Identifying a Trichloroethene Source Zone from Analyses of Rock Core in a Mudstone Aquifer

In fractured rock, Non-Aqueous Phase Liquid (NAPL) releases of trichloroethene (TCE) lead to complex contaminant distributions due to fracture connectivity and aperture variability. Continuous pool heights of NAPL can also result in TCE emplacement in both low permeability fractures and large pore throats in the rock matrix. Additionally, aqueous phase TCE diffuses into the rock matrix from fractures.

TCE analyses of rock core are an integral part of characterizing the magnitude and spatial distribution of contaminant mass in the rock matrix. Analyses of cores from seven closely spaced coreholes in a mudstone aquifer show a complex vertical and lateral TCE distribution. Phase partitioning calculations for TCE in the rock matrix show that most TCE is adsorbed to solid surfaces because of the large organic carbon fraction (f_oc) in the mudstone; however, large TCE content in some core samples also suggest the presence of NAPL in the rock matrix and proximity to a TCE source zone. A probability of NAPL occurrence, P_NAPL, in core samples can be calculated by accounting for variability in matrix porosity and f_oc in the phase partitioning calculations. The distribution of P_NAPL along the closely spaced coreholes identifies a configuration attributed to a TCE source zone.