Evaluating the Water Quality Benefits of Reconstructed Multi-purpose Oxbows
Oxbows are natural floodplain features formed when a river cuts off a meander loop as it migrates within its floodplain. Natural oxbows are among the most biologically diverse aquatic systems in the world, but over time, sediment and organic material often accumulate to fill the oxbow. Removing this fill material to restore habitat is considered oxbow reconstruction. To date, restoration of oxbows in agricultural areas has primarily focused on creating habitat for the federally endangered Topeka Shiner and waterfowl. More recently, new research by Schilling et al. (2017, 2018) is showing that restored oxbows can be used for nutrient reduction, with N load reductions ranging from 35 up to 76 percent. Oxbows reconstructed to receive inputs from tile drainage received considerably more nitrate than oxbows fed simply by groundwater discharge or from an occasional flood pulse. Overall, research on newly reconstructed oxbows suggests that these systems can reduce nitrate concentrations and loads delivered via tile drainage, groundwater seepage and overbank flooding. Although results are promising, at this point, monitoring has only been conducted at a few sites.
Researchers propose the term “multi- purpose oxbow” as the descriptor for the practice to indicate that the oxbow restorations be designed to meet water quality and ecosystem habitat goals. Maximum nutrient reduction effectiveness of these restored oxbows requires intercepting tile drainage before it reaches natural waterways, and future oxbows restored under the INRC program will be fed by tiles. However, there are still important questions regarding how the introduction of tile drainage may affect the conservation benefits of restored oxbows.
Researchers and project collaborators will monitor four reconstructed oxbow sites (two tile-fed, two non-tile) and assess their functions as a new conservation practice for nitrate reduction while also serving habitat goals. The primary objectives are to:
- characterize the hydrogeology of four new reconstructed oxbows that capture the range of input water and nutrient sources, including groundwater seepage, overbank flooding and tile drainage.
- evaluate the nutrient reduction capacity (including both nitrate and orthophophorus) of the four oxbows and quantify their capacity in terms of mass load reduction and retention.
- quantify the relationships of biotic responses to hydrological and nutrient dynamics.
- develop design criteria to implement oxbow reconstruction to maximize nutrient reductions while simultaneously enhancing ecosystem services.
Funding provided to the Iowa Soybean Association and The Nature Conservancy has been used to reconstruct new oxbow sites in the Des Moines Lobe region of north-central Iowa. This project will focus on hydrology and water quality assessment at four reconstructions: two sites fed by groundwater discharge and flood pulses, and two sites fed primarily by tile drainage. At all oxbow sites, a geophysical survey of the oxbow areas will characterize subsurface conditions. We will install a minimum of four water table monitoring wells at each site to characterize groundwater flow directions and rates, water levels (including in the oxbow and in the nearby river) and water quality in the floodplain. Precipitation will be monitored at all sites. Discharge from tiles into the oxbow will be measured, as will discharge from the oxbow. Dimensions (area, depth, volume) of the oxbows (as-built) will be determined. A depth-volume relationship will be developed to calculate the volume of water in the oxbow for a specified stage. Flood inundation and connection with surface water will be quantified between the stream and oxbow. Combined with turbidity and total suspended solid (TSS) measurements of stream surface water, the potential for oxbow sedimentation from flooding will be assessed and an estimate of oxbow life expectancy developed. Twice-monthly grab samples of groundwater, tile water and surface water will be collected using standard methods at all sites and analyzed at the Iowa Soybean Association certified testing laboratory in Ankeny. A mass balance will be developed for the reconstructed oxbow to quantify input and outputs of water and nutrients as well as concentrations.
Results of this project will be transferred in several ways to different audiences, including communicating directly with private landowners since private land is where most potential oxbow reconstruction sites exist. The Iowa Soybean Association and The Nature Conservancy will use articles in their newsletters. Communication with private landowners will include an extension publication and newsletter articles, and state state and federal level land management agencies ill be informed via annual and final reports, presentations and scientific publications.
Note: Project reports published on the INRC website are often revised from researchers' original reports to increase consistency.
Key Research Questions
The overall objective of the research was to monitor four reconstructed oxbow sites (two tile-fed, two non-tile) over a two-year period in north-central Iowa and assess their function as a BMP for nutrient reduction. The specific objectives of the research were to:
1) characterize the hydrogeology and water budget of four new reconstructed oxbows that capture the range of input water and nutrient sources, including groundwater seepage, overbank flooding and tile drainage;
and 2) evaluate the nutrient reduction capacity (NO3-N and OP) of the individual oxbows and quantify their capacity for nutrient mass load reduction.
Hydrology and water quality monitoring of tiles, shallow groundwater, oxbow and receiving streams were conducted at four reconstructed oxbow sites (two tile-fed, two non-tile) over a two-year period in north-central Iowa. An electromagnetic terrain conductivity (EM) survey of the oxbow areas was conducted at all sites to evaluate local patterns in subsurface geology using a Geonics EM-31 unit. Monitoring wells, well point piezometers, tile flow monitoring and nitratax sensors were used to evaluate water flux and NO3-N concentration patterns.
Study results indicated that the two tile-fed oxbows were dominated by tile drainage inputs compared to groundwater inflow and precipitation inputs. NO3-N concentrations were highest in the drainage tiles flowing into the tile-fed oxbows (mean 8-10 mg/l) and were much lower in floodplain groundwater at all sites (<1-2 mg/l). Thus, annual NO3-N loads into the tile-fed oxbows were substantially larger (4.4-154 kg) than input loads into the non-tiled oxbows (<1 kg).
For the two tile-fed oxbows, the two-year NO3-N retention efficiencies were very similar (0.76-0.77) On a monthly basis, greater retention efficiencies were measured in summer and fall. Low input NO3-N concentrations hampered quantification of retention at the non-tile sites but were estimated to be near 100%. Overall, dissolved reactive phosphorus concentrations concentrations and loads into the oxbows were too low to allow for meaningful estimates of DRP retention.
While not a part of this study, biologists at Iowa State University monitored paired tile-fed and non-tiled reconstructed oxbows to assess potential differences in fish populations between the two oxbow types. Leberg (2021) sampled 12 restored oxbows in 2019 and 2020, including six that received water from subsurface tile drainage systems (multipurpose oxbows) and six wetlands that had no direct connection to tile. The oxbows studied by Leberg included the four oxbows sampled in this project. While water temperature was somewhat lower in multipurpose oxbow sites, other measured habitat features were not significantly different between oxbow types. Most importantly, no significant compositional difference in fish assemblage was identified/measured between tile-fed and non-tiled oxbows, including no statistically significant differences in population abundances of Topeka Shiners. These study findings provided evidence that tile drainage has minimal impacts on oxbow ecology compared to other oxbows in central Iowa.
Adding results from the present study to previous work on tile-fed reconstructed oxbows (Schilling et al., 2019) indicates that NO3-N retention in “multipurpose oxbows” (i.e., those oxbows constructed for multiple ecosystem benefits including fish habitat) should now be considered 62± 19% if four additional site-years of data were included with previous assessments.
- 4 Presentations
Publications / Journal Articles
Pierce, S. 2021. Reconstructed multi-purpose oxbows improve water quality and provide ecosystem services. The IGS Geode, Activities of the Iowa Geological Survey, 2020-2021. Available on-line at https://issuu.com/uiowaengineering/docs/igs_geode_2021.
During this part of the project, bi-weekly water sampling of monitoring wells, oxbow surface and tributary surface water, and flowing tiles has commenced and analysis has been under way. Water level monitoring from installed transducers is also ongoing and as is collection of manual depth to water measurements. Real-time nitrate monitoring is underway as well at the two tile-fed sites.
Biweekly monitoring trips for water level measurements and sample collection ended in mid-October. Over the 2020 field season, IGS made 11 monitoring trips. Sample results processed by the Iowa Soybean Association have been received and are being analyzed. Water level data from June-October was collected in select wells at each site. Nitrate sensors were installed to monitor the two tile-fed oxbows, Sanders and McClellan, over the growing season. All transducers and monitoring equipment have been uninstalled or stabilized for the winter. In late October, level surveys were done at the oxbow sites to get elevation data for installed wells. An electromagnetic resistivity survey was conducted at the Sanders oxbow site, and surveys will be finished at the Hefty and McClellan oxbow sites in spring 2021.