Assessing Groundwater Perching Horizons Using Synthetic, Ground, And Airborne Tdem Data At The Pantex Plant, Texas

At the U.S. Department of Energy's Pantex Plant near Amarillo, Texas, recharging groundwater<br>encounters a perching fine-grained zone (FGZ) above the main Ogallala (High Plains) aquifer, a critical<br>agricultural, municipal, and industrial water supply. We conducted modeling and ground-based timedomain<br>electromagnetic induction (TDEM) measurements to examine whether TDEM might help assess<br>the integrity of the FGZ and its ability to retard the flow of groundwater to the main aquifer. Conductivity<br>models based on well logs and representative TDEM soundings demonstrated that changes in thickness<br>of the FGZ and perched aquifer are detectable if changes in other stratigraphic horizons are minimal.<br>Based on these results, we conducted an airborne geophysical survey using Fugro's GEOTEM system.<br>Conductivity-depth transforms (CDTs) from airborne measurements matched the nearest ground-based<br>profiles reasonably well. Pseudo-depth slices constructed from CDTs depicted apparent conductivity at<br>critical depths, enabling refinement of borehole-based stratigraphic and hydrologic models. Elevated apparent<br>conductivities were observed where interpreted FGZ integrity was good and the saturated zone is relatively<br>thick; low apparent conductivities are interpreted to indicate areas where the FGZ and associated saturated<br>zone are thin or absent. Survey results are being used to support groundwater investigations, development<br>of fate and transport models, and upcoming corrective action decisions.