Testing the Applicability of Using Groundwater Temperatures to Constrain Recharge Rates in Arid Intermontane Basins

Recent studies in the Wasatch Mountains and adjacent Salt Lake Valley (Manning, 2002; Manning and Solomon, 2005), located in the western United States, have shown that it is possible to constrain groundwater recharge rates by using groundwater temperatures measured within the basin in conjunction with a coupled groundwater flow/thermal transport model. In the Wasatch Mountains, where snowmelt is the main source of recharge, recharge rates are on the order of a few hundred millimeters per year, and recharge temperatures vary between 0° C and 11° C. The purpose of this study was to investigate if this method could be applied in a much drier and warmer setting. To accomplish this, a generic groundwater flow/thermal transport model of a typical Basin and Range watershed was constructed. The model was used to test the sensitivity of groundwater temperatures to different hydrologic and thermal parameters, and determine the minimum recharge rate within a mountain block that would produce a measurable thermal perturbation in an adjacent basin. Modeled conditions were based on those found in the southern Great Basin, where recharge rates are up to 2 orders of magnitude lower and air/ground surface temperatures are on average 2° C to 5° C warmer than those within the Salt Lake watershed. Model parameters tested include hydraulic conductivity, porosity, basal heat flux, thermal conductivity, and recharge rates. For recharge rates of 5 mm/yr or less, temperatures within the model were highly sensitive to the thermal parameters (average ±4° C in the saturated domain with a 10% change in thermal parameters ), indicating that at these recharge rates thermal transport is primarily conductive. Alternatively, at recharge rates of 50 mm/yr and above, model temperatures were no longer sensitive to the thermal parameters, indicating an advective thermal regime. Model results showed that hydraulic conductivity and recharge rates were the main controls on the distance to which the thermal perturbation extends into the adjacent basin. For any recharge rate, model temperatures were not sensitive to porosity. At intermediate values of hydraulic conductivity (0.1 m/day) model results indicate that a minimum recharge rate of 50 mm/yr in the mountain block is needed to produce a measurable thermal perturbation at shallow depths in the adjacent basin. Based on these results, it appears that the method put forth by Manning and Solomon can be used to constrain groundwater recharge rates in drier and warmer environments, with a few exceptions.