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Water quality criteria are descriptions of the chemical, physical and biological conditions necessary to achieve and protect designated uses. Water bodies that do not violate criteria are said to “support their uses.” Waters that do not meet their criteria or support their uses are said to be “impaired,” and in need of restoration.
Water quality criteria should contain more than just limits for a few common pollutants. They should be varied and strong enough to define complete success in achieving the Clean Water Act’s goal of “chemical, physical and biological integrity” for each water body in the state.
State-wide criteria associated with each designated use should be seen as starting points. Additional and more protective site-specific criteria can and should be developed for many waters. For example, the state’s acceptable pH range may be too broad; the minimum standard for dissolved oxygen may be too low; or the maximum temperature standard may be too high to protect the most sensitive organisms in some waters in your watershed.
“Numeric criteria” are measurable water quality benchmarks. They are extremely important, because they serve as the basis for developing pollutant limits for discharge permits. They are also invaluable in determining water quality problems and establishing specific, measurable goals for watershed restoration plans.
For most pollutants, numeric criteria are expressed as maximum acceptable concentrations (e.g. dissolved oxygen equal to or greater than 5.0 mg/L). For some other water quality measures, such as dissolved oxygen and temperature, they are expressed as a minimum or maximum acceptable level. For still others, such as pH, they are expressed as an acceptable range.
“Narrative criteria” are statements that establish water quality goals. Some narrative criteria describe a desirable biological condition, such as a balanced, healthy population of native aquatic life. Others express general statements about conditions that should or should not exist. For example, many states’ narrative standards say waters should be “free from substances that may cause adverse effects to aquatic life or human health.”
Since no state can ever set numeric criteria for all conceivable pollutants and water quality parameters, narrative criteria serve as an important backstop to numeric criteria. One of the greatest challenges, is translating narrative criteria into discharge permit limits or into measurable goals for protecting and restoring watersheds. Narrative criteria should always supplement numeric criteria, but they can never replace them.
States have been reluctant to set criteria for pollutants that are hard to measure, such as sediment and nutrients (nitrogen and phosphorus), even though they are the greatest problems in many watersheds. In addition, physical characteristics such as habitat, stream morphology and stream flow are seldom represented in state standards even though they are critical factors to support aquatic life uses.