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Published: 2002-03-14 00:00:00

Residual levels of pesticides in some Waikato soils could be high enough to be a potential hazard to environmental health.

That was the message to Environment Waikato's Environment Committee this week. A recent research project carried out in the Auckland region by Waikato University PhD student Sally Gaw, in conjunction with Auckland Regional Council, had implications for the Waikato.

The research, which found soil contamination at a number of horticultural properties in the Auckland region, could indicate potential contamination of Waikato horticultural and agricultural areas, and problems with changing patterns of land use.

While modern herbicides and pesticides were designed to break down quickly in soils, some of the traditional pesticides such as DDT and copper did not break down rapidly or were strongly fixed by the soil. Cumulative use of such persistent pesticides over several years could result in rising levels in soil, she said.

Use of DDT stopped in the mid-1970s, but use of copper-based formulations was ongoing and had become popular for use in "organic" growing. Some soils treated with DDT in the 1970s would still contain levels higher than modern guidelines for the specific land use. In the past, it had been an offence not to spray orchards, and chemicals such as DDT were used four to six times a year.

Sally Gaw said in some cases levels of residual pesticides were high enough to represent a potential hazard to environmental health if the sites were subject to sensitive land uses. This means that it is particularly important that contamination risks are considered before land is subdivided.

The study was driven by the fact that large portions of Auckland designated as greenfield areas for urban development had prior history as horticultural land. Residential suburbs had been built over land that was previously horticultural, usually without chemical analysis of the soil or reference to guideline values appropriate for residential use.

Similar changes from horticultural or agricultural to residential or intensive rural had also occurred in parts of the Waikato, particularly lifestyle blocks around Hamilton and Franklin.

Contaminated sites scientist Dr Nick Kim said parts of the Waikato may have been contaminated with organochlorines, and it was known DDT was widely used to control grass grub in some agricultural areas. DDT was also pre-mixed into some fertilisers. Accumulation of copper, particularly on horticultural soils, was a developing problem that required management.

Localised but highly contaminated ‘hotspots’ associated with former sheep dips sites and foot rot troughs contained dieldrin and arsenic, and were commonly near streams or over shallow groundwater. A study to be done in the Waikato will establish if contaminants identified in the Auckland survey were also likely to be an issue in the Waikato.

Committee chair Lois Livingston said she looked forward to the results of the study so Environment Waikato could ensure public safety in the future.”