How Clean Is Clean?

When a below ground site contamination assessment is made soil samples are collected and analysed for a range of substances. What is an acceptable result (acceptance criteria) depends on the intended use of the site (which is often not its current use), for example:

• Single house residential with poultry and home grown produce contributing more than 10% of the fruit and vegetable intake. (This criterion applies to most houses with yards even though poultry and / or significant home grown produce is not a real option).
• Residential, children’s day-care centres, preschools, primary schools with gardens and accessible soil where home grown produce contributes less than 10% and there is no poultry.
• Residential with minimal access to soil, including high-rise apartments and flats.
• Parks, secondary schools, recreational open space, playing fields.
• Commercial, industrial locations
• Garden beds

TYPICAL CONTAMINANTS

The types of potential contaminants found on a site depend on its history. Some examples are:

• Metals from foundries
• Lead from leaded petrol or from lead containing paints
• Petroleum and toxic volatiles from underground fuel storage tanks (UST’s)
• Polyaromatic hydrocarbons from timber preservatives or bitumen plants

NUMBER OF SAMPLING POINTS AND 95% UCL

The number of sampling points needed depends on the size of the site, with a minimum of 5 for a very small site, and 21 for a site of 1 hectare. The Guideline allowable level for each substance applies to the 95% Upper Confidence Level (95% UCL) of the average ie if the site passes the criteria then there is a 95% probability or better that the average concentrations of the contaminants are below the acceptance criteria. Thus it is allowable to have some results above the Guideline provided the 95% UCL of the average is acceptable and provided there is no significant hot spot.

COMPOSITING

It is permissible to composite samples for analysis of non volatiles thus reducing the cost, however the allowable Guideline is reduced accordingly. For example if 3 sub samples are composited, the Guideline is 1/3 that for plain samples. Hence if one is certain there is no contamination then compositing can save money, however it is not recommended for cases where contamination may be present.

The general rule is that samples should be taken from at least 2 depths and of each soil horizon. Any composite samples should be made from the same horizon.

DUPLICATES

Duplicate and other quality control samples are also required, with analysis of duplicates by two different laboratories.

HOW CAN NEW ENVIRONMENT HELP?

The above information is a simplified summary.

New Environment can provide detailed advice, site sampling and consultancy.