GROUND WATER/SURFACE WATER INTERACTIONS IN THE MEDANO CREEK FLOODPLAIN, GREAT SAND DUNES NATIONAL MONUMENT, SAN LUIS VALLEY, COLORADO

Natural resource specialists at the Great Sand Dunes National Monument in southern Colorado have realized the complexity of the interactions between ground water and surface water in the monument and their importance in protecting the monument's natural systems. Beginning prior to 1990, a number of research efforts have sought to define the temporal and spatial nature of these interactions. The main dune mass at the monument is bounded by Sand Creek to the north and Medano Creek to the east and south. The length and duration of live flow in these streams in the monument are controlled by complex interactions between surface flow and the underlying aquifer, composed of fine-grained eolian sand with varying percentages of silt and clay. The monument authorized a study completed in October 2001 during a period of no-flow in Medano Creek in order to better understand the hydrologic regime of the Medano Creek floodplain.

Water level data from a series of shallow monitoring wells in the Medano Creek floodplain indicate a water table approximately 6 feet below ground level. Deeper monitoring wells, however, display deeper water levels, implying series of unsaturated zones alternating with downward-moving wetting fronts in the eolian aquifer. Geophysical logging and lithologic data show there are no regionally extensive lithologic changes in the upper portion of the aquifer. Unsaturated zones in the aquifer are due to very slight variations in silt or clay content (perhaps undetectable in hand samples), coupled with a strong downward flow gradient. Siltier perching zones act to decrease saturation (to approximately 80-90% saturation) locally beneath these zones. The limited areal extent of these zones gives rise to a complex hydrogeologic setting of multiple water tables. It is likely that the seasonal onset of live flow in Medano Creek during spring runoff in most years completely saturates the eolian aquifer. Simulation of the groundwater/surface water interactions in the Medano Creek area will require additional data collection and construction of a multi-layer saturated/unsaturated flow model to simulate adequately the rapid temporal and spatial changes observed in the hydrologic system.