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Nitrification occurrence can be confirmed by monitoring for nitrite or nitrate. Wilczak et al (1996) suggest that an accurate nitrogen balance and specific biological monitoring for ammonia -oxidizing bacteria and nitrite -oxidizing bacteria are the best ways to determine whether nitrification is occurring. 4.0 Summary Nitrification is a microbial process by which reduced nitrogen compounds (primarily ammonia) are sequentially oxidized to nitrite and nitrate. Ammonia is present in drinking water through either naturally -occurring processes or through ammonia addition during secondary disinfection to form chloramines. Chloramine use is expected to increase in the near future as a result of more stringent disinfection by-product (DBP) maximum contaminant levels (MCLs) associated with the Stage I and Stage II Disinfectant/Disinfection By -Product Rule (D/DBP Rule). There are several symptoms of nitrification that can impact distribution system water quality. Of the water quality issues identified in the literature and summarized in Table 1, only the formation of nitrite and nitrate within the distribution system poses a potential direct public health threat, and is not addressed through current provisions of the Safe Drinking Water Act. The findings of this paper suggest that a nitrite or nitrate MCL violation with the distribution system is unlikely, unless treated water nitrite and nitrate levels are near their respective MCLs. Additional research may be warranted to determine if nitrification episodes can cause direct violations of the Lead and Copper Rule. The causes of nitrification and prevention and mitigation approaches have been thoroughly documented within the literature. 5.0 Secondary Considerations 5.1 Increased Disinfection By -Products during Mitigation Nitrification mitigation techniques such as break-point chlorination or temporarily switching from chloramines to free chlorine can result in increased levels of disinfection by-products. Disinfection by-products are regulated under the Stage 1 Disinfectants and Disinfection By - Product Rule. Under the Disinfectants and Disinfection By -Product Rule compliance monitoring samples are be collected under routine operating conditions. Thus, disinfection by-product samples collected during a nitrification mitigation episode are not typically included in MCL compliance calculations. It would also be possible to exceed a Maximum Residual Disinfectant Level (MRDL) under the Disinfection By -Product Rule during a nitrification mitigation episode. However, the USEPA specifically allows short-term exceedence of MRDLs to control microbiological contamination problems. 5.2 Source Water Nitrification Under certain circumstances nitrification can have a beneficial impact on drinking water quality. This would be true in the controlled conditions at a water treatment plant. Rittmann and Snoeyink (1984) reported that nitrification of ammonia -containing groundwater resulted in "biologically stable" water that did not permit bacterial growth in the distribution system. Kurtz - Crooks et al. (1986) showed that the chlorine demand of ammonia -laden groundwater was .Prepared by AWWA with assistance from Economic and Engineering Services, Inc. 13