Preliminary measurements of stress-dependent hydraulic conductivity of Santa Fe Group aquifer system sediments from the 98th St core hole, Albuquerque, New Mexico

Measurements made on undisturbed and recompacted samples from the 98th Street core hole in western Albuquerque show that the hydraulic conductivity of Santa Fe Group sediments is sensitive to the effective confining stress. The hydraulic conductivity of consolidated, undisturbed, and typically fine-grained sediments decreased two to three orders of magnitude between vertical effective stresses of about 50 and 1,000 kPa. That of unconsolidated, recompacted, and generally more sandy samples decreased by less than one to two orders of magnitude over about the same magnitude of effective stress change. Bulk density (and its surrogates, void ratio and porosity) appears to be the single most important controlling factor in the reduction of hydraulic conductivity. A simple regression model shows that changes in porosity can account for about three-fourths of the variability of hydraulic conductivity extrapolated at zero confining pressure. The results of our work imply that heavy ground water pumping may measurably reduce the hydraulic conductivity of the Santa Fe Group aquifer system in the vicinity of large water wells, and also that changes in effective stress (for example, due to burial and tectonic uplift over geologic time or heavy pumping over human time spans) may result in a nonlinear increase in the basin-scale hydraulic conductivity anisotropy.