Disinfection Byproducts

All locations within the City of Napa Water System are meeting the regulatory standard for Disinfection Byproducts in Drinking Water (PDF). The particular byproducts at issue are Trihalomethanes (THMs). If you have ever in the past received a special notice in the mail regarding the presence of THMs caused by the reaction of chlorine with organic matter in potable water, the Water Division is pleased to report that the running annual averages for all sites have been in full compliance since April 2016, below the stringent regulatory threshold of 80 parts per billion (ppb).

This issue has led to inquiries to the Water Division for more information. Water Division staff have spoken at length with many customers about the complex nature of this new water quality regulation (called DBPR-II, or Stage II of the Disinfection Byproduct Rule), the presence of THMs, the causes of THMs, and steps the Water Division has been taking to prevent their formation. No specific corrective actions are needed by customers who receive a DBPR-II notice (e.g., no need to boil water).

The Water Division held a public meeting in 2015 to discuss water system operations and preventative measures and answer questions about disinfection byproducts. Dr. Bruce Macler, a drinking water toxicology specialist from EPA Region IX, was on hand along with City staff. View a recording of this meeting on the City Council page.

More recently, you may have heard about the release of a 2019 National Tap Water Database by the Environmental Working Group (EWG), which deals with disinfection byproduct data in a misleading and unnecessarily alarming manner. For a more realistic perspective, read the City of Napa Response to EWG (PDF).