January 19, 2025. In March 2022, the North Carolina Environmental Management Commission (EMC) adopted a numerical water quality standard for a toxic chemical called 1,4-dioxane. The water quality standard would have established the safe level of 1,4-dioxane in rivers, lakes and streams used as a drinking water source. The standard would also become the benchmark for setting permit limits on wastewater treatment plant discharges of 1,4-dioxane to those waters. The 2022 water quality standard never went into effect and there is no immediate prospect of EMC adoption of a numerical 1,4-dioxane standard. This post concerns the health risks associated with 1,4-dioxane in North Carolina drinking water. Later posts will relate the tangled history of the unsuccessful effort (to date) to adopt a 1,4-dioxane standard and discuss the cost of controlling the toxic pollutant.
What are the health effects of exposure to 1,4-dioxane? The U.S. Environmental Protection Agency (EPA) has identified 1,4-dioxane (a chemical often used in solvents) as a toxic substance that affects the liver and nasal tissue. Chronic or acute exposure to 1,4-dioxane can also have toxic effects on the kidneys, brain and lungs. In addition, EPA studies link exposure to 1,4-dioxane with increased cancer risk in humans.
EPA issued a risk determination for 1,4-dioxane in 2020 that largely focused on worker exposure to the chemical. In November 2024, EPA released a revised Unreasonable Risk Determination for 1,4-dioxane that also considered the risk to the general public of exposure through drinking water. In the 2024 report, EPA concluded that:
1,4-dioxane presents an unreasonable risk of injury to health under the conditions of use…EPA’s final determination is based on cancer and non-cancer risks (from liver toxicity and effects in the olfactory epithelium) to workers and occupational non-users (ONUs) from inhalation and dermal exposures, and cancer risks to the general population, including fenceline communities, from exposures to 1,4-dioxane in drinking water sourced from surface water contaminated with industrial discharges of 1,4-dioxane (including when it is generated as a byproduct).
Unreasonable Risk Determination, page 3. Based on the risk data, EPA established a health guideline of 0.35 nanograms per liter (μg/L) in drinking water and 80 μg/L in other waters. Note: nanograms per liter and parts per billion (ppb) are essentially equivalent measures; some sources cited here refer to 1,4 dioxane levels in parts per billion.
Is 1,4 dioxane a problem in North Carolina drinking water? North Carolina’s Department of Environmental Quality (DEQ) began investigating 1,4-dioxane levels in the Cape Fear River Basin more than ten years ago. The Cape Fear River Basin extends from tributaries of the Haw River in north-central North Carolina to the coast. Water bodies in the Cape Fear River Basin provide drinking water for a number of public water systems serving hundreds of thousands of citizens, including systems serving Pittsboro, Fayetteville, Wilmington and New Hanover County. Data collected in 2014-2016 documented levels of 1,4 dioxane exceeding acceptable health exposure levels and identified wastewater treatment plants in the upper Cape Fear River Basin as the likely source:
…four areas of high 1,4-dioxane concentration were identified. Three of the four areas were located immediately downstream of domestic wastewater treatment facilities (WWTF), and one was further downstream of a WWTF, as well as downstream of an inactive textile manufacturing site and other possible legacy source.
1,4-dioxane Monitoring in the Cape Fear River Basin, page 4.
Later monitoring found wastewater treatment plants (WWTPs) in Greensboro, Burlington, Reidsville and Asheboro to be sources of 1,4-dioxane releases to surface waters. 1,4-dioxane is not a product or by-product of wastewater treatment. Targeted monitoring has generally traced 1,4-dioxane in municipal wastewater to an industrial or waste disposal facility discharging to the municipal system.
In May 2024, DEQ issued a report to the N.C. legislature on 1,4-dioxane in North Carolina drinking water sources and the potential health effects. The report documented the presence and likely sources of 1,4-dioxane in state waters in excess of health guidelines. The report concluded that North Carolina residents are:
- Exposed to 1,4-dioxane concentrations that may be two times the national average in drinking water and as much as 4 times national averages in surface and groundwater; and
-
Based on earlier sampling in public water systems nationwide, North Carolinians only received half the protection from 1,4-dioxane as citizens in rest of the nation.
1,4-dioxane in Drinking Water Legislative Report.
Effect on downstream drinking water systems. Conventional drinking water treatment does not remove 1,4-dioxane. On multiple occasions in the last five years, the 1,4-dioxane level in the Town of Pittsboro’s drinking water spiked in response to high levels of 1,4-dioxane released by an upstream WWTP. Following a 2022 release from Greensboro’s T.Z. Osborne WWTP to a tributary of the Haw River nearly 50 miles upstream, Pittsboro measured 1,4-dioxane at concentrations as high as 11.9 ppb in Pittsboro’s treated drinking water. Those levels significantly exceeded EPA’s health guideline for drinking water of 0.35 μg/L (or 0.35 ppb). The Town of Pittsboro has at times been forced to temporarily shut off the water system’s Haw River intake and provide bottled water to residents until a slug of 1,4-dioxane has passed further downstream.
The well-known toxicity and likely cancer-causing effects of 1,4-dioxane as well as the documented impact on drinking water in the Cape Fear River Basin led to the EMC’s 2022 adoption of a water quality standard based on the EPA health guideline of 0.35 μg/L per liter for water bodies used as a drinking water source. As noted above, the rule has never gone into effect. More about that in the next post.