SITE INFORMATION AND CORRESPONDENCE_FILE 1

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Excerpts on Ground Waters and Site Cleanup Page 12 from the'Nater Quality Control Plan receiving water and effluent data to determine whether there is a reasonable potential for interactive toxicity. Pollutants which are carcinogens or which manifest their toxic effects on the same organ systems or through similar mechanisms will generally be considered to have potentially additive toxicity. The following formula will be used to assist the Regional Water Board in making determinations: n [Concentration of Toxic Substance]i -- ------- ----- <1.0 i= 1 [Toxicologic Limit for Substance in Water]i Tae concentration of each toxic substance is divided by its toxicologic limit. The resulting ratios are added for substances having similar toxicologic effects and, separately, for carcinogens. If such a sum of ratios is less than one, an additive toxicity problem is assumed not to exist. If the summation is equal to or greater than one, the combination of chemicals is assumed to present an unacceptable level of toxicologic risk. For example, monitoring shows that ground water beneath a site has been degraded by three volatile organic chemicals, A, B, and C, in concentrations of 0.3, 0.4, and 0.04 µg/1, respectively. Toxicologic limits for these chemicals are 0.7, 3, and 0.06 µg/1, respectively. Individually, no chemical exceeds its toxicologic limit. However, an additive toxicity calculation shows: C.3 + 0.4 + 0.04 = 1.2 0.7 3 0.06 The sum of the ratios is greater than unity (>1.0); therefore, the additive toxicity criterion has been violated. The concentrations of chemicals A, B, and C together present a potentially unacceptable level of toxicity. For permitting purposes, it is important to clearly define how compliance with the narrative toxicity objectives will be measured. Staff is currently working with the State Water Board to develop guidance on this issue. 9. Policy for Investigation and Cleanup of Contaminated Sites The Regional Water Board's strategy for managing contaminated sites is guided by several important principles, which are based on Water Code Sections 13000 and 13304, the Chapter 15 regulations and State Water Board Resolution Nos. 68-16 and 92-49: State Water Board Policy &Regulation The Regional Water Board will require conformance with the provisions of State Water Board Resolution No. 68-16 in all cases and will require conformance with applicable or relevant provisions of 23 CCR, Division 3, Chapter 15 to the extent feasible. These provisions direct the Regional Water Board to ensure that dischargers are required to clean up and abate the effect of discharges in a manner that promotes attainment of