THE ROLE OF STRUCTURE AND STRATIGRAPHY ON GROUNDWATER/SURFACE-WATER INTERACTIONS IN A GAINING REACH OF THE EL RITO WATERSHED

Understanding the interactions between groundwater and surface water is critical to the future sustainability of communities in semi-arid watersheds. Streamflow is the primary source of water for acequias and irrigation in many semi-arid watersheds and sustained perennial streamflow is thought to depend on greater fractions of deep groundwater following the snowmelt pulse. The persistent perception is that deep groundwater is not a significant component of streamflow generation despite recent observations in the Saguache Creek and Rio Hondo watersheds refuting this perception. Recent research indicates that groundwater/surface water interactions are very complex in the El Rito watershed, a mountainous, sedimentary watershed in northern New Mexico. The El Rito watershed can be broken into four distinct hydrogeological zones: 1) perennial streamflow in the headwaters maintained by springs and groundwater discharge, 2) losing conditions downstream of the headwaters, 3) a small, persistent 500 m gaining stretch in the mid-reach, and 4) losing conditions from the mid-reaches to the outlet. In this poster, we investigate the processes controlling zone 3. We hypothesize that extensional faulting associated with the Rio Grande Rift combined with the westerly dip of stratigraphic units are responsible for the creation of the small gaining reach. We tested this hypothesis using high-resolution stream gauging, radon measurements in streams and springs, electrical resistivity surveys, geologic mapping, and temperature logging of streamflow. Our data show that the upwelling occurs near a small east-west trending fault characterized by a sharp contrast in water table depth (higher water tables downstream of the fault), persistent and spatially confined cold spots in streamflow associated with the discharge of cooler groundwater, and small spikes in radon and decreases in tritium concentrations in the cold spots. These data provide support for the hypothesis and indicate that structural geologic and stratigraphic features can have profound effects on groundwater/surface water interactions. Without the fault, there would be no gaining reach and without the gaining reach, the acequia may not contain water. This research also highlights the need to properly characterize the 3D geologic structure of watersheds.