Soil contamination

A site is considered to be contaminated when the levels of hazardous substances found at a site are significantly higher than that of ‘normal’ levels, and there is likely to be a risk to health or the environment.

So what are ‘normal’ levels? Surely any amount of hazardous substance is bad? Normal levels are often referred to as background levels.

Some hazardous substances occur naturally in the soil from weathering of rocks or geothermal activity, such as arsenic, mercury and lead. Many minerals are needed by organisms to be able to exist but they are often needed in tiny amounts, and these chemicals can become toxic if they are in the soil at high concentrations.

Other hazardous substances are created by people so do not occur naturally. In the past, it was not known that many of these chemicals were hazardous and so they have accumulated in our environment. One example is the organochlorine DDT, which was widely used as a pesticide but is now banned.

Levels will be set by scientists that are considered ‘safe’ for that compound. Different levels may be set depending on what the land is used for. For example, the amount of dioxin in the soil is set at a one in a million chance of causing cancer in a residential area, and a one in one hundred thousand chance in a non-residential area.

How does the land get contaminated?

Land can become contaminated when hazardous substances are leaked, spilt or disposed of. Often the contamination was unintentional or occurred despite following recommended management practices. In the past, the use, handling and storage of hazardous chemicals were not as controlled as they are today. People in those days were not so aware of the damage being done to the environment or the effects these chemicals could have on health.

There are many potential sources of contamination – in fact, the Ministry for the Environment has put together a list of 52 land uses that can possibly cause contamination. A few common land uses that could well be in your area include.

• sheep dips
• timber treatment sites (including sawmills)
• scrap yards
• service stations
• airports

The contamination from these sites can be released into the environment by stormwater runoff, leaching into ground water, wind blown dust or evaporation to the air.

It is especially important that residential areas are free from contamination – why do you think that is?

Other things to think about:

• How do scientists decide what is a safe level of a toxin in the soil, air or water?
• What happens if the risk turns out to be greater than previously thought?
• Is the level going to be the same for soil, air and water?
• What does it mean to have one chance in a million of causing cancer? How do scientists work out this level? Do you think this is an acceptable risk?