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

Wednesday, August 9, 2017: 9:25 a.m.
Thomas Mohr, PG , Santa Clara Valley Water Distrcit, San Jose, CA

1,4-Dioxane samples were collected and analyzed from 4,864 U.S. public water systems for the 3rd 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.

Thomas Mohr, PG, Santa Clara Valley Water Distrcit, San Jose, CA
Thomas Mohr is Senior Hydrogeologist at the Santa Clara Valley Water District, where he manages groundwater monitoring programs and groundwater studies for a major indirect potable reuse project. His independent study of 1,4-dioxane began after the publication of the District’s Solvent Stabilizers White Paper in 2001, which generated many inquiries and dialogs and ultimately led to his six-year personal quest to prepare his book, “Environmental Investigation and Remediation: 1,4-Dioxane and Other Solvent Stabilizers” (Mohr, DiGuiseppi, Stickney; CRC Press, 2010). Mohr has published on the use of solid phase extraction for 1,4-dioxane analysis (now used in EPA Method 522), on perchlorate isotope forensic investigations, on cataloging and prioritizing investigation of past dry cleaner operations, on salt and nutrient management planning, on differentiating sources of nitrate using stable isotopes of water, and on one of the earliest applications of anaerobic bioremediation of gasoline compounds. He previously worked as Environmental Engineering Coordinator for the City of Sunnyvale, and Senior Hydrogeologist for the County of Yolo. Mohr was President of the Groundwater Resources Association of California in 2006 and 2007, and served as a Director on GRA’s Board for 10 years.