Evaluating the Potential for Drainageways to Serve as Test Sites for Innovative Grass Waterway Designs
Grass waterways, a common conservation practice used to reduce gully erosion throughout much of Iowa, are typically located where surface runoff water is concentrated. They are often planted with sod-forming grasses to reduce runoff, sediment transport and gully formation by slowing water flow. As a surface erosion-prevention practice, the benefits of grass waterways are well understood. However, the subsurface characteristics and potential nutrient reduction benefits of these systems have not been well investigated. Schilling et al. (2013) noted that drainageways in the region are typically filled with organic-rich alluvium and post-settlement materials and have high water tables and anaerobic conditions that could serve as natural bioreactors for nitrogen (N) reduction. Most first-order drainageways are tile drained, a recommended grass waterway design practice as it prevents long-term buildup of excessive wetness, maintains the vegetative cover, prevents formation of gullies and facilitates accessibility of farm equipment into fields. However, tiling beneath drainageways results in groundwater nitrate-N concentrations bypassing denitrification hot spots, increasing nitrate-N export from the landscape.
Researchers will study the potential to alter drainageway tiles to reconnect upland groundwater with organic-rich drainageway deposits for N reduction, which could allow them to better function as “natural bioreactors” on the landscape. In many cases, drainage tiles are installed on the edges of and run parallel to grass waterways. At a concept scale, we believe that these tile drains could be reconfigured (similar to saturated buffer designs) to allow tile water to reconnect with the organic-rich waterway soils for N treatment. This project represents a first step toward developing a new grass waterway design by characterizing potential candidate sites for testing at the Kirkwood Community College farm in Cedar Rapids, Iowa, at the largest two-year agricultural department in the nation.
This project will:
- use soil mapping and LiDAR, along with the Agricultural Conservation Planning Framework (ACPF) tool to identify current and potential grass waterways located at the Kirkwood farm;
- characterize the alluvium contained within the grass waterways and collect soil samples to verify the characteristics of the deposits;
- install shallow wells to document water table depths and biogeochemical conditions in the alluvium;
- rank the suitability of the current and potential grass waterways to serve as a test site for evaluating new grass waterway designs; and
- develop the conceptual design for a new grass waterway practice that could be installed and tested at Kirkwood starting in 2021.
During this period, routine sampling of groundwater within the waterway wells continued. Coe College has continued to analyze water samples and deliver data promptly. Samples were also analyzed at the time of sampling for multiple water quality indicators. Pressure transducers that were installed previously were monitored and data was collected routinely. In addition, slug tests were performed on each well to estimate the hydraulic conductivity of the geological materials. Well surveys were conducted to inform researchers about relative elevation changes within each waterway. Well instrumentation was removed in late December and data is now being summarized.
In early March 2020, the Iowa Geological Survey installed 12 more shallow groundwater monitoring wells in grassed waterways at Kirkwood Community College, for a total of 18 wells. Four wells were instrumented with pressure transducers to monitor continuous water levels. At 12 locations, soil cores were collected during well installation. These cores have been described and are currently being analyzed for particle size, total carbon/nitrogen and other soil nutrients.
Due to the shutdown this spring, we were unable to collect the remaining six soil cores before crop planting. We plan to collect these cores and finalize the waterway characterization after harvest this fall. The Kirkwood Community College and Coe College teams have been very helpful to work with us during this strange time. We have been able to collect bi-weekly water samples from these wells starting in April and will continue bi-weekly sampling through at least October 2020. Even with the COVID 19 shutdown, we have successfully driven this project forward with minimal shortfalls.
During the first six months of this project, the team successfully identified three independent fields containing grassed and potentially grassed waterways. Waterways were selected for investigation and a detailed field investigation has commenced. Twelve monitoring wells were installed in two fields during the fall of 2019. With the wet fall and late harvest, the final well installation will occur in March.
A field conductivity survey was completed detailing the location of coarse sands and gravels and areas of finer materials within the waterway soils. Mapping potential best management practice locations using the ACPF toolbox was also completed for the entire Hoosier Creek watershed. This valuable data will help determine the total potential for waterway nutrient processing within the watershed.