STRUCTURAL CONTROLS TO SUCCESSFULLY MODEL GROUNDWATER FLOW WITHIN THE MANTLED KARST OF THE SAVOY EXPERIMENTAL WATERSHED, NORTHWEST ARKANSAS

The structural geology of northwest Arkansas was shaped by the uplift of the Ozark Dome and subsequent formation of the Ouachita Mountains. Ordovician through Pennsylvanian marine sediments deposited on the flanks of the Ozark Dome were fractured and faulted as a result of the Ouachita Orogeny forming a primary series of southwest to northeast trending faults and a secondary orthogonal set of fractures trending southeast to northwest. The Savoy Experimental Watershed (SEW) is bounded by a set of these faults to the northwest and southeast and by a set of the fractures to the northeast and the southwest. A third fracture set breaks the area into smaller structural blocks within this larger bounding area. Groundwater flow in the mantled karst formed in the Mississippian Boone/St. Joe limestone aquifer is controlled by the topography, lithology and structural geology of the area.

Identification of fractures, sinkholes, sinking streams, caves, springs, seeps and other karst features within SEW was completed using field reconnaissance and inspection of aerial photography, satellite imagery, and topographic maps. Digital data sets for elevation, landuse, location of karst features, and orthophotoquads were compiled in ArcView GIS. These digital data provided the basis for defining physical controls on groundwater flow which in turn were used for model discretization using Visual Modflow.

The model was initially calibrated to low-flow discharge data for two springs draining a portion of SEW. This steady-state calibration was followed by calibration to a transient condition resulting from a significant storm pulse. Model verification was completed for subsequent low-flow periods and storm induced transient events. The calibrated Visual Modflow model was used in conjunction with MT3D to reproduce breakthrough curves for low-flow tracer data for the conservative tracers Rhodamine WT dye and chloride. Using various boundary conditions within the model, flow and conservative contaminant transport were represented showing that the majority of the flow travels within fractures. Given sufficient control of the structural parameters, mantled karst aquifers can be successfully modeled.