Three Ways a Chlorine Mapping Study Identifies Legionella Bacteria Risk

By Hilary Nardone, Lead Environmental Group Manager

Chlorine mapping is the process of gathering and analyzing disinfectant residuals from outlets, mainly faucets and showers, in your facility.A Chlorine Mapping Study is a useful tool to understand and improve distribution system optimization.The goal is water quality optimization in the entire water system, knowing that chlorine disinfection levels often vary throughout the building’s drinking water systems.Chlorine levels measured throughout the domestic water systems directly correlate to the building’s risk of harmful waterborne pathogens, including Legionella bacteria.Below are three ways a Chlorine Mapping Study identifies your facility’s potential areas of highest bacterial growth:

This diagram is an example chlorine mapping project.At the time of sampling, chlorine residuals are highest at the point of entry, then begin to lessen as the water moves through the distribution system.

1.If the chlorine level from the municipal water supply cannot be detected at the building’s point of entry, it is sensible to consider supplemental disinfection:

A study by Kuchta et. al. found that Legionella bacteria can survive in low levels of chlorine for relatively long periods of time – twice as long as other bacteria like E. coli.1In fact, the National Academies of Sciences, Engineering, and Medicine recommends that the Environmental Protection Agency require minimum disinfect residual throughout the public water systems to prevent the growth of Legionella bacteria.2 Therefore, if the chlorine residual in the incoming potable water supply is found to be less than 0.5 ppm, supplemental addition of disinfectant at the building’s entry point may greatly minimize likelihood of biofilm and Legionella bacteria.3

2.Chlorine breakdown may vary based on water temperature:

A Chlorine Mapping Study of your facility might show sufficient disinfectant residuals on your cold-water samples but a noticeable decrease in residual on your hot-water samples (even if both samples are taken from the same outlet!).This is because free chlorine often reacts more quickly in hot water than in cold water.Hot water heaters and hot water storage tanks may use up the disinfectant, allowing the Legionella bacteria that entered to flourish if not controlled.4


3.Chlorine residuals may be lowest at the plumbing distribution systems’ furthest points, indicating stagnant conditions and/or complex piping:

All buildings, including hospitals and hotels, experience unique and variable plumbing systems that contribute to stagnation or dead ends.Chlorine degrades as it reacts, leaving stagnant areas with little to no protection against bacteria.In fact, water age is a major factor in water quality deterioration within distribution systems.5 To decrease periods of stagnation, regularly flush your outlets, including sinks, showers, bathtubs and whirlpool spas.A Chlorine Mapping Study will identify areas that need more attention to flushing.

A Chlorine Mapping Study is a useful tool, pinpointing opportunities for improvement to minimize Legionella bacteria growth.Contact Barclay Water Management for your chlorine mapping needs.



1.Kuctha JM, States SJ, McNamara AM, Wadowsky RM, Yee RB.Susceptibility of legionella pneumophila to chlorine in tap water.Applied Environmental Microbiology.Nov. 1983;46(5):1134-1139.

2.National Academies of Sciences, Engineering, and Medicine 2019.Management of Legionella in Water Systems.Washington, DC: The National Academies Press. doi: 10.1722625474

3.CDC.Legionella (Legionnaires’ Disease and Pontiac Fever): What Owners and Managers of Buildings and Healthcare Facilities Need to Know about the Growth and Spread of Legionella.CDC. April 30, 2018.Accessed September 4, 2019.

4.CDC.Developing a water management program to reduce Legionella growth & spread in buildings.CDC.Published June 2017(1.1).

5.EPA.Effects of Water Age on Distribution System Water Quality.EPA. April 15, 2002.Accessed September 5, 2019.

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