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n [Concentration of Constituent]i Individually,no chemical exceeds its toxicologic < 1.0 limit.However,an additive cancer risk calculation Y=1 [Toxicologic Limit in Water]= shows: The concentration of each constituent is divided by its 0.3 0.4 0.04 toxicologic limit.The resulting ratios are added for + + – 12 constituents having similar toxicologic effects and, 0.7 3 0.06 separately,for carcinogens. If such a sum of ratios is The sum of the ratios is greater than unity(>1.0); Iess than one,an additive toxicity problem is assumed therefore,the additive toxicity criterion has been not to exist.If the summation is equal to or greater than violated.The chemicals together present an unaccept- one,the combination of chemicals is assumed to. able level of toxicity(in this case,cancer risk). present an unacceptable level of risk to health. Fol'example,monitoring shows that ground water CONCLUSION AND STATUS beneath a site has been degraded by three volatile A Compilation of Water Quality Goals has been organic chemicals in the following concentrations: developed to provide a uniform method and a con- Perchloroethylene 0.3 Rg/l venient source of numerical limits for determining Trichloroethylene 0.4 compliance with California's water quality standards. 1,1-Dichloroethylene 0.04 It has been used for severalyears by the staff of a One-in-a-million incremental cancer risk estimates, number of the California Regional Water Quality calculated from cancer potency factors found in the Control Boards as a reference for numerical water U.S. EPA's IRIS database,are as follows: quality limits. Perchloroethylene 0.7 gg/l A Compilation of Water Quality Goals will be up- Trichloroethylene 3 dated and expanded,as needed,to account for newly 1,1-Dichloroethylene 0.06 developed numerical water quality information. I G LOSSARY Beneficial Uses--Uses of water that must be stringent of either(a)the numerical limit which imple- protected against water quality degradation.They are ments all applicable water quality objectives or(b)the established by the Water Boards in the Water Quality background level. Control.Plans.See"water quality standard." Water Quality Objectives—Numerical or narra- Water Quality Criteria—Numerical or narrative tive limits on constituents or characteristics of water limits for constituents or characteristics of water designed to protect designated beneficial uses of the designed to protect specific designated uses of the water.California's water quality objectives are estab- water.California's water quality criteria are called lished by the State and Regional Water Boards in the "water quality objectives."See"water quality stern- Water Quality Control Plans.See"waterquality stan- dard." dard." Water Quality Goal—The most stringent,appli- Water Quality Standards—An combination of cable,numerical water quality limit for a constituent or the designated beneficial uses of water and water parameter of concern in a specific body of ground or quality objectives(criteria)to protect those uses.In i surface water at a specific site that is chosen to protect California,water quality standards are promulgated either(1)existing water quality or(2)beneficial uses by the State and Regional Water Boards in the Water of water.In the first case,the water quality goal is set Quality Control Plans.Water quality standards are equal tb the background level in the body of water.In enforceable limits for the bodies of surface or ground the second case,the water quality goal is set at the less waters for which they are established. Selecting Water Quality Goals Page 13