2012 NGWA Ground Water Summit: Innovate and Integrate

Characterizing Chlorinated Solvent Diffusion and Degradation Processes in a Clay Aquitard

Wednesday, May 9, 2012: 2:30 p.m.
Terrace Room A-C (Hyatt Regency Orange County)
Adam M. Gilmore, University of Guelph;
Beth L. Parker, Ph.D., University of Guelph;
Steven W. Chapman, University of Guelph;

A detailed research study is ongoing to evaluate effects of mass storage and release from low permeability zones on downgradient plume persistence (referred to as back diffusion) following treatment of a DNAPL source.  At the study site, small releases of mixed organic chemicals occurred decades ago into a surficial sandy aquifer, underlain by an organic-rich clayey aquitard.  In 2007, the source zone was treated via soil mixing with zero-valent iron and bentonite, with the goal of reducing mass discharge from the source zone via abiotic degradation and source zone permeability reduction. The goal of this study is to collect clear performance data for evaluating downgradient response to this treatment including back diffusion effects.  Several high resolution site characterization techniques have been employed including: (1) membrane interface probe (MIP), (2) continuous coring for assessing small-scale heterogeneity with detailed VOC subsampling to quantify contaminant distributions, and (3) microbial characterization and application of compound-specific isotope analyses (CSIA) for assessment of degradation.

High resolution characterization results show much contaminant mass occurs in low permeability zones within the aquifer and underlying aquitard, providing an expectation of strong back diffusion effects. However, strong degradation occurs within the aquitard, which can greatly reduce the magnitude and longevity of back-diffusion.  The detailed characterization results are applied in numerical model simulations to evaluate the positive effects of degradation on 1) preventing the downward flux of contaminants in the clay aquitard to underlying aquifers, and 2) reducing the mass flux of contaminants stored in the aquitard to the overlying aquifer following source treatment.  This study shows the benefit of high resolution site characterization at key locations to identify mass distributions and processes in both low and higher permeability strata due to the importance of mass transfer between these zones, which can strongly affect remediation system performance and downgradient plume persistence.