Evidence for natural attenuation of 1,4-dioxane in a glacial aquifer system

Abstract

Although 1,4-dioxane is generally thought to be recalcitrant, recent studies suggest it may degrade in the subsurface under ideal conditions. A fuller understanding of natural attenuation processes affecting 1,4-dioxane is therefore needed to assess its potential for in situ bioremediation. This investigation employed multiple lines of evidence to evaluate attenuation of 1,4-dioxane at the Gelman Site beneath the city of Ann Arbor, Michigan, USA. Data from a network of groundwater monitoring wells were used to determine attenuation metrics at individual wells and at the scale of a prominent 1,4-dioxane plume. A series of plume maps and historical remediation data were used to calculate changes in aqueous phase mass storage, mass influx rates, and mass removal rates over a 12-year period (2005–2017). Individual point and plume-scale metrics indicate that attenuation may be occurring at rates too small to meaningfully contribute to remediation results at the site. Conversely, plume-scale mass balance calculations reveal a 1,4-dioxane storage surplus for the first 6 years, followed by a storage deficit during the remaining 6 years that cannot be explained by mass influx or removal estimates, respectively. Mass balance deficits observed in this aquifer system are attributable to biodegradation and/or unrecognized discharge to surface water and storm drain systems at rates similar to remedial pump-and-treat mass removal during 2011–2017.