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f of the Organic Constituents section contain numerical Benzene 1µg/1 California Primary MCL ' limits relating to human health and welfare. Ethylbenzene 29 Taste&Odor Threshold 4 Other than the values cited above,applicable listed Toluene 42 Taste&Odor Threshold values for benzene include a proposed Recommended Xylene(s) 17 Taste&Oa o*Threshold Public Health Level,a 10-day exposure health advi- Gasoline 5 Taste&Ocor Threshold sory,a Cal/EPA Cancer Potency Factor,a Proposition The reader is cautioned that these values would 65 Regulatory Level,and 10-`cancer risk estimates. apply to ground water at the hypothetical site in this 10-day advisories are not protective of human health example,and not necessarily to water resources in in the long term and are,therefore,not applicable to other locations.Water resources at other sites may have protecting a ground water resource.The current Prop. different beneficial use designations and water quali-? 65 criterion for benzene is 35µg/l.The cancer risk objectives,which could alter the assessment of relevant estimates and the limit calculated from the Cal/EPA water quality goals for these chemicals. Cancer Potency Factor range from 0.35 to 1 ug/l.The In the above example,the constituents of concern proposed Recommended Public Health Level is also are not normally found naturally in ground water,so 0.35 gg/l.The most Iimiting values for benzene appear aquifer-specific background levels are not relevant to to.be the cancer risk-based limits.However,since these beneficial use protective water quality goal selection. limits are in relatively close agreement and are also Where background concentrations(out of the influence numerically close to the Primary MCL(within the of waste management activities at the site)are higher limits of sampling and analytical precision at this than the water quality goals selected to implement ' concentration),the Primary MCL of 1 gg/l could be applicable water quality objectives,the Regional Water chosen as a reasonable water quality goal for benzene. Board would not normally require the site owner or For ethylbenzene,other relevant human health- operator to improve upon the background conditions. and welfare-related limits from the tables include a In such cases,the background concentrations would federal Primary MCL and an MCL Goal of 700 gg/l, become the applicable water quality goals. a proposed Secondary MCL of 30 pg/1,a proposed In addition,strict application of California's Recommended Public Health Level of 680µg/l,an Antidegradation Policy would require that background EPA Health Advisory of 700µg/1 (which is in agree- levels of chemicals in ground water("zero"for anthro- meet with the IRIS RfD),and an& A National Ambi- pogenic substances at most sites)be selected as appro- ent Water Quality Criterion of 3100µg/l.In reviewing priate water quality goals if water quality degradation these limits, the most stringent applicable water quality is not found to be consistent with the requirements of igoal for ethylbenzene appears to be the taste and odor that policy,as discussed above. threshold of 29µg/l,discussed above. Similarly,a review of other relevant values from ADDITIVE TOXICITY CRITERION { the tables of the Organic Constituents section of this FOR MULTIPLE POLLUTANTS manual shows that all listed limits for toluene and When multiple constituents have been found in xylene are higher(less protective)than their taste and ground or surface waters,their combined toxicity must odor thresholds discussed above.Limits for gasoline be evaluated.In the absence of scientifically valid data other than the taste and odor threshold do not exist in to the contrary,§2550.4(8)of the Chapter 15,Article 5 the tables.Therefore,relevant water quality goals for regulations discussed above requires that theoretical toluene,xylenes,and gasoline appear to be the taste risks from chemicals found together in a water body and odor thresholds of 42, 17,and 5 gg/l,respectively. "shall be considered additive for all chemicals having (The proposed Secondary MCL for toluene is slightly similar toxicologic effects or having carcinogenic lower,at 40 Rg/1;however,it is very-close to the taste effects."This requirement is also found in California's and odor threshold and is currently a proposed value.) hazardous waste management regulations [Title 22 of In summary,the water quality goals chosen to CCR,§66264.94(f)],and in U.S. EPA's ,77s;:Assessment implement the applicable water quality objectives for Guidance for Superfund(RAGS). the protection of all beneficial uses of ground water at The commonly used toxicologic formula for the site being studied are: assessing additive risk is: Page 12 A Compilation of Water Quality Goals i Ii