STRONG MATRIX DIFFUSION EFFECTS ON CONTAMINANT BEHAVIOR IN FRACTURED SEDIMENTARY ROCKS

In fractured sedimentary rocks such as sandstone, shale and carbonates, contaminant transport in groundwater occurs primarily or exclusively in the fracture network, which has a very low bulk porosity (<0.001). In these rocks the matrix blocks between fractures have much larger porosity, generally >0.02. In theory, molecular diffusion causes contaminant mass in the fractures to be transferred to the rock matrix where groundwater is nearly immobile. In practice, there has been a paucity of field data showing that this diffusion-driven mass transfer is important. A new method for measuring the contaminant mass in the rock matrix was developed and applied at seven industrial sites where chlorinated solvent contamination has been present for decades in fractured sandstones, shales, limestones and dolostones. The method measures total mass concentrations of individual analytes that are converted to pore water concentrations based on rock properties. The contaminant analyses were done on thousands of samples taken at closely spaced intervals from continuous rock core through contaminated zones. These measurements show that the matrix diffusion halos along fractures are large and measurable and that nearly all of the contaminant mass at these sites now exists in the rock matrix rather than in the fracture network. These results provide insight into the distribution of hydraulically active fractures and have important implications for monitoring, transport and fate of contaminants in sedimentary rocks.