A study examining contamination of total coliform samples

The Area-Wide Optimization Program’s (AWOP’s) initial focus on managing disinfectant residuals in distribution systems (DSs) started in the 2008-2009 time-frame. The program recognizes the opportunity to apply the multiple barrier concept to the DS to improve water quality and optimize public health protection. The AWOP program’s DS optimization (DSO) goals include maintaining > 0.2 mg/L of free chlorine throughout a drinking water distribution system to minimize the impacts of microbial contamination (e.g., total coliform positive events), thus improving regulatory compliance and public health protection. For systems utilizing chloramination, AWOP adopted a goal of maintaining a monochloramine residual ≥ 1.50 mg/L as Cl2 at all locations in the DS at all times to provide protection against microbial pathogens and nitrification.

An observation during the implementation of the DSO approach has been that coliform-positive samples were sometimes erroneously attributed to poor sampling technique rather than a call to action to identify and address the source of contamination. With this in mind, Andrew Waite, a scientist from US EPA Region 6, conducted a study on the reliability of total coliform testing. His study confirmed the conclusions drawn from the 1982 Pipes & Christian1 study, affirming that coliform-positive samples should be taken seriously and not be dismissed without further investigation.

Results of Mr. Waite’s study were presented during a July 2018 EPA training webinar regarding the Revised Total Coliform Rule. The study used blank (reagent water) samples and attempted to purposely contaminate them by exposing them to a variety of conditions. One blank and one coliform positive control were analyzed as a baseline for true positive and negative samples. The positive controls consisted of reagent water samples spiked with coliform bacteria. The field samples and controls were analyzed by the Region 6 laboratory using EPA-approved methodology.

Only 3% of all samples yielded positive results for total coliforms following attempts at intentional contamination. Mr. Waite’s study reinforces earlier findings suggesting that total coliform positive samples are indications of a potential water quality issue and should be treated as valid unless the state finds appropriate basis for invalidation. States could use the information from Mr. Waite’s study and the Pipes and Christian (1982) study to work with water system staff to better assess the nature of water sample contamination as they pursue DS optimization.


1 W. Pipes and R. Christian, Sampling Frequency: Microbiological Drinking Water Regulations Final Report, 1982, EPA R-8050637


Ouro Koumai (US EPA | Technical Support Center, Koumai.ouro@epa.gov)