Geophysical tomography visualizes amendment delivery and transport in fractured rock
Rutgers University Newark, with the support of the United States Geological Survey (USGS) and Pacific Northwest National Laboratory (PNNL), has successfully demonstrated how cross-borehole time-lapse electrical resistivity tomography (ERT) can image the delivery of an amendment injection into a TCE-contaminated fractured rock system. Time-lapse ERT is a high-resolution subsurface imaging method that is analogous to medical tomography. Conventional methods for monitoring flow and transport rely on sparse borehole observations that can miss important flow paths, especially in heterogeneous fractured rock systems. In contrast, ERT can image beyond the boreholes (see prototype electrode array right) and provide spatially continuous information at scales representative of the structural heterogeneity exerting the primary control on flow and transport.
The field demonstration at NAWC involved the injection of a 60 L solution containing molasses (365 g/L) spiked with NaBr (102 g/L) to enhance the electrical conductivity contrast required for time-lapse ERT monitoring. A dense dataset of electrical measurements using 143 electrodes distributed across 7 boreholes (shown right) was inverted for images of changes in electrical conductivity due to the amendment injection using the code E4D developed by PNNL. The resulting time-lapse images provide unprecedented information on the evolution of the amendment into an important bedding plane feature controlling TCE transport at the site. The images reveal strong heterogeneity in bedding plane transport with implications for quantifying bedding plane surface area impacted by the amendment injection.
In contrast, prior amendment injections at this site were poorly characterized by local borehole measurements alone. The ERT monitoring is being continued to assess whether electrical changes due to biodegradation induced by the amendment can be detected.