The preservation of water quality and environment is increasingly important among the industrialized society. Effluent discharge standards employed by the Bureau of Environmental, Japanese Federal Government have prevented rivers, lakes, and oceans from the pollution by human activities. The number of novel man-made hazardous substances produced by industries is increasing according to the year, resulted in conventional discharge standards ineffective to preserve natural ecological environment. A novel monitoring parameters to detect unknown pollutant sensitively are need to be investigated. In this study, a novel parameter, ecological co-inhabitance index (ECI), was proposed in order to evaluate the water quality for creatures that were actually living the environment. This parameter utilized the population change of benthic organisms. Benthic animals in rivers areknown to be sensitive for environmental change, and was selected as test organismsfor this study. Four typical rivers with different characteristics were selectedto apply to ECI. These rivers receive different kinds of pollution streams. The results indicated that ECI estimated the pollution correctly that enable to reflect changes of pollutants, seasonal environment, and physical conditions.
