Individuals may differ in what they consider to be acceptable water quality, even for the same water and intended use. Others point out that all of nature uses water to absorb and transport waste products and nutrients, so humans ought to be able to do the same thing in a responsible way. They believe that a demand for zero tolerance is unreasonable and are prepared to accept an approach that follows guidelines to maintain contaminants at levels below which our lives and the health of ecosystems are at reasonable risk. Yet others feel that this attitude does not address concerns about the possible additive and synergistic effects of multiple trace contaminants. They feel it relies too heavily upon traditional approaches instead of ones that might deal better with the actual risk.
Risk assessment is at best an imprecise science.
It combines information on the level of exposure to a substance, as well as its toxicity, to characterize what is likely to happen to humans or other animals that may be exposed. Confusion often arises from the scientific uncertainties associated with this assessment. People tend to take risks when they are self-imposed or well known, but resist involuntary risk. For example, models have been developed to simulate the movement and fate of pesticides in the subsoil and predict concentrations over time and depth. Although these tools are useful, results can seldom be used to say for certain whether real problems exist, unless they are validated using actual data.