EVIDENCE FOR HEAT TRANSPORT BY DEEP GROUNDWATER FLOW ON THE SOUTH FLANK OF GRAND MESA NEAR CEDAREDGE, COLORADO

Three-dimensional finite-difference groundwater flow models on the south flank of Grand Mesa suggest that bedrock flow paths discharge to surface streams, to natural springs, and into near surface alluvial material that serves as a local aquifer for domestic water wells. This upward discharge is driven by over 6,000 feet of topographic relief which produces flow regimes with a wide range of scale. Using the base flow of the Tongue Creek drainage, and measured depths to water table as constraints, a lower limit is derived for the average hydraulic conductivity of the strata that equals 0.004 ft/day. Variations in hydraulic conductivity of the major stratigraphic units are found by perturbing their values while preserving the base flow and water table constraints of the system. The results indicate that topography of the water table is the dominant controlling factor in groundwater flow at this site, with lithology contributing second-order perturbations. The major hydrological features of the system can be predicted from a constant hydraulic conductivity model.

Water temperatures in shallow domestic wells at low elevations are significantly warmer than would be expected from the mean annual surface temperature at their corresponding elevations. Finite-difference models of regional heat flow give a nearly uniform temperature at shallow depths that is independent of elevation, and therefore cannot explain the observed temperature distribution of groundwater. If three-dimensional groundwater flow is combined with a heat transport simulation, qualitative agreement between predicted and observed water temperatures is obtained. It appears, therefore, that the distribution of groundwater temperatures results from the interaction of regional heat flow and the transport of heat by groundwater. This provides evidence for the existence of these groundwater recharge pathways. In addition, the relative importance between the processes of thermal conduction and groundwater heat transport appears to be consistent with the derived average hydraulic conductivity and thermal conductivity.