Mapping contaminant-transport structures in karst bedrock with ground-penetrating radar

Ammonium perchlorate, a risk to human health, was used formerly to manufacture rocket fuel at the Naval Weapons Industrial Reserve Plant (NWIRP) McGregor, McLennon County, Texas. Perchlorate exists in several groundwater contaminant plumes, whose geometries were suspected to be influenced by transmissive bedrock structures. To identify these possible contaminant-transport pathways, a towed-array ground-penetrating radar (GPR) system was used to acquire 118 line-km of data across 478hectares of the property. The shallow geology consists of bedrock limestone overlain by 0to3m of clay soil. For the 50-MHz antennas used, the conductive clay limits depth penetration to less than 3m and yields a 1.4-m wavelength, reducing lateral and vertical resolution. Nevertheless, GPR data resolved the top of bedrock in many areas. Linear discontinuities in bedrock were interpreted as weathered fracture zones, and linear areas of signal loss were attributed to deeper clay weathering along fracture zones. GPR-interpreted fractures have orientations corresponding to known lineament and fault trends, appear to control plume geometries, and tend to have higher hydraulic transmissivities. GPR results led to a more complete contaminant-transport model and were used to optimize the positions of monitoring wells needed to define the extent of contamination. This reduced the cost and time required for an environmental investigation at the site. GPR was helpful also in positioning remedial trenches across contaminated structures, resulting in plume containment at the property boundaries.