Diffusive Disappearance of Immiscible‐Phase Organic Liquids in Fractured Geologic Media

Abstract

AbstractIn a new conceptual model for immiscible‐phase organic liquids in fractured 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 lost by diffusion from the fractures into the porous matrix between the fractures. Theoretical calculations for one‐dimensional diffusive fluxes from single, parallel‐plate fractures using parameter values typical of fractured porous geologic media establishes the concept of immiscible‐phase disappearance time, which is the time required for a given volume of immiscible liquid in a specified aperture to disappear following its arrival in the fracture. Nonlithified surficial clayey deposits with matrix porosities ranging from 25 to 70% are extensive across many regions of North America and Europe, and at shallow depth, typically have fractures with apertures in the range of 1 to 100 microns. The common chlorinated 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 with much lower matrix porosities (5–15%), disappearance times for these solvents are generally less than several years for fracture apertures ranging from 10 to 200 microns typical for shales, siltstones, sandstones, and carbonate rocks. This is sufficient time for the immiscible phase of chlorinated solvent contamination to have disappeared at many industrial sites. This conceptual model has important implications with respect to ground‐water monitoring, diagnosis of the nature and degree of contamination, and expectations for ground‐water remediation at many contaminated sites. Proposed methods for enhancing immiscible‐phase mass removal using hydraulic manipulation, surfactants, or alcohols will be futile where the immiscible phase has disappeared into the clay or rock matrix, and reverse diffusion and desorption will control clean‐up time frames. Therefore, prospects for permanent restoration of many DNAPL and LNAPL sites in fractured porous media are more limited than previously thought.