GRPU Information & Updates

Legionella pneumophila Monitoring

GRPU is conducting weekly Legionella pneumophila sampling in the drinking water system and/or in community buildings. The objectives of this Legionella monitoring plan are to establish the following:

Determine if Legionella pneumophila occurs in the distribution system at levels of concern
Identify Legionella pneumophila areas of concern in buildings
Share and report data to GRPU customers in a timely manner
Inform customers of potential Legionella pneumophila growth in drinking water so they can proactively work to prevent exposure

To gain a better understanding of where the Legionella pneumophila is growing and proliferating, samples are being collected from a variety of locations within the city. Samples are collected at locations representative of the distribution system, as well as from hot and cold water locations within buildings. Approximately 20 samples are being collected each week. Sample locations may vary from week to week and repeat sampling will be conducted when Legionella pneumophila positives are identified.

To request sampling, please fill out the Volunteer Legionella Testing Form online and email it to info@GRPUC.org. For your convenience, printed forms are also available at the GRPU service center.

Results containing the general sample locations and Legionella pneumophila concentrations are shown on the GRPU Monitoring Map. The concern level and corrective actions are shown below so customers are able to have a point of reference of the implications for the data. See Homeowner and Building Manager Tips for steps to help avoid risk of exposure to waterborne germs.

**Table 1. Example of Legionella pneumophila reporting for customers¹**
Legionella pneumophila (MPN/100mL)	Concern level	Corrective action
Non-detect (<10)	Baseline or normal	Keep maintaining best management practices.
<100	Potential for growth	Repeat monitoring. Measure and document incoming water quality.
100 – 1,000	Possible growth	Investigate and mitigate risks of growth, system-wide assessment.
>1,000	Indicates growth	Investigate, mitigate risks of growth, and enhance control measures.

¹This table is adapted from AIHA guidelines and Bartrand, T. LeChevallier, M., Clancy, J., and G. Burlingame. (2024) Protocol for responding to the detection of Legionella pneumophila in drinking water distribution systems. AWWA Water Science. 2024;6:e1370.