The discovery of high levels of heavy metals and DDT in soil at a pre-school in Hamilton led to the realisation that many of the toxic chemicals used previously as horticultural sprays had accumulated in soil that was now classified as residential. This led to wider concerns about residential soil contamination and prompted some councils to insert comments on Land Information Memorandum (LIM) reports that certain residential properties may be potentially contaminated due to previous activities on the site. This issue was further complicated by the unwitting transfer of contaminated topsoil to new subdivisions, and to new developments on older properties that were unlikely to have contamination from previous activities. Potential contamination of a residential property has of course, major implications for its value.
Contamination sampling and interpretation of results should always be carried out by a consultant if the results are required for legal purposes, such as the removal of a comment from a LIM report. If the results will be used for non-legal purposes (i.e. personal interest only), the Ministry for the Environment National Environmental Standard for Assessing and Managing Contaminants in Soil to Protect Human Health (NES) User's Guide is a helpful document to refer to, especially Tables B2 and B3. Please note that the correct interpretation of results is dependent on the site, the sampling methodology, and the exposure pathways.
The following table lists common contaminants of residential soil, their source and guideline values:
Arsenic sprays were used to control sheep parasites, but can be found naturally at high levels. Also used as a timber preservative.
Contaminant in imported Phosphate rock which is used as a fertiliser.
Used as a timber preservative.
Used in horticultural sprays and as a timber preservative.
High levels in Lead based paints, also used as an anti-knock additive in petrol.
Previously used in temperature thermometers.
Used extensively in industry.
Previously used to control grass grub and other pests.
Used as a timber preservative.
Previously used in sheep dips.
Insecticide, as well as timber treatment chemical.
Previously used as a timber preservative
Polycyclic Aromatic Hydrocarbon (PAH)
High levels found in creosote (used for timber treatment), bitumen and tar oil (roading). PAH shouldn’t be a concern for high water content plants such as lettuce, however plants with high oil content may be susceptible to uptake.
Compounds in Suite
Heavy Metals + Mercury
As, Cd, Cr, Cu, Ni, Pb, Zn and Hg
Includes DDT, Dieldrin, Lindane etc
PCP and its main metabolite, TCP
Includes benzo[a]pyrene and a number of other PAH
Taking a Sample
Make sure that your sample is representative of the soil from your property. The best way to achieve this is to take a number of small samples from different places using a small hand trowel, and put them into a resealable plastic bag. Mix well, remove any weeds and rocks, and send to the lab along with a request form.
If there appears to be an area of likely contamination, treat this as a separate sample.In terms of depth, sample the top 7.5cm (3 inches) of soil unless you have reason to sample at a deeper level.
A small hand trowel and some resealable plastic bags.