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Selecting Numerical Limits - 2 - • 20 August 2002 3) To account for the Controllable Factors Policy,3 select the larger of a) the numerical limit chosen in step (2) and b) the natural background level of the constituent.4 These steps should provide a numerical limit which, if equaled or exceeded in ambient water, indicates that pollution has occurred. This is the least stringent limit below which ambient water would be in compliance with applicable water quality standards.5 [Note: Antidegradation principles may require that more stringent limits be applied to ambient water quality, where the natural background level was not selected in step (3) above.] In step (1), especially with respect to toxicity information, there should normally be a preference for: • Purely risk-based limits over risk-management based limits,' unless the water quality objective mandates the use of a risk-management based limit(e.g.,the Chemical Constituent objectives mandates compliance, at a minimum,with California Primary and Secondary drinking water MCLS); • Limits developed and/or published by California agencies, over those developed by federal agencies or other organizations, to be consistent with regulatory actions of our sister state agencies; • Limits that reflect peer reviewed science (avoid using draft or provisional limits,unless nothing else is available); • Limits that reflect current science(e.g., Public Health Goals are normally more recent than IRIS criteria, which are normally more recent than USEPA health advisories). These principles are consistent with the manner in which DTSC and OEHHA select toxicity-based criteria for health risk evaluations.8 3 See Controllable Factors and Antidegradation Policies,on pages 8 and 9 below. Natural background concentrations are clearly not controllable water quality factors. 4 For the NPDES program and for other situations where it is not clear that background conditions represent true"natural background",(i.e.,they have not been influenced by controllable water quality factors),the limit chosen in step(2) should be imposed even where background levels are less stringent. According to the SWRCB Policy for Implementation of Toxics Standards for Inland Surface Waters, Enclosed Bays, and Estuaries of California(SIP),the water quality objective becomes the effluent limit in such cases. In SIP Section 1.4,Calculation of Effluent Limitations, Step 2(page 6),when the water quality criterion(C),is less than the background concentration(B),then the effluent limit (ECA)is set at the criterion(C),not at the background concentration(B). 5 Water quality standards=beneficial use designations+water quality objectives or promulgated criteria to protect such uses. 6 Limits based only on health risk information. 7 Limits that include economic and/or technologic factors in addition to health risk. The balancing of these factors may not be relevant to protecting beneficial uses of ambient water resources and may not comply with the language of the narrative toxicity objective,which prohibits"toxic substances concentrations that produce detrimental physiological responses in human,plant, animal,or aquatic life." Additional discussion of this issue appears in the section"Selecting a Water Quality Goal From Among Available Numerical Limits"on pages 15 to 17 of the text at the beginning of A Compilation of Water Quality Goals. 8 See page 15,right hand column of the text at the beginning of A Compilation of Water Quality Goals.