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

• Published in: Journal of contaminant hydrology Vol. 217; pp. 32 - 42
• Main Authors: Shapiro, Allen M., Brenneis, Rebecca J.
• Format: Journal Article
• Language: English
• Published: Netherlands Elsevier B.V 01.10.2018
• Subjects: adsorption; aquifers; mudstone; organic carbon; porosity; remediation
• ISSN: 0169-7722; 1873-6009; 1873-6009
• DOI: 10.1016/j.jconhyd.2018.09.001

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 (foc) 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 foc, with a maximum value of 0.0396, and units associated with shallow water deposition have the smallest foc. Even though foc correlates with depositional conditions, foc still varies over more than an order of magnitude in continuous mudstone layers between boreholes, and there is large variability in foc over short distances perpendicular to bedding. Simulations of diffusion and linear equilibrium adsorption of TCE using spatially variable foc 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 foc do not capture the range of TCE mass that can be retained in a rock matrix characterized by spatially variable foc. Bounds on TCE mass within the rock matrix can be obtained by simulations with spatially uniform values of foc equal to the maximum and minimum values of foc for a given mudstone unit. •Organic carbon fraction (foc) in a mudstone varies over two orders of magnitude.•In continuous layers of the mudstone, foc varies over an order of magnitude.•The largest foc is associated with carbon-rich mudstone layers.•Simulations TCE in the rock matrix show large variability in adsorbed TCE over small distances.•Representative values of foc can be defined to constrain TCE mass retained in the rock matrix.