Water quality criteria are expectations that assure that the designated uses are protected. Originally, criteria were a narrative description of water quality. Today, criteria are quantifiable and a system based on long-term, statewide water quality monitoring data has been developed for Ohio.
A number of states are in the process of developing numeric biological criteria in their water quality standards. Ohio is one of the few states that has completed criteria development and implemented their use.
Water chemistry, whole effluent toxicity, and the health of aquatic communities are measured to determine whether water quality criteria have been met. Chemical criteria for individual sites are derived from laboratory studies of biological organisms' sensitivity to specific chemicals or combinations of chemicals. Whole effluent toxicity measures the harmful effects of an effluent on living organisms. Biological criteria are based on aquatic community characteristics that are measured both structurally and functionally. These criteria are used to evaluate the attainment of aquatic life uses. Biological indices are used to measure the populations and diversity of fish and insects living in streams or lakes (biological indices are not applied to ground water areas).
The indices often produce one "score" that indicates the overall degree of health or toxicity. If the scores of biological indices fall short of numeric criteria, then the use designation has not been protected and the water quality standards have been violated. The degree of use attainment is based primarily on aquatic life use designations because they provide the most accurate and comprehensive evaluation of overall water quality. The degrees of attainment are: