Numeric Water Quality Criteria
Numeric criteria are required where necessary to protect beneficial uses. Individual numeric criteria are based on specific data and scientific assessment of adverse effects. Numeric criteria represent limits and/or ranges of chemical concentrations, like dissolved oxygen, or physical conditions, like water temperature and turbidity.
Although most people focus on the face value or concentration component of numeric criteria, most numeric criteria incorporate a duration and frequency of exposure in addition to a magnitude. A typical numeric statement for an aquatic life criterion usually consists of a pair of concentrations and averaging periods (i.e., duration—acute or chronic). Criteria for toxics substances, for example, typically use a one-hour averaging period for an acute (short-term exposure) concentration and a four-day average for a chronic (long-term exposure) concentration. These criteria are values that should not be exceeded more than once in three years (frequency) if aquatic life is to be supported. Human health criteria are based on lifetime exposure and bioaccumulation of substances; thus, the averaging period is a 30-day harmonic mean.
The duration and frequency components of criteria, while grounded in toxicological mechanisms, present challenges in monitoring and assessment. To fully and accurately assess compliance requires abundant data. Typically, abundant data are not available and so various assumptions and extrapolations need to be made. Learn more.
Idaho has numeric criteria for temperature, dissolved oxygen, pH, turbidity, bacteria, ammonia, and 121 toxic substances (both metals [including mercury] and human-made organic chemicals). The criteria values and the applicability of the criteria to a specific water body depend on the beneficial use of the water.
- Temperature, dissolved oxygen, pH, turbidity, and ammonia criteria have been developed to protect aquatic life (fish and the aquatic insects they feed on).
- Bacteria criteria are designed to protect human health and are applied to waters designated for recreation use.
- Toxics criteria are developed to protect aquatic life and human health. Those based on protection of human health from exposure due to fish consumption are applied to waters where recreation is a beneficial use, which are all waters in Idaho.