EPA publishes recommended water quality criteria corresponding to a number of key designated uses. For aquatic life uses, criteria for both short-term (acute) and long-term (chronic) exposures are provided. Different criteria for freshwater systems and marine (saline) systems are often provided. Most human health criteria, except certain pathogens, address chronic exposures. (The OST WQS Web site).

States, tribes, and territories are not required to adopt the exact numbers that EPA has published, but once EPA has issued a criterion for a parameter, they must adopt a corresponding criterion. Such criteria must provide the same level of protection as EPA's, and state/tribe must document that this is the case.

The table to the left illustrates several basic principles regarding WQC. Note that the toxicity of pollutants differs depending on whether they are in fresh or salt water environments. However, there is no predictable pattern as to whether a pollutant is more or less toxic in fresh vs. salt water (copper is more toxic in marine water, cadmium in fresh water).

On the other hand, the chronic criterion for a pollutant is always more stringent than the acute criterion, as shown by the cadmium numbers in the table to the left. This is because of the well-known fact that long-term exposure to lower concentrations of contaminants can cause exactly the same negative effects as short-term exposure to much higher pollutant levels.

Finally, the table illustrates the fact that the form (or species) a pollutant is in changes its toxicity. Hexavalent chromium is much more toxic than trivalent chromium.

The following table is another illustration of how environmental conditions can affect the impact of a pollutant in aquatic life. As the temperature of the water increases, the toxicity of ammonia (NH3) also goes up -- the criterion gets "lower." To further complicate matters, the acidity (pH) of the water also affects the toxicity of ammonia.

EPA is currently developing and issuing technical guidance that can be used to help set WQC for nutrients (nitrogen, phosphorus). (The OST Nutrient Criteria Web page).