ESTIMATION OF HYDRAULIC CONDUCTIVITY IN FORMERLY SATURATED ALLUVIUM USING NUCLEAR MAGNETIC RESONANCE

Nuclear magnetic resonance (NMR) borehole logs collected from 11 PVC-cased monitoring wells are used to estimate hydraulic conductivity (K) of formerly saturated alluvium overlying an aquifer contaminated with hexavalent chromium, Cr(VI), in the western Mojave Desert, California. Groundwater elevations have declined as much as 60 feet from the time of the Cr(VI) release in 1952, exposing coarse-grained river and beach deposits in the present-day unsaturated zone. Presently saturated alluvium typically consists of finer-grained lake margin deposits with lower K values, which may not be representative of K values at the time of the release. NMR data from auger-drilled monitoring wells are used to develop an inverse relationship (R² = 0.78) between immobile water content and the NMR-derived K of saturated alluvium. This relationship is used to estimate K from the immobile water content of formerly saturated alluvium. The geometric means of NMR-derived K values from saturated alluvium across the screened interval of measured wells are similar to slug test K values, although the slug test K values were generally less than the NMR-derived K values. The K of formerly saturated alluvium near the Cr(VI) source area was estimated at 300 feet per day (ft/day), as compared to model-derived K values of 50 to 60 ft/day at the current water table. NMR-derived K is consistent laterally throughout the aquifer. High calculated K values in formerly saturated alluvium suggest that contaminated groundwater had the potential to move further from its source than is indicated when only present-day saturated K values are considered.