Evaluation of 1,4-Dioxane Natural Attenuation After In Situ Propane Biosparging: Bench-Scale Microcosm Tests and Field Metadata Analysis

Monitored natural attenuation (MNA), primarily via biodegradation, is a cost-effective approach to managing large and dilute groundwater plumes formed by 1,4-dioxane. However, impacts of prior treatments, such as aerobic bioremediation strategies, on the subsequent attenuation of 1,4-dioxane are poorly understood. In this study, bench-scale microcosms were conducted with groundwater collected from seven monitoring wells located along the 1,4-dioxane plumes at two sites where propane biosparging (and bioaugmentation) were previously employed. Over 4 weeks of incubation, significant 1,4-dioxane removal was observed in microcosms prepared with groundwater samples from four locations within the proximity of historical treatment zones. First-order attenuation rates were in the range of 0.02 to 1.11 day⁻¹, corresponding to half-lives between 0.6 and 32.0 days. Concurrently, a meta study was conducted by compiling groundwater data from > 900 monitoring wells across the U.S. Bulk 1,4-dioxane degradation/attenuation half-lives were estimated in the range from 109 to 6405 days (R² > 0.5). The majority (96%) of the estimated half-lives were less than 3000 days with the shorter half-lives (i.e., faster rates) generally observed at higher 1,4-dioxane concentrations (i.e., > 20 µg/L). Propane biosparging exhibited significantly shorter half-lives (i.e., a median half-life of 445 days) than MNA and pump and treat technologies. Additionally, 1,4-dioxane half-lives after the start of propane biosparging treatment were an order-of-magnitude shorter than those prior to this treatment, which may also explain the additional orders-of-magnitude decreases in half-lives in MNA-mimicking microcosms. Together, these findings indicate that propane biosparging not only accelerates immediate 1,4‑dioxane removal but also “primes” microbial communities for enhanced long‑term attenuation.