High Resolution Field Characterization and Numerical Modeling of Contaminant Storage and Release from Low Permeability Zones

Within the past decade it has become widely recognized that contaminants stored in low permeability zones can sustain plumes long after primary sources have been depleted by natural processes or isolated or reduced by remedial efforts. This can be of particular consequence for chlorinated solvents, where regulatory limits are generally several orders of magnitude lower than historical source concentrations. Recognition of this problem has lead to efforts to improve site characterization methods to collect appropriate data on lithology and contaminant distribution to assess mass storage in low permeability zones, and to apply numerical models to better predict effects on plume persistence. A primary challenge is that diffusion processes are controlled by small-scale concentration gradients and the distribution and heterogeneity of lower permeability zones, and recognition that most of the mass may be in the sorbed phase, require incorporating much higher resolution for site characterization and model application than what is commonly practiced.

This presentation will focus on a case study at a site in Florida, where several complementary commercially available high resolution techniques have been applied: MIP tool for initial lithology and contaminant screening to select detailed investigation locations, the Waterloo APS™ (Advanced Profiler System) for hydrostratigraphy and depth-discrete groundwater sampling targeting higher permeability zones and interfaces, Geoprobe HPT™ (Hydraulic Profiling Tool) for hydrostratigraphy, and continuous cores for lithology and subsampling for mass distribution. Sampling at appropriate scales and positions within the system is essential, recognizing the processes at work (diffusion, sorption, degradation) and interplay between high and low permeability zones, spatial scales of variability and time scales for evolution or changes. High resolution numerical modeling, using the detailed mass distribution for ground-truthing, provides insight into potential long-term impacts of mass storage and release from low permeability zones, allowing more informed decision making on site remediation and management options.