Department of
Environmental Quality

Numeric criteria are required where necessary to protect designated uses. Individual numeric criteria are based on specific data and scientific assessment of adverse effects. Numeric guidelines assign numbers that represent limits and/or ranges of chemical concentrations, like oxygen, or physical conditions, like water temperature.

A typical numeric statement for an aquatic life criterion usually contains a concentration and averaging period (acute – short term or chronic – long term). For example, for toxics, a one-hour averaging period applies for an acute (short-term) concentration, while a four-day average applies for a chronic (long-term) concentration. The criteria are values that should rarely be exceeded if uses are to be supported.

Idaho has numeric criteria for temperature, dissolved oxygen, pH, turbidity, bacteria, ammonia, and a list of 121 toxic substances (both metals [including mercury] and humanmade organic chemicals). The criteria values and the applicability of the criteria to a specific water body depend upon the designated use of the water.

• Temperature, dissolved oxygen, pH, turbidity, and ammonia criteria have been developed to protect aquatic life (fish as well as the aquatic insects they feed upon).
• Bacteria criteria are designed to protect human health and are applied to waters designated for recreation use.
• Toxics criteria are developed for protection of both aquatic life and human health.