Geophysical Characterization Of A Fractured-Bedrock Aquifer And Blast-Fractured Contaminant-Recovery Trench

Borehole- and surface-geophysical methods were used to characterize the hydrogeology and the effects of<br>blast fracturing an in-situ recovery trench in a contaminated fractured-bedrock aquifer. The recovery trench is located<br>at the former fire-training area of Loring Air Force Base in Aroostook County, Maine. Borehole-geophysical<br>methods, used in six wells at the site, included video, acoustic televiewer, heat-pulse flowmeter under nonpumping<br>and low-rate pumping conditions, natural gamma, electromagnetic induction, tluid temperature and conductivity,<br>caliper, deviation, and borehole radar. Borehole radar was used in a single-hole reflection configuration with<br>directional and non-directional 60-MHz (megahertz) antennas and in a cross-hole tomography configuration with 22-<br>MHz antennas. One surface-geophysical method, azimuthal square-array direct-current resistivity, also was used.<br>Geophysical surveys were conducted before and after blast fracturin, 0 the recovery trench. Integrated<br>interpretation of the geophysical data collected before blasting indicates that most transmissive fractures are steeply<br>dipping and are oriented northeast and southwest. Analysis of azimuthal square-array-resistivity data indicates that<br>the secondary porosity of the fractured-bedrock aquifer is about I percent. The borehole-geophysical data and crosshole<br>radar tomography data indicate that more fractures are present in the upper 20 to 25 m (meters) of bedrock than<br>in bedrock below this depth. Interpretation of the geophysical data collected after blast fracturing the recovery trench<br>indicates that the blast created an intensely fractured zone about 3 m wide, 26 m deep, along the 50-m length of the<br>recovery trench. Blast-induced porosity in the recovery trench is estimated from the borehole-radar data to be 13.5+ 5<br>percent at the midpoint of the trench, decreasin, u to 7.3+ 6 percent at the northwestern end. Post-blast effects on the<br>hydrology of the area adjacent to the recovery trench include (1) a decline in static water levels, (2) order-ofmagnitude<br>increases in upward flow in two wells, (3) reversal of flow directions in two wells, (4) order-of-magnitude<br>increases in the estimated transmissivity of three wells, and (5) an estimated increase in aquifer secondary porosity to<br>2 percent near the trench. The increase in secondary porosity is estimated on the basis of azimuthal square-array<br>resistivity data collected over the recovery trench and cross-hole tomography collected parallel to but outside the<br>trench. These effects are consistent with increased porosity and permeability in the blast-fractured recovery trench<br>and with increased fracture transmissivity near the recovery trench. The increased fracture transmissivity resulted<br>from an apparent hydraulic cleaning that occurred when water was ejected out of wells near the trench during the<br>blast.