Explaining 1,4-Dioxane Occurrence in America’s Public Water Supplies

1,4-Dioxane samples were collected and analyzed from 4,864 U.S. public water systems for the 3^rd Unregulated Contaminant Monitoring Rule (UCMR3) to understand the nature of its occurrence and the basis for establishing drinking water standards. 1,4-Dioxane was detected in samples from 21% of 4864 PWSs, and exceeded EPA’s Health Advisory Level (0.35 mg/L) in 6.9% of PWSs. 1,4-Dioxane ranked second for frequency of detection among the 28 UCMR3 contaminants.

1,4-Dioxane is now familiar as a groundwater contaminant, but the detection frequency for 1,4-dioxane in surface water was only marginally lower than in groundwater (by a factor of 1.25). Groundwater concentrations were higher than those in surface water and contributed to a higher frequency of exceeding the reference concentration (by a factor of 1.8), indicating that surface water sources tend to be more dilute. Sampling from large PWS increased the likelihood of 1,4-dioxane detection 2.18 times relative to small systems.

What are the implications of these findings? This presentation reviews the nature of 1,4-dioxane detections and evaluates co-contaminant association patterns to confirm the likely sources of 1,4-dioxane in PWSs. The consequences of 1,4-dioxane detections are evaluated in the context of health risk and drinking water treatment costs. Some water supply systems may need to improve their treatment capabilities in response to 1,4-dioxane detections, which can be an expensive challenge, owing to 1,4-dioxane’s infinite solubility, lack of volatility, and low propensity to adsorb to granular activated carbon. The UCMR3 data suggest there remain many unresolved 1,4-dioxane release sites, for which there will be a growing market for consultant remediation services.

This presentation will profile 1,4-dioxane occurrence, explain why its historical use manifests today as a drinking water contaminant, and how water utilities nationwide are addressing the problem.