Extending saturated buffers to additional landscape positions
Issue
Nitrate loss from artificial subsurface drainage tiles underlying agricultural fields is a major source of reactive nitrogen, especially nitrate (N03), in surface waters. A novel approach for reducing NO3 loss is to intercept a field tile where it crosses a riparian buffer and divert a fraction of the flow as shallow groundwater within the buffer. This practice is called a saturated buffer, and comprehensive monitoring at a network of sites has demonstrated its utility as an edge-of-field nitrate reduction practice. To date, all saturated buffer establishment has been within the riparian zone, either within existing buffers or in conjunction with filter strip planting. However, saturated buffer establishment may be able to be extended to other landscape positions with sufficient soil organic carbon where it would be possible to intercept a field tile and divert a fraction of the flow as shallow groundwater. Two potential locations would be in combination with grass waterways and prairie strips. We predict that soils within grass waterways and prairie strips contain sufficient organic carbon to facilitate nitrate loss and that they could be modified to act as saturated buffers for the removal of nitrate from tile drainage.
Objective
This project seeks to extend the utility of saturated buffers through establishment in additional landscape positions, in combination with existing grass waterways and prairie strips. If successful, this would dramatically increase possible sites for saturated buffer establishment and increase the potential of the practice for nitrate removal. Additional objectives will be to translate this information into site evaluation and design criteria to optimize the public investment in saturated buffers and provide feedback to NRCS for modifications to the federal practice standard for saturated buffers (CPS #604) where needed.
Approach
To achieve the objectives, the team will monitor NO3 loss and groundwater flow within a grass waterway and a prairie strip modified to function as saturated buffers. Locations planned are in Blackhawk County and Grundy County, Iowa. The team plans to work on private land with interested, innovative landowners and will also coordinate with another INRC-funded project, Evaluating the Potential for Drainageways to Serve as Test Sites for Innovative Grass Waterway Designs to assess soil hydraulic properties within the waterway.
Project Updates
Note: Project reports published on the INRC website are often revised from researchers' original reports to increase consistency.
July 2025
This project explores the viability of using a grassed waterway as an in-field practice for nitrate reduction in Northeast Iowa. Traditionally, saturating soils to improve nitrate reduction has been an edge-of-field practice. This project aims to test whether similar water quality benefits can be achieved by diverting tile water into the subsurface of a vegetated grassed waterway, an underutilized practice in Iowa’s nutrient reduction strategy. This work supports the broader goal of identifying nitrate reduction strategies for upland landscapes where stream-adjacent buffers are not feasible and the possibility of stacking practices where applicable.
The field site is located in Black Hawk County, Iowa, on a 130-ha field with poorly drained Clyde silty clay loam and an existing 21-m-wide brome grass waterway. A segment of the waterway was retrofitted in Fall 2023 with: three Agri Drain control structures; 275-m perforated distribution line parallel to the waterway; 10 groundwater monitoring wells in five transects, including pressure transducers in four wells; and tile flow monitoring via radar sensors in the upstream and downstream control structures.
Flow monitoring began May 2023, but no tile flow was diverted in 2023 due to drought. Tile flow resumed in January 2024 and continued at more typical levels through June 2025. Water samples were collected weekly from control structures and wells for nitrate analysis during the flowing season and bi-weekly or monthly during the fall and winter months with little or no flow. From January 2024 through June 2025, 106,005 m³ of tile drainage water was delivered to the saturated grassed waterway system, with 19,687 m³ (18.6%) diverted into the buffer’s subsurface soils. The practice was active for 103 flow days across two seasons. Nitrate-N concentrations in incoming tile flow averaged around 13.5 mg/L, resulting in a combined load of 2,877 kg NO₃⁻N over the monitoring period. Of this, 451 kg was diverted into the waterway, with an estimated 440 kg removed via subsurface processes, resulting in a 96% removal efficiency of the diverted nitrate and a 16% total system load reduction.
Efficiency metrics indicate consistent performance between years, with nitrate removal rates of 13 g/m/day in 2024 and 20 g/m/day in early 2025. When scaled to the treatment area, the system removed 4.0 kg NO₃⁻N/ha in 2024 and 3.9 kg/ha in early 2025. On a per-unit basis, the system removed 0.81 kg of nitrate per linear meter of distribution pipe in 2024 and 0.79 kg/m in 2025. These early results suggest that the saturated grassed waterway is a promising upland nitrate reduction strategy under suitable hydrologic conditions.
Related activities and accomplishments
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