Surface water quality criteria

Numeric criteria

We developed numeric criteria to protect designated uses. Numeric criteria are numbers that specify limits and/or ranges of chemical concentrations like oxygen, physical conditions, or water temperature.

Individual numeric criteria are based on specific data and scientific assessment of adverse effects. A typical numeric criterion for protecting aquatic life usually contains a concentration (e.g., 5 mg/L) and averaging period.

• Example: For some toxic pollutants, 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 in order to support designated uses.

Narrative criteria

The surface water quality standards include narrative criteria to supplement numeric criteria. The narrative criteria are statements that describe the desired water quality goal, such as waters being "free from" pollutants like oil and scum, color and odor, and other substances that can harm people and fish. These criteria protect water bodies from pollutants for which numeric criteria are difficult to specify.

Toxics standards and criteria

Toxics standards are different than conventional water quality standards such as pH, temperature, and dissolved oxygen. Toxics standards contain criteria for chemicals and compounds such as metals, pesticides, and other organic compounds found in the environment. Toxics criteria were developed to protect aquatic life and people who consume surface water, fish, and shellfish.

Although most toxics criteria are discrete values that do not vary with water conditions, the freshwater aquatic life criteria for ammonia, pentachlorophenol, and many metals must be calculated for each specific sample taken. These criteria are dependent on ambient water chemistry conditions, such as temperature, pH, or hardness. We maintain a spreadsheet to help calculate these criteria for you. Please contact us for the current version.

The complete list of aquatic life and human health toxics criteria for Washington's surface waters is included in Table 240 - Toxics substances criteria in the reader-friendly version of the water quality standards. This document also provides clarity on which human health toxics criteria are applicable for Clean Water Act programs such as NPDES permits. Read more about Washington's human health criteria.

Temperature standards and criteria

Water temperature is an important element for the health and survival of native fish and aquatic communities. Temperature can affect embryonic development, juvenile growth, adult migration, competition with non-native species, and the relative risk to — and severity of — disease. Complex life cycles within the biological community require complex temperature standards.

Different species require different criteria

Our surface water quality standards encompass a broader range of aquatic life temperatures to best protect fish populations. This range reflects the intricacy of aquatic life cycles throughout the year.

Fish — such as salmonid species including bull trout, Dolly Varden, and Char — have stringent temperature criteria. Redband trout and warm water fish — like sucker, chiselmouth, and dace — have less stringent temperature criteria.

At key life stages — such as adult migration and holding, spawning, incubation, rearing, and smoltification — salmonids have different temperature needs. The surface water quality standards accommodate these different needs with a range of designated uses. The standards identify specific stretches of streams and rivers where these uses occur. In areas where more than one use occurs, the most protective criteria apply.

Extra protections for early life stages

Temperature requirements are particularly critical during spawning and egg incubation periods. Those life stages need a cooler temperature to ensure that fertilized eggs have high survival success and the embryos develop into healthy emergent fry.

For the window of time when that life stage occurs, the surface water quality standards apply more protective temperature criteria to streams known to have spawning and egg incubation areas. The time frame varies depending on the geographic location and the subspecies present in the area.