Hidden Carcinogen Taints Tap Water, Consumer Products Nationwide
September 6, 2017

Hidden Carcinogen Taints Tap Water, Consumer Products Nationwide: Methodology


For this analysis, EWG combined data from the EPA’s Third Unregulated Contaminant Monitoring, or UCMR3, program – nationwide water testing conducted primarily between 2013 and 2015 – together with results of water testing that some utilities conducted from 2010 to 2015. UCMR data are available on the EPA website, and state drinking water agencies provided the additional data in EWG’s Tap Water Database.

All water testing data are accessible through the Tap Water Database. The database contains information for 1,060 community water systems that have tested for and detected 1,4-dioxane. Additionally, 1,167 water systems, classified as “consecutive” systems, buy or otherwise receive some or all of their finished water from one or more water utilities that have detected 1,4-dioxane, resulting in the spread of 1,4-dioxane contamination beyond the water utility where it was originally detected. In such cases, purchasing utilities are not required to test for or report 1,4-dioxane, but they likely have the contaminant in their water supplies. 

Individual test results show 1,4-dioxane levels for the day and the test site where the sample was collected. Calculation of water system averages included all available samples from 2010 to 2015. Samples reported as “no detection” were assigned a value of zero. These averages reflect the overall level of concern for 1,4-dioxane contamination for a given water utility. The levels of the chemical in finished water served to customers at the tap may fluctuate depending on the specific water source.

For consecutive systems, EWG’s database and interactive 1,4-dioxane map display testing results taken by the supplying system. In cases where the consecutive system also tested for 1,4-dioxane, the system’s own tests are shown. The Tap Water Database displays, based on publicly accessible records, where the system likely purchases its water and provides links to the water quality information for supplier systems.

Some water systems also blend water from different sources depending on their service needs. This mixing may reduce the average contaminant amount in the finished tap water served to consumers. While contaminant levels are often slow to change from year to year, the presence of 1,4-dioxane may have led some utilities to remove contaminated well or sources from use. Changes in water sources and water supply, especially those after 2015, may not be reflected in EWG’s analysis.

EWG mapped public water systems according to the information available in the EPA’s Safe Drinking Water Information System. The mapped locations may not exactly match the service area; they are intended to visualize the general location of a public water system.