Cross-borehole ERT monitoring of a tracer injection into chlorinated-solvent contaminated fractured mudstone

There is a need to monitor remedial injections in contaminated fractured rock to determine if targeted areas have been reached and to monitor treatment effectiveness. While detailed information can be obtained at boreholes, these locations are limited; determining connectivity in fracture networks is difficult and borehole monitoring locations may miss the injection entirely. The primary and secondary domains in fractured rock have hydraulic conductivities that differ by orders of magnitude such that tracer injections commonly have rapid breakthrough followed by extended tailings. Often, it is presumed that the tracer is transported into dead-end pore spaces or unknown inter-connected networks and/or sorbed into the primary porosity. Cross-borehole electrical resistivity tomography (ERT), guided by information from borehole geophysical logging and hydraulic testing, has the potential to monitor the fate of tracer injections between borehole locations. ERT has been under-exploited in fractured rock due to: (1) a lack of available 3D codes, (2) a lack of computing resources to accommodate a large number of model parameters, and (3) limitations of regularization constraints used in ERT modeling for representing fractured rock settings along with a full understanding of these constraints. Recognizing numerous advances in ERT imaging and building on our previous studies, we present results from a field-scale ERT experiment in fractured rock. We use ERT to monitor a conductive tracer injection in a fractured mudstone at the Naval Air Warfare Center (NAWC) in New Jersey. A custom-built electrode array included inflatable bladders to isolate fractures within each borehole and allowed for discrete water sampling and injection. By injecting the tracer in pulses and collecting 3D ERT measurements following each pulse, we were able to (1) avoid rapid breakthrough and large dilution rates and thus maintain a high conductivity contrast, and (2) characterize ambient flow by simulating static conditions after each pulse. Our findings show unanticipated tracer migration pathways between borehole locations and evidence for diffusion of tracer mass into the rock matrix over time. Identifying such transport processes using advances in ERT imaging will facilitate remedial injections to targeted areas.