EVIDENCE FOR 1,4-DIOXANE NATURAL ATTENUATION IN A GLACIAL AQUIFER SYSTEM

Conventional wisdom suggests that properties of 1,4-dioxane, such as its miscibility in water, low retardation factor, and resistance to microbial decomposition, limit the ability of processes such as sorption and biodegradation to attenuate dioxane concentrations. Consequently, 1,4-dioxane remains a persistent contaminant of concern. This investigation employed multiple lines of evidence to evaluate the natural attenuation of 1,4-dioxane in groundwater at the Gelman site beneath Ann Arbor, Michigan, USA. Over a period of more than three decades, site characterization has revealed a series of dioxane plumes expanding throughout a complex glacial aquifer system. At the same time, remedial pumping and ex-situ treatment have reduced concentrations by one to two orders of magnitude through the removal of more than 100,000 pounds (55,000 kg) of 1,4-dioxane. A recent reduction in the Michigan groundwater standard for 1,4-dioxane has prompted the evaluation of alternative remediation efforts that can contribute to the further reduction of contaminant concentrations below 7.2 µg/L.

Utilizing data from an extensive network of groundwater monitoring and remediation wells and a series of historical plume maps, we calculated mass-in-place, mass influx rates, and mass removal rates for the primary segment of 1,4-dioxane contamination in the Eastern Area of the site over a twelve-year period (2005-2017). Our results show increasing quantities of aqueous phase dioxane for the first six years, followed by decreasing mass storage for the remaining six years. The mass of dioxane removed by extraction wells does not account for the decrease in concentrations and total mass estimates observed within the Eastern Area. More detailed models were subsequently developed to quantify mass flux into the Eastern Area and evaluate the potential influence of heretofore unrecognized sinks in the system. Potential sinks that could account for the remaining discrepancies in our mass balance calculations include biodegradation and/or unrecognized discharge to surface water and storm drain systems. Both possibilities challenge existing approaches to contaminant mapping in discretely heterogeneous aquifer systems and established views on the potential for biodegradation of 1,4-dioxane under anaerobic conditions.