INVESTIGATING GEOPHYSICAL TECHNOLOGIES TO CHARACTERIZE PORE GEOMETRIC PROPERTIES CONTROLLING CONTAMINANT MASS TRANSPORT IN FRACTURED ROCK

Contaminated fractured rock aquifers pose unique challenges that limit the effectiveness of enhanced remediation strategies. The efficiency of targeted amendment injections is limited by the slow diffusion of aqueous phase contaminants into the low-permeability rock matrix between fractures or near dead-end fractures. Understanding the distribution of physical properties controlling this fine scale distribution of contaminant mass away from fractures, and how this mass changes in response to remediation efforts, is critical for improving remediation in fractured rock. We have investigated the measurements of two geophysical methods, (1) complex resistivity (CR), and (2) nuclear magnetic resonance (NMR), to define key transport characteristics at two sites contaminated with chlorinated ethenes for several decades. Each distinct fractured rock environment has been part of the G360 Institute research program on the behavior of chlorinated solvents in fractured rock since 1997 and 2003. One site is an interbedded sandstone, siltstone and shale sequence of marine turbidite origin that has been uplifted and faulted in a tectonically active area in southern California, and the other site has horizontally bedded sandstone, siltstone and dolostones located in south-central Wisconsin. The spectral polarization responses measured from CR and the T₂ relaxation times measured from borehole NMR show sensitivity to pore size distribution, surface area and air permeability from numerous depth-discrete rock core samples selected to represent the variability observed in continuous core logs. We demonstrate how borehole geophysical measurements might be used to estimate mass transfer rates local to fractures controlling fluid flow by minimizing the need to measure these properties on numerous core samples in the laboratory.