Statement of critical regional or State water problem: Concentrations of copper in effluents from publicly-owned treatment works and other dischargers in Massachusetts regularly exceed permitted levels. It is generally agreed that current water quality criteria for copper, based on laboratory toxicity tests, are often stricter than intended by regulatory guidelines. The main reason for this is that complexation (binding) of copper by naturally present dissolved organic compounds decreases bioavailability of copper, resulting in lower toxicity in the natural waters than would be observed in the laboratory tests. Conducting toxicity tests in individual receiving waters to develop site-specific water quality criteria is possible, but prohibitively expensive. The U.S. Environmental Protection Agency (EPA) Office of Water has therefore supported the development of a chemical-biological model predicting copper toxicity in a given water as a function of easily measurable chemical parameters including pH, concentration of dissolved organic carbon, and concentrations of major ions such as calcium, magnesium, and sulfate. The EPA, the Massachusetts Department of Environmental Protection (MADEP) and other states' regulatory agencies, as well as the region's dischargers, are all interested in the speedy validation of this "Biotic Ligand Model" as a low-cost technique for developing site-specific water quality criteria for copper.