Agricultural and horticultural practices have included the use of various pesticides and agrichemicals. Some of these chemicals can persist in the environment such that hazardous residues can build up and remain in soils. In severe cases this may pose a direct hazard to people and the environment. However, of greater concern is when land use change occurs, particularly the development of residential housing. Land that is suitable for agriculture and horticulture may not be suitable for residential purposes.
A wide variety of pesticides have been used in New Zealand over the last 100 years. New Zealand studies have shown that some pesticide residues such as arsenic, lead, copper and DDT remain in the soil as contaminants. Pesticides containing these chemicals were used extensively in the Tasman District under Government registration until they were withdrawn from sale around 1975. Some tobacco growers continued to use DDT up until about 1985.
These pesticide residues are persistent in the environment and tend to bind tightly to the soil, most often in the top 10 cm. Consequently they may be present in the soil as a contaminant long after they were applied. The more soluble arsenic may leach slowly into underlying groundwater.
Generally, commercial horticultural crops grown on this soil comply with the level of residues permitted in the Food Standards Code as these particular contaminant residues are not taken up by most plants. The exposure pathway of concern for human health is the ingestion of soil. People are much more likely to ingest soil when in their home environment such as via home-grown vegetables and, children in particular, handling and playing in the soil. Dairy and organic farming may also be restricted on impacted soils.
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The present day hazard that former sheep dip sites present ranges from little to no hazard up to a significant hazard (such as where stock health is affected or a drinking water supply is impacted). The more that is known and understood about a former sheep dip site the better it can be managed to limit the hazard it presents to people, stock and the environment.
To assist land owners and farmers to safely manage former sheep dip sites and any associated contaminated soils four Factsheets have been produced. The general premise is that land owners or farmers should identify former sheep dip sites and exclude stock from around the dip site and any surrounding contaminated land as a precaution. The Factsheets also detail other actions to protect the health of stock (sheep, cattle, pigs and poultry), water supplies, edible crops, wild food and, importantly, children and visitors to the farm.
Any changes to the use of sheep dip sites that may create a risk to human health, or any subdivision of these sites, may require a resource consent to be obtained from the Council.
Historically, New Zealand farming practices required the dipping of sheep to control external parasites. It is estimated that are about 500 old and disused sheep dips in Tasman District. These sheep dips were last used 50 or so years ago. The common chemicals used in the dips included arsenic and dieldrin. These pesticide chemicals can persist in soils for long periods of time and can potentially leach into ground and surface waters.
In 2009, with funding from the Ministry for the Environment, Council ran a pilot project assessing 20 former sheep dip sites across the Nelson and Tasman region. Site soils were tested for arsenic and dieldrin residues. This study identified varying concentrations of these chemicals remaining in soils surrounding these former sheep dip sites.
Council is working towards identifying former sheep dips in the District so that they can be managed safely. There is some urgency to locate these old dips while the knowledge about where they are and the stories about their use are still accessible from land owners and farm workers.
Council is also concerned about the inappropriate use and disposal of marine antifouling paints. Research in many parts of New Zealand (and internationally) has shown that when antifouling paint is cleaned off vessels and is not properly disposed of, or is spilt during application, metals and chemicals present can concentrate in soils (and marine sediments) and reach levels which pose a danger to people and the environment.
A project to investigate the nature and extent of contamination at ten historic sheep dip sites throughout Tasman Bay and Golden Bay was undertaken in 2013. The project used a hand held X-ray fluorescence (XRF) meter, supplemented with limited laboratory testing for arsenic and dieldrin. The dips tested included both plunge and spray sites representing private and community dips.
Investigation of nine formal and informal boat maintenance areas was carried out in conjunction with the sheep dip project. These sites were sampled with the XRF to characterise the nature and extent of contamination present and to provide information to council managers regarding these sites.
The XRF proved to be a very useful tool in this project allowing greater flexibility of sampling and providing a good indicator of contaminated areas.
The results for the sheep dip sites indicated the highest level of contamination were around old community dips, with results exceeding guidelines for levels of dieldrin and arsensic. Private plunge dips also breached human and stock health guidelines although usually for only one or the other of the contaminants. Private shower dips tended to show the lowest level of contamination over a smaller area.
Testing around slipways showed copper and zinc to be the metals found in the highest concentrations. Areas without hardstand tended to have low level results around background levels. This is thought to be due to the limited maintenance carried out at these sites in part due to the limited window of opportunity due to tides. Natural coastal processes are thought to redistribute the contaminants in these areas. Larger more commercial slipway sites including both ramps and hardstand areas tended to be very high, exceeding both ANZECC 2000 Interim Sediment Quality Guidelines (ISQG) and human health guidelines.
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Prior to its clean up, the former Fruit Growers Chemical Company (FCC) site at Mapua was considered some of the most contaminated land in New Zealand. The remediation and clean up of the former pesticide factory site was completed in 2007 with funding from both the Ministry for the Environment and the Tasman District Council.
Much of the history of the site and the remediation process are detailed in the Site Audit Report. The site audit has determined that the site is now suitable for its intended land uses, residential in one part, and commercial and open space in another, provided that the Site Management Plan is adhered to.
As recommended by the Site Audit Report monitoring of the site is ongoing. Reports on this monitoring are available via the links listed below. Additional information is also available from the Ministry for the Environment's website.
A condition of the funding required that at least 40% of the site remains as public land. Planning is underway for the development of a substantial waterfront park adjacent to the Mapua Wharf.
Whilst all historic agricultural and horticultural land has the potential to be impacted, land that has historically been used for orcharding is considered particularly vulnerable.
Orcharding practices prior to the 1970s entailed the use of various pesticides and agrichemicals that persist in the environment. A survey undertaken in 2003 found that concentrations of arsenic in soils under the trees and around the spray sheds and storage and handling areas at some historic orchards exceeded the Ministry for the Environment’s residential soil guidelines.
The Council has mapped areas that were established orchards in mid 1975. These properties are included in the Council’s HAIL Register.
Soil testing and, if necessary, remediation is required before such land can be used for residential purposes (e.g. subdivided for housing or construction of a second dwelling).
There may be costs for landowners for testing and remediation.
Soil testing guidelines for assessing contaminated land, including Pre 1970s orchard land, are set out in the National Environmental Standard for Assessing and Managing Contaminants in Soil to Protect Human Health (NES). Only 'suitably qualified and experienced professionals' are able to undertake such assessments for the purposes of submitting reports to the Council under the NES.