UNDERSTANDING THE ROLE OF TILL FRACTURES IN WATER AND NUTRIENT FLUX AT THE WATERSHED SCALE

Despite evidence that fractures control solute transport in shallow till deposits, hydrologic models in Midwest glacial terrain have generally neglected this characteristic in understanding water and nutrient flux in watersheds. The reasons for this neglect are not obvious, but we believe that it is due to the lack of a standardized method and a suitable model for simulating fractures at that scale. In this study, we hypothesize that till fractures make a significance difference in water and nutrient flux, specifically decreasing transport times to streams and tile drains in these landscapes. We envision three steps to test this hypothesis. First, we will choose a representative watershed in central Iowa within the Des Moines Lobe, in which there are multiple stream gauges, extensive water quality data, and where there is an opportunity to take multiple till cores. We will then quantify fracture density, orientation, and aperture from those cores at multiple depths using x-ray computed tomography (CT) images. Column tracer experiments will be performed to corroborate transport parameters from earlier studies on fracture transport in the till. Second, we will statistically upscale the fractures to the watershed scale, making decisions about what fracture sets are most important at the larger scale. Third, we will use a fully-coupled, surface-subsurface model, HydroGeoSphere, to simulate water and nutrient flux in the watershed, using hydraulic heads, stream gauge data, and water-quality data for calibration. This model has the ability to simulate fracture flow, streams and drainage ditches, and even tile drains simultaneously, which will show how preferential flow through the fractures combines with other elements of the hydrologic system to influence water and nutrient flux. By utilizing a fully-coupled modeling approach, we hope to show that real-time feedback between surficial and subsurface processes provides a more realistic representation of water and nutrient flux in watershed-scale investigations.