The U.S. EPA addresses the influence of hardness on both copper and cadmium toxicity in their development of freshwater quality criteria
For the copper and cadmium toxicity data used in this risk assessment, hardness was also considered in the ranking of sensitivities of various freshwater species.
To realistically compare freshwater toxicity data among species, all data were standardized to a hardness of 50 mg/L CaCo3. This value was selected because it is the mean hardness value of 24 Maryland coastal plains streams sampled five time over a 2-year period from 1992 to 1993