Low Permeability Does Not Mean Impermeable: Diffusion of Chlorinated Solvents Into a Clay Aquitard

Chlorinated ethenes and ethanes diffused into a low-permeability clay layer 90 feet below ground surface have complicated remediation efforts at a site on Long Island.  The clay layer is overlain by a gravel and sand aquifer with relatively high hydraulic conductivity that is consistently aerobic.  The clay layer is assumed to be approximately 10 feet thick, and is the aquitard between the gravel and sand aquifer and the underlying Magothy aquifer, an important drinking water supply for Long Island. 

The site was a solvents recycling facility, and mixed contaminants leaked from storage tanks over two decades ago.  During the remedial investigation, it was noted that the contamination was only detected in wells screened directly above the clay layer; with limited contamination identified greater than a few feet above the clay.  No contamination was identified in the underlying Magothy formation, indicating that the clay was indeed serving as an aquitard.  After careful consideration of the data, assisted by the use of 3D visualization software, CDM developed a conceptual site model for the site.  The diffusion of contaminants into such a deep clay layer was the key point to consider in remedy selection.  A draft feasibility study has been prepared with remedial alternatives based on the conceptual site model. 

The original data indicating the presence of the matrix diffusion problem will be presented, followed by the conceptual site model.  The complexities and complications created by the diffusion into the clay layer will be discussed.  Lastly, the remediation options developed during the feasibility study will be presented, including a novel approach to a permeable reactive biobarrier.