Chlorine Residual as a Predictor of Trihalomethane Values

The Initial Distribution System Evaluation (IDSE) requirement of the Stage 2 Disinfectant/Disinfection By-Products Rule requires all public drinking water systems to identify monitoring locations that are characterized as "near the entry point", and "average residence time", and locations of "high total trihalomethanes" (THM) and "high haloacetic acids" (HAA) concentration. While a distribution system network model is the preferred method to identify these locations, many public water systems to not maintain a calibrated model. In such cases, an alternative approach may be used that applies statistical analysis of existing chlorine residual data to the IDSE requirements. Moreover, statistical analysis can test hypotheses that chlorine residual is a surrogate for water age for individual systems, in cases where both THM and chlorine residual were analyzed for the same sample. If the hypothesis is true then average residence time, high THM locations for the IDSE can be identified using existing chlorine residual data. This paper was presented at the 8th Annual Water Distribution Systems Analysis Symposium which was held with the generous support of Awwa Research Foundation (AwwaRF).