A water quality index for river management

The theoretical basis underlying the development of a new series of water quality indices is outlined. The main features of their development are the incorporation of legally adopted water quality standards and criteria and the inclusion of information on potential water use and toxic determinands directly within their structural format. Thus an indication of a change in the economic potential of a river through a gradual change in water quality (and hence water use) can be obtained. The General Water Quality Index (WQI) has been applied to all the data for an annual or longer time series as a means of detecting cycles and trends in river water quality. An examination of the lowest determinand ratings for each data set highlights the specific determinand(s) responsible for these deteriorations. A 5 percentile WQI score has been calculated for the time series of data and 90 per cent upper and lower confidence limits calculated around this score. The results of these applications indicate the ability to the index accurately to reflect both changes in water quality and potential water use.