In the past, hazardous substances have been manufactured, used and disposed of without full knowledge of the potential impacts of these activities on human health and the environment. Examples of hazardous substances associated with activities known to have caused an unacceptable risk to human health or the environment through land contamination include:

• Polychlorinated biphenyls (PCB's);
• Organochlorine pesticides (OCP's);
• Heavy metals such as arsenic, lead, chromium, cadmium and mercury; and,
• Hydrocarbons.

In most cases, land contamination is the result of past activities that have been poorly managed or where there was a general lack of knowledge of the potential impacts that certain substances may have on human health and the environment. At present, although hazardous substances are generally managed in a responsible manner, there is still potential for land contamination to occur due to unforeseen circumstances, accidents or criminal acts.