Introduction

Sources of contamination

Many kinds of industrial and commercial activity result in materials being deposited on the ground. Some of these materials can migrate from the site of deposition. If the contaminants also have hazardous properties which damage the environment, the impacts upon ecosystems, water resources and man-made structures can be significant. Where this occurs the land is said to be contaminated.

Persistence

Many man-made materials are not broken down by natural processes when they are released into the land. These materials are said to be persistent in the environment. If the material in question is capable of causing some kind of impact, then that impact may continue until the material is removed.

Types of contamination

Many materials of human origin (sometimes called ‘anthropogenic materials’) have properties which make them hazardous to the environment.

These include metals such as lead, mercury, cadmium and zinc, which have been widely used in industry. Compounds of these metals are generally toxic to plants and animals. Some metal compounds are water soluble and highly mobile when released into soil and groundwater. Other compounds are immobile, insoluble and, therefore, have less of an impact than might initially be supposed.

Most metal ions are more mobile in an acidic environment, so the pH of the soil is a very important factor in predicting the spread of contamination.

Organic chemicals are another major category of pollutant in contaminated land. In terms of the number and size of spills, fuels such as diesel and heating oil are highly significant. The impact of the more volatile hydrocarbons, such as petrol, on the ground is often less since a significant proportion of a spill will usually evaporate (more so when the weather is warm) and the quantity of material seeping down into the soil will be reduced.

Some hydrocarbons are not persistent, and will be broken down by the action of soil micro-organisms, but this process takes time and some of the by-products will in themselves cause problems of odour and toxicity.

Chlorinated hydrocarbons are persistent in the environment because micro-organisms are incapable of breaking them down. Most are toxic to soil organisms and other forms of life which will encounter them. Indeed many chlorinated compounds which cause problems as ground contaminants are used as pesticides.

Pathways

Damage will only occur if there is a pathway that links the contaminant to the receptor (part of the natural or man-made environment) and causes harm.

The three elements in this linkage are summarised below:

• contaminant – a material from a man-made source with the potential to cause harm;
• receptor – a living organism, group of organisms, eco-system or piece of property that could be harmed by the contaminant, or water polluted by it; and
• pathway – a route for the contaminant to reach and affect the receptor.

The likely pathway in any contamination incident varies according to the nature of the contaminant. For example, if the contaminant is water-soluble, then it can dissolve in the groundwater. It is then as mobile as the groundwater itself.

Other contaminants, despite being insoluble in water may also be highly mobile. For example, an oil-based material (hydrocarbon) will sink into the ground until it reaches a water-saturated soil stratum – the groundwater or ‘aquifer’. It may move along above the groundwater in the same general direction of the groundwater flow.

The level of groundwater below the surface varies depending on other environmental conditions such as rainfall. Any layer of oil-based material will rise and fall with the groundwater, so contamination can be spread vertically and horizontally.

The pathways are represented in the following diagram: