Town of Friday Harbor
San Juan Island

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60 Second Street
PO Box 219






Notice:  Friday Harbor Drinking Water System Users

Trihalomethane (THM) Update:  January 2011 through March 2011

Trihalomethane Levels - Quarterly Measurements





1st Quarter

77.5 ug/L

72.0 ug/L

76.1 ug/L

63.1 ug/L

2nd Quarter

82.9 ug/L

77.2 ug/L

83.7 ug/L


3rd Quarter

96.7 ug/L

103.3 ug/L

119.5 ug/L


4th Quarter

62.5 ug/L

64.8 ug/L

62.8 ug/L


The Town is required to monitor your drinking water for specific contaminants on a regular basis, including the presence of Trihalomethanes (THMs), a byproduct of disinfection process. Friday Harbor water system users have the right to know about the quality of their drinking water. We have included information below that you should be aware of and what the Town is doing to correct the situation.

What are Trihalomethanes?

The problem we are experiencing continues to be the result of using Chlorine as a disinfectant for our water. Chlorination has made the U. S. water supply safe from illness and disinfection has almost completely eliminated risks of deadly waterborne diseases. However, when chlorine has long-term contact with any organic material, such as leaf foliage present in the Trout Lake reservoir or growth in the aged water distribution mains, it produces a byproduct known as Trihalomethanes (THMs). The THMs produced may have adverse health effects at high concentrations, and the federal government has set limits on the amount permissible in drinking water.

How are Trihalomethanes calculated?

THM levels are measured on a quarterly basis to calculate a "12-month" average. The State requires that we notify you if the level of THMs in the water exceeds 80 micrograms per liter (ug/L) or 80 parts per billion in said time period. Town recently measured a THM level of 63.1 ug/L, which equates to an average of 82.3 ug/L for the past twelve months.

What can you do in the home to reduce THMs?

You may wish to filter your drinking water by using activated carbon filters or simply leave your tap water standing in a pitcher in the refrigerator overnight. These simple methods will help lower THM levels.

What is the Town doing to alleviate the problem?

Filtration systems that use carbon adsorption are the most effective technology for our situation. As water comes in contact with the carbon, organic particles are adsorbed to the surface area of the carbon granules. Construction is almost completed for installation of a granular activated carbon adsorption system at the Water Treatment Plant. The project is being constructed by Faber Brothers Construction at a cost of $600,000.  The new filters will not only reduce levels of THMs, but we anticipate overall improvement of the taste and odor of our drinking water quality.

If you have any questions regarding water quality related issues, please contact us at (360) 378-2810.




Neither the Town of Friday Harbor nor any agency, officer, or employee of the Town warrants the accuracy, reliability or timeliness of any information presented here, and shall not be held liable for any losses caused by such reliance on the accuracy, reliability or timeliness of such information.
Portions of such information may be incorrect or not current. Any person or entity who relies on any information obtained from the system, does so at his or her own risk.

Copyright 2017 Town of Friday Harbor Washington.  All rights reserved.