Evaluation of 1,4‐Dioxane Natural Attenuation After <i>In Situ</i> Propane Biosparging: Bench‐Scale Microcosm Tests and Field Metadata Analysis

ABSTRACT 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 &gt; 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 ( R 2 &gt; 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., &gt; 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.