Developing design criteria to test a new saturated waterway conservation practice
Issue
Grassed waterways planted in upland drainageways are a common conservation practice to slow rainfall runoff and reduce soil erosion from agricultural fields. Less recognized is that the subsurface materials in these lowland areas are often filled with organic-rich deposits that could serve as natural bioreactors for reducing nitrate concentration in groundwater and tile drainage. Previous work funded by the INRC documented that grass waterway locations at an eastern Iowa site contained soil and biogeochemical conditions conducive for denitrification to occur, and nitrate concentrations in waterway groundwater were 70% lower than observed in groundwater beneath cropped fields.
Objective
In this new study, subsurface soil and groundwater conditions will be characterized across a wider range of waterway sites in Iowa and a hydrologic model will be developed to test the potential for development of a new conservation practice that utilizes the waterway deposits for nutrient reduction. The proposed practice would alter drainageway tiles to reconnect upland groundwater to the organic-rich drainageway deposits for nitrate reduction as saturated buffers do in riparian zones.
Approach
Up to 30 different waterway sites will be investigated by Schilling and his team at the University of Iowa and information from the field investigation will be used by Arenas at Iowa State University to model the performance of a new saturated waterway practice under a wide range of geologic and hydrologic conditions. At the conclusion of the project, the design for a new grass waterway practice will be developed that could be installed and tested at an appropriate site.
Project Updates
Note: Project reports published on the INRC website are often revised from researchers' original reports to increase consistency.
September 2025
FINAL REPORT
The main questions this project hoped to answer:
Although geomorphic and geologic mapping has shown that drainageways in loess-mantled areas of the Midwest share a similar geologic history and sedimentology across the region, there are questions regarding the uniformity of hydrogeologic properties across different glaciated landform regions and topographies that could influence the capacity of the sediments to process subsurface water from groundwater and tiles. For example, if sediments are to be exploited for subsurface NO3-N reductions, the rate of groundwater flow in the waterways must be slow enough for denitrification reactions to occur, but not be so slow that water ponds in the waterways or only a small N load is reduced. Hence, the purpose of our project was twofold: 1) evaluate a broad population of existing grass waterways found in Iowa to assess the range of hydrogeologic conditions typically found in these settings; and 2) construct a physical model to quantify the relation between surface and subsurface flows under a range of drainageway topographic and hydrogeologic conditions. The overall goal of this effort was to identify appropriate catchment and hydrogeologic conditions where subsurface drainage beneath waterways could best be modified to reduce tile NO3-N loads.
Findings
Researchers utilized a combined field investigation and numerical modeling approach to evaluate subsurface hydrogeological conditions beneath grass waterways within major landform regions across Iowa. Field monitoring at 22 waterway sites across Iowa revealed a consistent stratigraphy comprised 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 but it was higher in NE Iowa where sand contents were higher.
Groundwater quality was largely anaerobic in the lowland waterway areas, where high water tables contributed to low DO and NO3-N concentrations in shallow groundwater. Numerical modeling parameterized using the field data quantified landscape controls on subsurface water flux in grass waterway catchments. On an annual basis, more water was found to be exported from grass waterways in surface runoff compared to subsurface tile and groundwater flow.
This result is consistent with the primary purpose of the grass waterways to slow surface water runoff and reduce soil erosion. 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 landform region MLRA 104 (NE Iowa) may be the best region to test a new conservation practice that aims to capture and remediate tile NO3-N export from grass waterways.
Related accomplishments and activities
4 presentations and 1 field day was held near Reinbeck, Iowa, in September 2025, associated with the new grass waterway practice installed near Reinbeck, Iowa.
1 publication: Schilling, K.E., Streeter, M.T., Gibertini-Diaz, V., Betret, E. and Arenas‐Amado, A. 2024. Hydrogeology and subsurface water flow beneath grass waterways: implications for exploiting waterways for nitrate reductions. Agricultural Water Management 298:108847.
Results of the INRC project were used to apply for additional funding from INRC to construct a new grass waterway conservation practice that features subsurface processing of tile nitrate.
1 graduate student was employed to assist with this project.
June 2023
Analyzing soil samples collected at 22 waterway locations for particle size and TN and TC. Developing Hydrogeosphere model to test design criteria for waterways (Arenas lead).
Other Activities
Presentation on project at SWCS meeting in Des Moines on August 8, 2023 (abstract at link).
swcs_abstract_developing_design_criteria_to_test_a_new_saturated_waterway_conservation_practice.docx
June 2022
During the initial project period, a project planning meeting was held where it was decided to focus study sites on Iowa State University research farms. During the fall of 2021, researchers Schilling and Streeter visited multiple potential sites. In all, eight sites were selected on ISU research farms across Iowa. Three non-ISU sites were also selected. In early April of 2022, the project team installed 22 shallow monitoring wells across the set of selected sites. At approximately 50% of the locations (where truck access was possible), continuous soil cores were collected using a Giddings soil sampler. Soil samples were also collected from each representative zone for lab analyses. Soil samples were analyzed for soil particle size (texture) and Total Nitrogen and Carbon. Following well installation, all wells were purged by overpumping with a Waterra and sampled for groundwater quality parameters.
December 2021
During the initial project period, a project planning meeting occurred. It was decided to target Iowa State University research farms for project study sites since there is a strong working relationship between ISU and the Iowa Geological Survey and ISU farms are spatially dispersed across Iowa. During fall 2021, researchers visited multiple potential sites. In all, eight sites were selected on ISU research farms across Iowa. Two non-ISU sites were also selected.
In the spring, shallow monitoring wells will be installed, and waterway stratigraphy will be characterized at each of the selected study sites.