This paper seeks to evaluate the USEPA Water Quality Criteria for copper, where copper is measured by the total recoverable method and the criteria are based on water hardness. Photo- oxidized stream water (pH 7.69 +/- 0.22, water hardness 81 mg/L) was used to determine the LC50 (43.2 ug/l) of copper for Daphnia pulex. Graded dilutions of nonphoto-oxidized stream water were used in a toxicity test with constant hardness (81 mg/l CaCO3) to determine the effects of organics on Daphnia mortality. It was found that mortality was inversely related to dissolved organic matter; mortality was directly related to cupric ion content; and cupric ion content was inversely related to organic matter. These data suggest that dissolved organic matter (in addition to hardness) has a significant impact on copper toxicity. A probabilistic deterministic methodology was used to determine if the EPA Criterion Maximum Concentration (CMC) for copper was being violated in streams at five sites that received urban runoff. Each site exhibited some degree of CMC exceedence. A water quality criterion for copper based solely on hardness can lead to gross misrepresentations of the toxicity in the water body. This uncertainty could result in regulatory as well as private sector inefficiencies should the criteria be used in any type of standard setting, and leads the scientific community to ignore further interactions among water quality parameters that may mitigate or strengthen the toxic effects of copper to aquatic organisms. Criteria concerning individual speciation of copper have not been promulgated and individual species impacts are not distinguishable given the total recoverable metal measurement method.