COMPARISON OF TEMPERATURE-PROFILE METHODS FOR ESTIMATING PERCOLATION RATES AND POTENTIAL RECHARGE IN THE SOUTHWEST

Percolation rates and potential recharge are estimated using sediment temperature profiles for three prominent topographies in the Southwest: (1) beneath stream channels, (2) within basin fill materials, and (3) underlying mountainous terrain. Beneath stream channels, vertical temperature profiles vary over time in response to the rate of downward heat transport, which is dominated by conductive heat transport during dry periods, and by advective transport during channel infiltration. A relatively simple heat and liquid-water transport simulation code is coupled with an optimization code to inversely fit simulated sediment temperatures to observed temperatures during periods of streamflow. Percolation rates focused beneath stream channels were examined for a perennial, seasonal, and ephemeral channel in central New Mexico. At greater depth, water percolating through the vadose zone perturbs the local geothermal gradient to create a steady concave upward temperature profile, while upward fluxes of water perturb the gradient to create a concave downward profile. An analytical solution to the heat transport problem was developed for generating a family of temperature profiles, based on a series of recharge rates. Temperature profiles were matched to observed borehole temperatures to determine a net recharge rate in basin fill material beneath Frenchman Flat, NV. The complexities of stratigraphy and vapor movement were incorporated into a heat and water transport model for matching simulated temperature profiles to observed profiles for estimating deep percolation rates. Deeper percolation beneath expansive inter-channel basin fill sediments was examined below Frenchman Flat, NV. Deep percolation was examined in the complex bedrock beneath Yucca Mountain, NV. Temperature-derived results suggest that deep potential recharge represents a fraction of 1% of percolation rates beneath stream channels, however, more conclusive results are limited by the lack of shallow vs. deeper temperature data at the same site.