Fault-controlled contaminant plume migration: Inferences from SH-wave reflection and electrical resistivity experiments

A contaminant plume migrates within a shallow late-Tertiary gravel aquifer in an anomalous curvilinear pattern aligned with the general orientation of an underlying early-Paleozoic fault system. The bedrock faults are masked by ~100-m of overlying Late Cretaceous through Quaternary sediment. Fault reactivation has been as late as Pleistocene with near-surface deformation affecting the aquifer. Near-surface geophysical attributes indicate a potential structural influence for the anomalous plume migration pattern. Specifically, the fault zones exhibit lower shear-wave velocity (100 m/s) and electrical resistivity (80 Ω-m) relative to surrounding sediment. In addition, shear-wave splitting across a well-constrained fault strand indicates the sediment has an average 3.6% azimuthal anisotropy coincident with fault strike. Variation in elastic and electrical properties, as well as spatial coincidence of the plume and geologic structure, suggest neither mechanical (i.e., consolidation) nor chemical (i.e., cementation) “healing” of the sediments have completely occurred since deformation cessation. The variation in geophysical characteristics are also indicative of a non-uniform hydraulic conductivity, thus providing a structurally controlled preferential pathway for fluid migration. These results demonstrate that integrated non-invasive geophysical methods are effective for evaluating complex geologic conditions in environmentally sensitive areas.