DIFFUSIVE DISAPPEARANCE OF IMMISCIBLE-PHASE ORGANIC LIQUIDS IN FRACTURED GEOLOGIC MEDIA

In a new conceptual model for immiscible-phase organic liquids in frac tured porous media that specifically includes the effect of molecular diffusion on the persistence of organic liquid in fractures, dissolved contaminant mass from the liquid in fractures is test by diffusion fr om the fractures into the porous matrix between the fractures. Theoret ical calculations for one-dimensional diffusive fluxes from single, pa rallel-plate fractures using parameter values typical of fractured por ous geologic media establishes the concept of immiscible-phase disappe arance time, which is the time required for a given volume of immiscib le liquid in a specified aperture to disappear following its arrival i n the fracture. Nonlithified surficial clayey deposits with matrix por osities ranging from 25 to 70% are extensive across many regions of No rth America and Europe, and at shallow depth, typically have fractures with apertures in the range of 1 to 100 microns. The common chlorinat ed solvents such as dichloromethane (DCM), trichloroethene (TCE), and tetrachloroethene (PCE) are expected to completely disappear in these deposits within a few days to weeks. For fractured sedimentary rocks w ith much lower matrix porosities (5-15%), disappearance times for thes e solvents are generally less than several years for fracture aperture s ranging from 10 to 200 microns typical for shales, siltstones, sands tones, and carbonate rocks. This is sufficient time for the immiscible phase of chlorinated solvent contamination to have disappeared at man y industrial sites. This conceptual model has important implications w ith respect to ground-water monitoring, diagnosis of the nature and de gree of contamination, and expectations for ground-water remediation a t many contaminated sites. Proposed methods for enhancing immiscible-p hase mass removal using hydraulic manipulation, surfactants, or alcoho ls will be futile where the immiscible phase has disappeared into the clay or rock matrix, and reverse diffusion and desorption wilt control clean-up time frames. Therefore, prospects for permanent restoration of many DNAPL and LNAPL sites in fractured porous media are more limit ed than previously thought.