Common-depth (CD) radar surveys and cross-hole radar tomography methods were used to monitor the movement of a saline tracer in a dual-porosity dolomite aquifer at Belvidere, Illinois. The tracer test was conducted using an array of six open-hole bedrock wells at the Parson’s Casket Hardware Superfund site. The injection and recovery boreholes were about 20 m (meters) apart, and the imaging boreholes were arranged to provide planar coverage across and along the anticipated tracer path. A hydraulically conductive zone identified during previous investigations was isolated using straddle packers and pumped to establish a hydraulic gradient between the injection and recovery wells. A sodium chloride (NaCl) solution was continuously injected into this zone to move the tracer across the tomographic image plane.

CD cross-hole radar surveys and cross-hole tomography surveys were conducted before and periodically during the tracer injection. Background tomograms contain similar radar velocity and attenuation changes with depth, consistent with a layered dolomite that has variable porosity and electrical conductivity. Slow changes in attenuation associated with low tracer velocity permitted the acquisition of multiple CD surveys and two cross-hole tomography surveys during injection. CD surveys were used to rapidly identify the presence of tracer between wells. Attenuation-difference tomograms contain attenuation increases that delineate the spatial distribution with time of the saline tracer and show the progressive movement of the tracer within the tomographic image plane. Formation porosity and resistivities calculated from radar velocity and attenuation tomograms were used to estimate changes in fluid resistivity and tracer concentration in the tomographic image plane.