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

Abstract

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 [Formula: see text] of the property. The shallow geology consists of bedrock limestone overlain by [Formula: see text] of clay soil. For the [Formula: see text] antennas used, the conductive clay limits depth penetration to less than [Formula: see text] and yields a [Formula: see text] 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.