Hill AFB: 1,4‑dioxane detected near South Weber; base uses conservative screening level

Julie McNeil, an environmental staffer at Hill Air Force Base, presented the base's 1,4‑dioxane remedial-investigation findings and next steps. McNeil said the plume's vertical extent is about 15 to 55 feet below ground and that the base applied EPA regional screening levels, choosing a conservative trigger of 0.46 micrograms per liter for groundwater evaluation.

"We utilized the EPA's tap water regional screening levels... and chose 0.46 parts per billion, as the level we were, we were looking for in the groundwater," McNeil said. She noted that three effluent points tested above the regional screening level and that some municipal-sewer outfalls showed increased concentrations farther from the OU‑2 discharge, indicating additional sources beyond the base.

McNeil described planned next steps: step‑out sampling at the toe of the plume, continued groundwater monitoring, and collection of surface-water and seep samples when conditions permit. She said the work is intentionally conservative because there is not yet a federal MCL for 1,4‑dioxane and that future EPA numbers could expand or shrink the delineated plume and the required response.

Council members pressed about the potential for the chemical to reach the Weber River and whether it bioaccumulates. McNeil said 1,4‑dioxane is miscible and moves in water but, unlike PFAS, does not bioaccumulate according to current information; she advised continued monitoring and coordination with state authorities as the base refines the plume boundary.