EVALUATING PALEOSOL CONTINUITY USING GROUND PENETRATING RADAR FOR MORE ACCURATE REGIONAL-SCALE GROUNDWATER MODELING ON THE KINGS RIVER ALLUVIAL FAN

Ground penetrating radar (GPR) data, collected on the Kings River alluvial fan southeast of Fresno, California, shows the lateral distribution of near surface paleosols. These laterally continuous paleosols, along with similar paleosols at depth, act as sequence boundaries in the alluvial fan succession and as low K barriers to fluid flow and contaminant transport (Weissmann et al., in press). Discrete breaks in these paleosols may act as fast paths for fluids and contaminants into lower aquifer units. Using the Pulse-Ekko 100 system with 50-Mhz antennas, we recognize the shallow paleosols due to signal attenuation from the high clay content of the paleosols. Paleosol breaks are recognized by locating areas along GPR transects with increased depth of signal penetration. Preliminary results suggest the breaks in paleosols range in width from approximately 100m to 3km; with the widest break occurring across a previously described incised valley fill (Weissmann et al, 2002). Other breaks correspond to channel widths observed in soil surveys. Therefore, the discrete breaks in the paleosol appear to be created as the Kings River switched its position on the alluvial fan during initial reactivation of the alluvial fan surface. In the future, paleosol character and break distributions, determined from this work, will be incorporated into revised groundwater models of the Kings River aquifer system.