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Selecting Numerical Limits - 3 _ 20 August 2002 Avoid using Proposition 65 limits to interpret narrative toxicity objectives. Proposition 65 limits are in conflict with other health-based limits for drinking water in California(i.e., PHGs and other health- based criteria from which MCLs are derived).9 The intent of Proposition 65 is to do two things: • Provide warnings to persons prior to significant exposure to carcinogens and reproductive toxicants, and • Prohibit significant discharges of carcinogens and reproductive toxicants into sources of drinking water. The "safe harbor"regulatory limits developed by OEHHA pursuant to Proposition 65 are used to determine what is "significant" in these two contexts. The intent of Proposition 65 is not to designate "safe" levels of chemicals in drinking water. Other programs exist in California for that purpose, namely the Public Health Goal program of OEHHA. The above principles may be used to generate algorithms to help select the most applicable or relevant and appropriate water quality numerical limits. Because water quality standards for groundwater and surface water differ significantly, separate algorithms are presented below. An Algorithm for Groundwater For chemicals in groundwater, the following water quality objectives and numerical limits normally apply to the receiving water: ❖ Chemical Constituents Objective (each of the following three items apply separately) ➢ Drinking Water MCLs; select the lowest of the fol owing • California Primary MCL ■ California Secondary MCL ➢ Numerical water quality objective from the Basin Plan ➢ Concentrations that indicate impairment of any designated beneficial use; select the lowest of the following ■ Agricultural use protective limits ■ Federal Primary MCL, if lower than California Primary MCL10 s• Toxicity Objective ➢ Human health-risk based limits for drinking water use, normally in the following hierarchy • OEHHA Public Health Goal ■ Cal/EPA cancer potency factor at the one-in-a-million risk levels i 9 For carcinogens,Proposition 65 uses the one-in-100,000 risk level,rather than the one-in-a-million level used to derive Public Health Goals, to form the basis of Primary MCLs,and mandated for human health protection in surface water by the California Toxics Rule. For reproductive toxicants,Proposition 65 arbitrarily divides the no-observed-adverse-effect level(NOAEL)from laboratory studies by 1000,regardless of the degree of certainty that the NOAEL is relevant to human toxicity. 10 Indicates that the California MCL will be lowered in the near future to at least as low as Federal MCL. Compliance with the Federal MCL is needed to protect the MUN beneficial use in the long term.