Plume persistence following full-scale source zone remediation due to aquitard back diffusion with insights on degradation effects

ABSTRACT: Contaminant diffusion into low permeability silt and clay-rich zones, commonly present within or at the base of sandy aquifers, can result in significant dissolved and sorbed mass accumulation over decades. This is generally accentuated for chlorinated solvent sites, where DNAPLs preferentially accumulate along low permeability zones placing high concentrations in the source and downgradient plume at interfaces. Subsequently if source concentrations decline, due to natural depletion or engineered remediation, declining concentrations at the interfaces initiates back-diffusion, releasing the stored mass resulting in long-term plume persistence. Abiotic and/or biological degradation within these low permeability strata can reduce the strength and longevity of back diffusion effects. A detailed study is ongoing at a site where small releases of mixed chlorinated solvent DNAPL occurred decades ago into a heterogeneous sandy aquifer overlying a thin clay-rich aquitard. Source zone remediation was implemented in 2007 using insitu soil mixing with zero-valent iron and bentonite (ZVI-bentonite). Characterization of the contaminant mass distribution was conducted in 2008-2009 using the membrane interface probe (MIP) and subsampling of continuous cores. A multilevel monitoring network was installed to track groundwater plume conditions along two transects perpendicular to flow and a longsect along the plume centerline. In 2018, a comprehensive field episode was conducted to assess conditions again a decade later, including subsampling of continuous cores, magnetic susceptibility for ZVI distribution, combined MIP-HPT (Hydraulic Profiling Tool) characterization and compound specific isotope analyses (CSIA). The high-resolution datasets show that back-diffusion is a strong contributor to plume persistence as well as incomplete source treatment. Strong degradation is occurring within the aquitard, which will reduce the magnitude and longevity of back diffusion. Numerical modeling provides insights into the time-scales. This presentation will focus on insights from the high resolution measurements and modeling as tools to improve the site conceptual model and decision making.