3. A risk that is difficult to perceive or quantify

From an environmental point of view, the same source of pollution can have variable consequences depending on how the site or its surroundings evolve over time. For example, a given source of pollution may be tolerated for several years on an industrial site, then become unacceptable if a residential development is envisaged, or if the water table on which the site is located becomes strategic for the region's drinking water supply.

For this reason, the French authorities have chosen not to define strict standards defining the concentration of pollutants in soil or groundwater that must be treated. Like most industrialized countries, the French approach is based on the principle of proportionality between action and potential risk. This approach has proved to be the most sensible way for public authorities to focus the considerable sums of money involved in pollution...