Natural background concentrations in the Waikato region

The natural background concentration of an element is the concentration that would exist in the absence of human input.  Some elements exist naturally over a wide range of concentrations and it is important to know whether the amount present at a site is a result of human contamination or natural processes.

Scientists use background concentrations for two main reasons:

• Organisms adapt and are tolerant to the background concentrations in which they evolved.  Scientists need to be able to determine whether a high level is actually toxic to the surrounding environment.
• We need to know when a set guideline value or soil standard is lower than what is naturally occurring to avoid incorrectly describing a natural state as ‘contaminated’, or imposing unrealistic cleanup standards.

Background soil concentrations are also specifically mentioned in the National Environmental Standard for Assessing and Managing Contaminants in Soil to Protect Human Health (2012) which is implemented by district and city councils.

Sampling and deriving background concentrations

The most common method of estimating natural background values is sampling soil at native reserve sites where no direct human use has occurred (i.e. well removed from urban areas and fertilised land).  This approach is applicable in New Zealand where we have experienced negligible atmospheric inputs of trace elements to soil. 

Soil samples from 38 sites have been chosen to represent background concentrations for the Waikato region. Their soil types represent the most common in our region but do not include peat soils as these have quite different properties. 

Each sample was analysed for acid recoverable trace element content (except for total fluorine) and the 95th percentile has been calculated for each element to give an upper limit for the natural range of concentrations. 

Some compounds relevant to contaminated land investigations (e.g. DDT, PCP, Dieldrin and dioxin) are not naturally occurring, so any detection of these compounds indicates a human input.  Small amounts of hydrocarbon compounds (e.g. BaP, PAHs and TPH) may be present naturally due to natural bush fires, volcanic eruptions and decaying organic matter but, as these are extremely variable, background data is not presented for these compounds.

Upper limit background concentrations for selected elements in soil of the Waikato region, acid recoverable data

Please note:

• This table may be updated periodically as more sampling data becomes available.
• If you require information for other trace elements not shown here, please contact us on 0800 800 401.
• If you are conducting contaminated land investigations, local background values may be significantly different to those shown below (especially in mineralised areas).  You might like to consider completing your own background sampling.