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The blending of distribution system waters containing different disinfectants may also have an impact on nitrification. Typically, the blended C12:NH3-N ratio will increase after blending. Most often this is due to dilution of the ammonia fraction in the chloraminated water. The impact on the blended total chlorine residual will be a function of the initial chlorine and chloramine residuals. A few studies have assessed the feasibility of blending chloraminated and chlorinated water in distribution systems (Mahmood et al. 1999; Muylwyk et al. 1999; and Barrett et al. 1985). Documented problems related to blending chlorinated and chloraminated water include the loss of disinfectant residuals, increases in tastes and odors, and increases in disinfection by-products, all of which are related to uncontrolled breakpoint chlorination that occurs in disinfectant blending zones (Barrett et al. 1985; and Mahmood et al. 1999). However, a few utilities do successfully blend their own chlorinated supplies with purchased chloraminated water. This is primarily accomplished when the chloraminated supply has excess ammonia that, when blended with the chlorinated supply, serves to form chloramines (Wolfe et al. 1988). In those cases, the blending ratios, and chlorine to ammonia ratios are strictly controlled to prevent breakpoint chlorination (Barrett et al. 1985). A new AwwaRF Report which updates the 1993 report Optimizing Chloramine Treatment (Kirmeyer et al. 1993) includes an international survey of chloraminating utilities and is expected to provide additional information on blending multiple disinfectants. 3.4 Monitoring The EPA Phase II Inorganic Contaminant regulations require water systems to sample for nitrite and nitrate at each entry point to the distribution system on at least an annual basis. Additional monitoring is required on a quarterly basis for at least one year following any one routine sample in which the measured concentration is greater than 50 percent of the MCL (EPA 2001). The MCLS are 1 mg/L for nitrite -N, 10 mg/L for nitrate -N, and 10 mg/L for nitrite + nitrate (as N). Monitoring distribution system water quality can help to predict the on -set of a nitrification event. According to Wilczak et al, (1996), nitrification is often indirectly identified by one or more symptoms including: ■ Loss of chloramine residual, ■ Increase in water temperature, ■ Decrease in dissolved oxygen, ■ Drop in pH and alkalinity, and ■ Increase in HPC population. Individual utilities use combinations of the indicators listed based on their system's experience managing nitrification to initiate nitrification control measures. Example criteria reported in the literature as triggers for breakpoint chlorination include nitrite action levels of 0.05 mg/L and falling chloramine residuals (Odell et al. 1996, Wilczak et al. 1996, Wolfe 1988). Prepared by AWWA with assistance from Economic and Engineering Services, Inc. 12