AWWA JAW69599

To investigate the effect of hardness variability on implementation of the U.S. Environmental Protection Agency's (USEPA's) hardness-dependent water quality criteria (e.g., metals), the authors superimposed effluent hardness and flow data from two publicly owned treatment plants onto 15 receiving water data sets. Although study results suggest that minimum effluent hardness may be used to develop protective effluent limitations for chronic cadmium, copper, chromium (III), nickel, and zinc, the results also suggest that in some cases significant data are necessary to develop protective effluent hardness- based limitations with 80% confidence. The study supports USEPA's recommendation that effluent limitations for hardness-dependent metals be developed with consideration for the dynamic conditions seen in effluent and receiving water bodies. The authors provide guidance for implementing USEPA's hardness-based metals criteria based on real effluent and receiving water data sets. This generalized guidance will help water managers and permitting officials understand how to set metals limits that are protective of receiving water quality. Includes 6 references, tables, figures.