Department of
Environmental Quality

A toxic substance is any substance, material, or disease-causing agent, which upon exposure, ingestion, inhalation, or assimilation into an organism will cause death, disease, malignancy, physical deformations, or other abnormalities in affected organisms or their offspring. Examples of toxic substances are metals such as cadmium, mercury and lead, and humanmade organics such as household and agricultural pesticides, many solvents, and other industrial chemicals.

Toxic criteria exist for the protection of aquatic life and human health.

Ambient Chemical Water Quality Criteria

Ambient water quality criteria have been established in Idaho for toxics and other chemicals. These criteria are defined explicitly, so definitive conclusions about compliance can be made. Many of the chemical criteria are complex with exact sampling, analytical, and quality control requirements.

Chemical criteria are defined in terms of concentrations and the frequency and duration of allowable exceedances of these concentrations. Concentrations are usually defined as maximum and average concentrations. These maximum and average concentrations are referred to as criterion maximum concentrations (CMC), or “acute” criteria, and criterion continuous concentrations (CCC), or “chronic” criteria. The allowable recurrence intervals and durations of exposure vary some among the different criteria. Maximum acute criteria are defined as one-hour or one-day (usually one-hour) average concentrations which, except possibly when a locally important species is unusually sensitive, should protect aquatic life if not exceeded more than once every three years. Average continuous, or chronic, criteria are usually defined as four-day or 30-day average concentrations, which should protect aquatic life if not exceeded more than once every three years.

Most numeric chemical criteria are based upon the “National Toxics Rule.” These criteria, including their rules on application (frequencies and duration), are published in Idaho's Water Quality Standards (IDAPA 58.01.02.210) by reference. All Idaho numeric chemical criteria use the one-hour and four-day durations for CMC and CCC, except ammonia. Ammonia criteria use one-hour and 30-day day durations for CMC and CCC.

By definition, if two or more exceedances of either a CMC or CCC occur less than three years apart, the criterion is violated. It follows, then, that no single exceedance within a three-year period can ever violate numeric chemical criteria. However, a single exceedance may violate narrative standards or call for further investigation.

Interpreting Numeric Criteria

Ammonia and several commonly monitored metals have criteria that are expressed as equations. In order to determine whether a criterion is met, the assessor must not only have the results of the monitoring, but must also have access to other information specific to the monitoring site and period. For example, the corresponding water hardness must be known for some metals. Temperature and pH must be known to calculate an ammonia criterion. If concurrent hardness, pH, or temperatures are not reported, the assessor may use typical values, if known, for the water body in question for the period of interest. If typical values are used, the assumptions concerning these values would need to be recorded in the assessment. For example, average summertime hardness might be estimated based on a review of annual USGS Water Resource Data water year reports.

Spreadsheets to easily calculate criteria values given hardness, pH, or temperature are available here for ammonia and metals.