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Jackson, L.E. and L.D. Lemke. | Hydrogeology Journal [Published 2 October 2019 prior to print]

Multiple lines of evidence were employed to evaluate attenuation of 1,4-dioxane at the Gelman Site beneath Ann Arbor, Michigan. Over three decades, site characterization revealed a series of dioxane plumes expanding throughout a complex glacial aquifer system. At the same time, remedial pumping and ex situ treatment removed >100,000 lbs of 1,4-dioxane. A recent reduction in the Michigan groundwater standard for 1,4-dioxane to 7.2 µg/L prompted the evaluation of alternative remediation efforts to further reduce contaminant concentrations. Utilizing data from a network of groundwater monitoring and remediation wells and historical plume maps helped determined 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 from 2005-2017. Individual point and plume-scale metrics indicated that attenuation may have been occurring at rates too small to meaningfully contribute to remediation results at the site. Conversely, plume-scale mass balance calculations revealed a 1,4-dioxane storage surplus for the first 6 years, followed by a storage deficit during the remaining 6 years that could not be explained by mass influx or removal estimates. Mass balance deficits observed in the aquifer system were attributable to biodegradation and/or unrecognized discharge to surface water and storm drain systems at rates like remedial pump-and-treat mass removal during 2011-2017.

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