Variability of organic carbon content and the retention and release of trichloroethene in the rock matrix of a mudstone aquifer

摘要

Contaminants diffusing from fractures into the immobile porosity of the matrix are subject to prolonged residence times. Organic contaminants can adsorb onto organic carbonaceous materials in the matrix extending contaminant retention. An investigation of spatial variability of the fraction of organic carbon (f(oc)) is conducted on samples of core from seven closely spaced boreholes in a mudstone aquifer contaminated with trichloroethene (TCE). A total of 378 samples were analyzed at depths between 14 and 36 m below land surface. Mudstone units associated with deep water deposition have the largest f(oc), with a maximum value of 0.0396, and units associated with shallow water deposition have the smallest f(oc). Even though f(oc) correlates with depositional conditions, f(oc) still varies over more than an order of magnitude in continuous mudstone layers between boreholes, and there is large variability in f(oc) over short distances perpendicular to bedding. Simulations of diffusion and linear equilibrium adsorption of TCE using spatially variable f(o)(c) in the matrix show order of magnitude variability in the adsorbed TCE over short distances in the matrix and residence times extending to hundreds of years following remediation in adjacent fractures. Simulations using average values of f(oc) do not capture the range of TCE mass that can be retained in a matrix characterized by spatially variable f(oc). Bounds on TCE mass within the matrix can be obtained by simulations with spatially uniform values of f(oc) equal to the maximum and minimum values of f(oc) for a given mudstone unit.