California and National Toxics Rule Implementation and Development of Protective Hardness Based Metal Effluent Limitations

The California and National Toxics Rules describe water quality standards for seven metals that vary as a function of hardness: (1) cadmium, (2) copper, (3) chromium III, (4) lead, (5) nickel, (6) silver, and (7) zinc. Some of the methodologies used by California Regional Water Quality Board, Central Valley Region, for assignment of metal effluent limits may be inconsistent with the narrative toxicity objectives of the Basin Plan (i.e., “All waters shall be maintained free of toxic substances that produce detrimental physiological responses in human, plant, animal, or aquatic life”). The purpose of this paper is to describe a methodology for the assignment of metal effluent limits that will be protective under all dilution conditions when the effluent/receiving water hardness is less than 400 mg/L.