1,4-Dioxane Assessment Strategies in a Fractured Rock Aquifer

The mass distribution and fate & transport of the emerging contaminant 1,4-dioxane in a sedimentary bedrock aquifer can be very different from the co-released chlorinated volatile organics (CVOCs) and requires a different strategy to assess. This presentation will detail our innovative assessment of 1,4-dioxane impacts in a fractured mudstone aquifer both in a source area and the distal plume. Two source areas had discharges of the 1,1,1-trichloroethane (1,1,1-TCA), which had been stabilized with 1,4-dioxane. The data set includes 20 years of data on CVOCs and 3 years on 1,4-dioxane. We used the 1,1,1-TCA/1,1-DCE/1,1-DCA forensic ratio method to date the releases of 1,1,1-TCA, and therefore also 1,4-dioxane, and used these estimates in fate & transport models. CSIA was also utilized on CVOCs and 1,4-dioxane to provide additional lines of evidence for sources. The network of 80+ monitoring wells, designed to characterize and monitor CVOCs in discrete highly transmissive bedding plane fractures, did not yield the same data set for the co-released 1,4-dioxane, which showed low concentrations over large areas of the 5,000-ft long plume. The more soluble and mobile 1,4-dioxane is depleted in monitored zones much faster than CVOCs, but is stored in the rock matrix, which informs the assessment strategy.