Hydrogeology and subsurface water flow beneath grass waterways: Implications for exploiting waterways for nitrate reductions

Although grass waterways are an effective conservation practice to reduce soil erosion, less is known about their subsurface hydrogeology. New research is highlighting the potential for NO3-N load reductions in waterways, but studies have not been done to characterize regional patterns. In this study, we used field investigation and numerical modeling to evaluate subsurface hydrogeological conditions beneath grass waterways found in major landform regions of Iowa. Our goal was to identify waterways that could be best utilized in a new conservation practice aimed at reducing tile NO3-N export from cropped fields. Waterway stratigraphy consisted of a layer of fine-textured, nutrient-rich sediment overlying glacial or post-glacial parent material. The mean hydraulic conductivity (K) of the alluvial sediments reflected a dominantly silty matrix and it was highest in northeast Iowa where sand contents were higher. Groundwater was largely anaerobic in lowland waterway areas, where high water tables contributed to low dissolved oxygen and NO3-N concentrations in shallow groundwater. Numerical modeling parameterized using the field data showed that in terms of annual water balance, more water is exported from grass waterways in surface runoff compared to subsurface tile and groundwater flow. In terms of subsurface flow, tile water yields were higher in smaller and steeper catchments and when the K of the waterway alluvium was higher. Based on regional patterns of sedimentology and landscape topography, smaller and steeper catchments within a landform region found in NE Iowa may be most appropriate to test a new conservation practice aimed to capture and remediate tile NO3-N export from grass waterways.