Contaminants diffusing from fractures into the immobile porosity of the rock 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 rock 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_oc in the rock 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 rock matrix characterized by spatially variable f_oc. Bounds on TCE mass within the rock matrix can be obtained by simulations with spatially uniform values of f_ocequal to the maximum and minimum values of f_oc for a given mudstone unit.