Evaluating Coliform Monitoring Strategies for Contamination Scenarios in a Distribution System Model

Large public water utilities currently are required to take several 100 mL water samples every month in their distribution systems and to test these samples for the presence of total coliforms to comply with the Total Coliform Rule. How and where they take these samples is not specified. The Total Coliform Rule is currently undergoing review and revision. One possible revision is more specific rule language or guidance of where and when utilities take their monthly coliform water samples. This project considers various drinking water distribution system monitoring strategies for a large drinking water utility using simulations within an EPANET computer model of that utility’s distribution system. A review of the literature and the use of best professional judgment help to inform the understanding of the primary causes of the occurrence of coliform bacteria in drinking water distribution systems. In this analysis, both contamination scenarios and coliform monitoring approaches are simulated in the EPANET computer model to better understand what types of contamination events might be detected by routine coliform monitoring and to what extent monitoring strategies affect observed results. In addition, statistical strengths of the various strategies and their respective results are considered in this project. Analysis of model output supports the assertion that water age may be a good factor to consider in designing a monitoring plan, especially if sampling cannot be conducted every day and that taking samples immediately after and in the vicinity of rapid changes in flow (e.g., during a firefighting incident) may help utility operators to better understand what associated risk, if any, exists to consumers of drinking water as a result of these flow conditions.