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water. A common example of incorrect application is the use of the total trihalomethane (TTF*A) MCL for the protection of ground water from chloroform. Chloroform is one of the four chemicals covered by the term "trihalomethanes". The TTHM standard of 100 99/1 is over 500-times greater than the 10-6 incremental cancer risk estimate for chloroform. EPA has stated that the MCL for TTHMs was based mainly on technology and economics22 Therefore, this standard does not clearly protect the beneficial use for domestic supply of waters of the State. The MCL for TrHMs was derived, for application to drinking water as it is delivered to consumers after disinfection by chlorination,by balancing the benefit provided by the chlorination process (elimination of pathogens in drinking water) with the health threat posed by the trihalomethane by-products of this process. In the case of contaminated ground water this type of cost/benefit balancing is not germain, and so the MCL does not apply to the protection of the ambient quality of domestic water supply sources. The 10-6 cancer risk estimate of 0.19 gg/1 should be used as the measure of potential impairment by chloroform of the beneficial use of ground water for domestic supply. Staff of EPA,Region 9 has stated that the application of the 10-6 cancer risk estimate, instead of the TTI M MCL, as a water quality goal for chloroform in ground water appears to be consistent with the federal Clean Water Act and the recently adopted Safe Drinking Water Act.23and that the TTkIM standard is not appropriate for protection of ambient water quality.24 In fact,virtually all primary MCLs are derived by balancing the technologic and economic concerns that are directly related to the use of water for domestic supply with the health effects information developed under the RMCL process. Thus primary MCLs are not necessarily reliable indicators of protection of beneficial uses of an ambient water resource and should not necessarily be relied upon as water quality goals in these situations. There are other instances where water quality criteria more stringent than MCLS are applied to protect the beneficial uses of a water resource. For example,it is common practice to require compliance with aquatic life criteria for heavy metal contaminants in surface waters that are often much lower than MCLs for the same contaminants. Once it has been decided that some degradation in water quality will be permitted (i.e., background water quality is not used for water quality goals), other factors may require water quality goals to be set below water quality-related standards and criteria. Care should be taken to consider other dischargers in the area and the contribution to the degradation of water quality that each imposes. If one discharger is permitted through the >{ disposal of his waste to degrade the water resource to just below the point where beneficial uses are impaired,then no additional capacity exists for further degradation by other discharges of waste. In addition,the knowledge of the health and environmental effects of chemicals or combinations of chemicals is constantly evolving. What is considered to be safe at or below 10 gg/1 today may be found to be harmful at 1 gg/1 tornorrow. a 3.3 Calculating Designated Levels Designated Levels are measures of the minimum concentrations of waste constituents which,upon accounting for environmental attenuation at the proposed site of discharge, have the potential to cause the water quality goals for the constituents to be equalled or exceeded in ground and/or surface waters. Designated Levels fall into two main types, Page 28 Designated Level Methodology