive and reliable database of sawmill sites that used PCP in New Zealand has been developed, which will be of assistance to local government in the management of these sites, particularly at times and in situations involving a change in land use. Although considerably more data are now available on PCP and dioxin contamination on these timber treatment sites, our knowledge is still limited from a statistical analysis perspective, for the following reasons.

• Due to the changes that have occurred in the industry and on particular sites in the time since PCP was last used, it is difficult to find locations that are still readily accessible for sampling.

• Many sites had more than one process using PCP, and therefore in trying to characterise contamination at a number of locations per site the total data set is still small.

• Storage and waste disposal areas, which may also have PCP/dioxin contamination, have not been covered in the study investigations.

The data generated from this study has been used to revise the existing estimate of the dioxin reservoir at timber treatment sites. However, for the reasons listed above, it still does not provide a definitive picture of contamination across timber treatment sites throughout New Zealand. As a result there will still be some inherent uncertainty in any reservoir estimate.

Significantly, the study confirms the use of the OCDD screen as an excellent technique to assess dioxin contamination at a site. The data suggest a relationship between PCP and dioxin concentrations which may allow PCP to be used as a useful surrogate for dioxin contamination. However, it will be important for the relationship to be validated on a site-by-site basis.