Establishment and Monitoring of Saturated Buffers
Riparian buffers are a proven conservation technology for reducing the movement of nutrients into receiving waters. However, in artificially drained land, much of the nitrate-laden water leaching from row crop fields is routed directly through the buffers in drainage pipe. One promising approach to address this issue is to intercept the field tile outlet where it crosses a riparian buffer and divert a fraction of the flow as shallow groundwater within the buffer - termed a saturated buffer. The infiltrated water raises the water table within the buffer into organic rich soil layers and provides an opportunity for the nitrate contained in the field tile drainage water to be removed by denitrification before entering the adjacent stream.
To support adoption of saturated buffers as a nitrate management practice within tile-drained watersheds, this project will establish saturated buffers within watersheds targeted for practice implementation by the Iowa Water Quality Initiative; quantify nitrate loss within these buffers established under a range of conditions; and inform the development of criteria under which saturated buffers would be implemented as a conservation practice.
This project will support monitoring of an additional three saturated buffer sites per year as well as continued monitoring of existing sites. Monitoring equipment will include a 4-chamber control box to divert water from the tile outlet into the buffer, groundwater monitoring wells, pressure transducers for monitoring flow diverted into the buffer and water depths within wells, soil moisture probes, and more. Water samples will be collected routinely and nitrate removal estimated by the change in nitrate concentration as the shallow groundwater moves across the buffer and the measured flow into the buffer.
Project partners worked with the North Raccoon River Water Quality Initiative project to assess and design two additional saturated riparian buffers in Buena Vista County IA. Installation is expected in fall 2018. Monitoring of the 11 previously established saturated buffers was ongoing though the reporting period. Flow was continuous at nearly all sites throughout the reporting period.
The project documented that saturated riparian buffers are a promising practice for nitrate removal in tile-drained landscapes. They retain the multiple benefits of traditional riparian buffers, while filtering much of the nitrogen in the tile water. They even reduce nitrogen concentrations in the winter, when plants are not growing. Although saturated buffers are relatively simple, cheap, and quick to install, and may require little maintenance and no active management — all attributes that are attractive to farmers — they are not suitable for installation on all tile outlets. Rather, saturated buffers require specific soils and landscape characteristics to function properly, and thus, may be more limited in placement within a watershed than other field-edge practices.
A major focus of the project was to summarize the effectiveness of saturated riparian buffers (SRBs) in removing nitrate at six sites installed across Iowa over a total of 17 site-years. Water flow and nitrate in the tile outlets, diverted into the buffers, and nitrate concentration changes within the buffers, were monitored throughout the year at each site. Results showed that all the SRBs were effective in removing nitrate from the tile outlet, with the average annual nitrate load removal ranging from 13 to 179 kg N for drainage areas ranging from 3.4 to 40.5 ha. This is nitrate that would have otherwise discharged directly into the adjoining streams. The annual removal effectiveness, which is the total nitrate removed in the SRB divided by the total nitrate draining from the field, ranged from 8 to 84%. Assuming a 40-year life expectancy for the structure and a 4% discount rate, the team computed a mean equal annual cost for SRBs of $213.83. Given the average annual removal of 73 kg for all site-years, this cost was estimated to equate to $2.94 kg−1 N removed, which is very competitive with other field-edge practices such as denitrification bioreactors and constructed wetlands. Thus, SRBs continue to be a promising practice for nitrate removal in tile-drained landscapes.
One concern of enhancing denitrification on agricultural landscapes is the potential increase in nitrous oxide emissions from incomplete denitrification. To assess this risk, nitrous oxide emissions from SRBs were compared to traditional buffers and bordering crop fields at two sites, Bear Creek Site 1 and Iowa Site 1, in Central Iowa. Nitrous oxide emissions were measured directly from the soil surface and dissolved in shallow groundwater and estimated indirect emissions from downstream denitrification from 2015 through 2017. Nitrous oxide emissions from soil surfaces were greatest from fertilized corn. Saturated riparian buffers were only significantly greater than traditional buffers in one out of six site-years. Dissolved nitrous oxide in shallow groundwater seeping from SRBs was not significantly greater than dissolved nitrous oxide from the tile outlets. Indirect nitrous oxide emissions from rivers and estuaries were significantly reduced from nitrate removal in both SRBs. Overall, total nitrous oxide emissions from SRBs were similar to those from traditional buffers and less than those from fertilized corn–soybean agriculture. Replacing cultivated land in riparian areas with a SRB shows potential to subsequently remove nitrate from surface waters and reduce nitrous oxide emissions from agricultural landscapes.
Project partners worked with Iowa USDA-NRCS to revise the Saturated Buffer Conservation Practice Standard (Code 604 – June 2018).
Project outreach included:
- Direct and Indirect Nitrous Oxide Emissions from Saturated Riparian Buffers and Woodchip Bioreactors: Are We Trading a Water Quality Problem for and Air Quality Problem? 2017. M. P. Davis, T.M. Isenhart, D.B. Jaynes, T. Parkin, T.A. Groh, M.L. Soupir, and K. Hofmockel. ASA/CSSA/SSSA International Annual Meeting. Tampa, FL. October 2017.
- Saturated Riparian Buffer In Situ and Potential Denitrification. 2017. T.A. Groh., M.P. Davis, T.M. Isenhart, D.B. Jaynes, and T. Parkin. ASA/CSSA/SSSA International Annual Meeting. Tampa, FL. October 2017.
- December 14, 2017. Siting and Designing Saturated Buffers. D. Jaynes. Training session at the Drainage Water Quality Practices Design Workshop. ISU Extension, Ft. Dodge, IA.
- Saturated buffer strips show promise as viable solution for preventing nitrogen losses. Drainage Contractor. November 17, 2017.
Monitoring of the 11 established saturated buffers was ongoing though the reporting period. Due to extended periods of low rainfall, several sites in central Iowa had zero flow. Sites farther to the east continued to flow throughout the reporting period.
Project partners worked with ISU Research Farms to complete the establishment of a saturated buffer at the ISU Uthe Farm. This is the first site established that includes multiple tiles connected to the same distribution tile. Monitoring of the 10 previously established saturated buffers is ongoing. At each location, subsurface drainage flow is intercepted and infiltrated as shallow groundwater at the top of the buffers. Flow and nitrate concentrations in the subsurface drainage is measured continuously. The fate of the nitrate in the buffers is assessed by sampling groundwater within the buffers from multiple transects of wells spanning the width of the buffers.
Project partners worked with ISU Research Farms to complete the establishment of a saturated buffer at the ISU Uthe Farm. This is the first site established as part of this project that includes multiple tiles connected to the same distribution tile. Monitoring of the 10 previously established saturated buffers was ongoing through this reporting period. Flow continued at several sites throughout the winter and resumed in all sites by the end of this period.
Installation of a saturated buffer was completed in fall 2016 on the Lehman farm as part of the Four Mile Creek Watershed Project in Polk County. Instrumentation installed includes monitoring wells to assess nitrate in shallow groundwater down-gradient from the distribution tile; pressure transducers to measure water level within the tile distribution box; and all data logging and telemetry equipment. Project partners also worked with ISU Research Farms on the design of a saturated buffer for the Uthe Farm. Monitoring of the nine previously established saturated buffers was ongoing through this reporting period.