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Selecting Numerical Limits • - 4 - • 20 August 2002 ■ California Drinking Water Action Level based on toxicity ■ USEPA IRIS criteria; select the lowest of one-in-a-million cancer risk estimate • reference dose for non-cancer toxicity ■ USEPA Health Advisory; select the lowest of • one-in-a-million cancer risk estimate • lifetime non-cancer limit ■ USEPA MCL Goals (non-zero values onlyl2) ■ Other health-risk based limits (check dates and basis before using these) • National Academy of Sciences criteria; select the lowest of ♦ one-in-a-million cancer risk estimate ♦ drinking water health advisory • Proposition 65 levels (if no other health risk-based limits are available) ❖ Tastes and Odors Objective ➢ Taste- and odor-based limits, normally in the following hierarchy ■ California Secondary MCL ■ Federal Secondary MCL ■ USEPA National Ambient Water Quality Criterion based on taste & odor (do not use if limit is based on tainting of fish flesh) ■ Taste & odor thresholds published by other agencies or from the peer reviewed literature First, select one limit for each of the items above that begins with an arrow (➢). Record your selections in a chart, such as the following: Water Quality Relevant Portion of Objective/Criterion Objective/Criterion Source Concentration Units Chemical Constituents Drinking Water MCL (lowest) DHS Numerical Water Quality Objective Basin Plan Beneficial Use Impairment Limit Toxicity Human Health—Drinking Water Tastes &Odors Taste & Odor Based Limits for Water Second, select the limit with the lowest concentration. The result should be a limit that satisfies all applicable water quality objectives. Consideration of natural background levels and antidegradation may require further modifications to this selection, as discussed below. 1 For volatile carcinogens,this limit is likely to be less stringent and less relevant to translating the toxicity objective than the Public Health Goal(PHG)because it considers only ingestion exposure. PHGs consider ingestion,vapor inhalation and skin adsorption exposures that are likely to occur from the use of drinking water in the household. 12 MCL Goals for carcinogens are set at"zero"to represent no health risk. No significant risk is used for PHGs.