Quantifying nutrient load reduction practices and export at field and landscape scales

FINAL REPORT 

This project included four sub-projects, and each had a key research question.

• Project 1: Installation and monitoring of a new saturated waterway conservation practice. Key Research Question: How should a new saturated waterway conservation practice be installed and monitored?
• Project 2: Characterization and monitoring of two new saturated buffer sites in eastern Iowa. Key Research Question: What type and rigor of pre-BMP monitoring data is needed to characterize saturated buffer sites?
• Project 3: Quantifying N and P loads at various spatial and temporal scales. Key Research Question: What are N and P loads rivers and speciation in Iowa's rivers and reservoirs?
• Project 4: Provide groundwater monitoring and geological services for Iowa State INRC projects and related nutrient-related studies. Key Research Question: How best can IGS assist with other researchers investigating Iowa's nutrient reduction issues?

Findings: 

Project 1: Installation and monitoring of a new saturated waterway conservation practice. Key Research Findings: Grass waterways are an effective conservation practice to reduce soil erosion but less is known about their subsurface hydrogeology. Recent research completed by the project team has characterized regional patterns in grassed waterway hydrogeology and shown the potential for NO3-N load reductions in waterways. We used field investigation and numerical modeling to evaluate subsurface hydrogeological conditions beneath grassed waterways and identified waterways that could be best utilized in a new conservation practice aimed at reducing tile NO3-N export from cropped fields. The method for NO3-N removal from the grassed waterways is similar to other practices like saturated buffers that utilize soil microbial respiration in anaerobic soils to perform denitrification. Saturated buffers work by using nutrient-rich upland tile flow to saturate organic-rich soils in riparian zones thereby forcing microbes to respire in anaerobic conditions. Grassed waterways typically consist of organic-rich soils and are relatively low on the landscape, although not in riparian zones. These waterways are within cropped fields. Since they are in fields and still must maintain their primary purpose, which is to convey surface water from the landscape without impacting implement traffic, saturating the entire soil profile like the method used for a saturated buffer would not work. Therefore, this project worked to design a practice that could saturate the deep soil profile of the grassed waterway soils for the purposes of denitrification. The new grassed waterway design consisted of two Agri Drain control boxes connected by a perforated tile running down the center of the waterway at approximately 4 ft depth. A two-chamber box was installed at the base of the waterway and provided the initial water table rise for the practice (approximately 2 ft but can be adjusted using weirs). A three-chamber box was installed at the head of the waterway and used to divert tile water into the practice and bypass excess water. It was further used to calculate total incoming tile flow rates and NO3-N concentrations. In a real-world application, this second box could be eliminated. To maintain the initial water table rise that was established in the two-chamber box while moving up an approximately 1% slope, the project team utilized Agri Drain Watergates, which use a system of floats to provide 1 ft of additional water table rise. In all, the new practice used 4 Watergates and constructed a practice that saturated approximately 2 ft of the soi profile above the waterway tile line over 6 ft of elevation rise in a cropped field. We installed flow and nitrate sensors in both control boxes as well as installed multiple monitoring wells to determine the effectiveness of the practice for nitrate removal. We have completed the first year of monitoring (2024) at the site and have found that the practice was able to take 60% of the total tile flow for the waterway and that the 40% of flow that bypassed the practice occurred over only 9 days during major rain events. In 2024, 286 kg of NO3-N flowed into the waterway and 171 kg was treated. Based on monitoring well data, approximately 60% of the treated NO3-N was removed, presumably via denitrification. The project team is continuing real-time monitoring of the grassed waterway practice in 2025 and 2026.

Project 2: Characterization and monitoring of two new saturated buffer sites in eastern Iowa. Key Research Findings: Saturated buffers have become a common practice for NO3-N removal in Iowa and are supported by NRCS in design and cost share. NRCS has multiple requirements that regulate the potential locations for successful saturated buffer implementation. One such requirement is that the contributing tiles that feed the saturated buffers cannot contain surface tile inlets due to the high likelihood of sediment and trash plugging the saturated buffer. Recently, the project team has designed and implemented a new modified blind inlet practice that eliminates standing surface inlets and provides effective trapping of sediment and nutrients in-field. This project was designed to test the effectiveness of the modified blind inlets at preventing sediment build-up in a new saturated buffer installation. The first phase of this project involves installation of a new saturated buffer, which will be intentionally placed in a field with traditional standing inlets. The saturated buffer additionally needs a second control box at the end of the buffer to serve as a cleanout once plugging/sedimentation occurs. This phase of the project was delayed significantly by NRCS, but the site has been selected, and designs have been completed. Installation of the saturated buffer will occur in 2025. Once installation is complete, the buffer control box will be implemented with turbidity, flow, and nutrient sensors in order to assess sediment and nutrient loads into the buffer over the course of a full growing season. Finally, the standing tile inlets will be converted to modified blind inlets, and we will assess nutrient and sediment reduction based on the new practice implementation. An additional component of this second project was to perform pre-BMP monitoring at a saturated buffer site near West Branch. The site has a saturated buffer installed on one side of a creek and the landowners intend to install a second buffer on the other side of the creek. The project team would like to develop a paired study comparing effectiveness of the two buffers under different management conditions. Therefore, to assess conditions before the second buffer is installed, the project team installed multiple shallow monitoring wells on both sides of the creek and collected bi-weekly samples of the wells in 2024. The team now intends to develop a research project that will involve manipulating the control weirs on both sides of the creek to assess the effect of management decisions on practice effectiveness.

Project 3: Quantifying N and P loads at various spatial and temporal scales.Key Research Findings: In this project, we completed two major studies investigating nutrient transport patterns in Iowa. In this first study, we modeled stream loads for three forms of N (nitrate [NO3−], organic N [ON], and ammonia [NH3]) and two forms of P (orthophosphate [OP] and particulate P [PartP]) at 46 Iowa sites from 1998 to 2022 to determine these forms’ contributions to overall nutrient loading. Statewide totals were determined by aggregating loads from 16 sites near Iowa’s border, and regional patterns were explored by examining yields at 41 watersheds throughout the state. The results revealed that Iowa’s statewide total N yield was 22 kg ha−1 y−1, which consisted of 81% NO3−, 18% ON, and 1% NH3. Iowa’s total P yield was 1.5 kg ha−1 y−1—73% PartP and 27% OP. Most notably, yields in individual watersheds varied considerably. Watersheds in the hillslope-dominated plains of southern Iowa had the greatest ON and PartP yields, and ON was the dominant N form at five sites. Watersheds in the heavily tile-drained landscapes of north-central Iowa had the greatest NO3− and OP yields. Additionally, concentrations of particulate nutrients (PartP and ON) were strongly correlated at individual sites, suggesting these nutrients have similar sources and transport pathways. While high NO3− and OP yields tended to coincide, their concentrations were not related, and these dissolved nutrients are likely entering Iowa’s waters under different seasonal and hydrologic conditions. This study’s results may help inform remediation efforts by identifying which nutrient forms are most prevalent throughout Iowa. In the second study, we used riverine load estimation models to quantify the historical retention of sediment and nutrients in Iowa’s three large flood-mitigation reservoirs (Coralville, Red Rock, and Saylorville) and determine the impact of reservoir residence time on loss rates. Water quality data collected by the US Army Corps of Engineers was used to estimate inputs and outputs of total suspended solids (TSS), along with the same N and P forms. Historical records of incoming flow and water storage were used to calculate annual residence times. These residence times were largely consistent across the basins, ranging from roughly 6 to 100 days (mean of 19 days). Our analysis period spanned from 2001 to 2023. Over this timeframe, most TSS (~ 80%) entering the reservoirs was retained. This sedimentation corresponded to average volume losses in the reservoirs' normal storage pools of 0.37%–0.85%/year. About 40% of P and 12% of N were likewise retained—mainly due to decreases in particulate P and nitrate. Residence time appeared unrelated to removal rates of TSS and particulate nutrient forms but longer residence times coincided with increased nitrate loss. We also found that reservoir impact on statewide nutrient export was significant, with loads in Iowa's major rivers being reduced by 9.8% (for P) and 4.7% (for N) due to reservoir capture. These findings suggest that reservoir operators may be able to facilitate further nitrate removal by lengthening storage durations without incurring additional sedimentation or generating other nutrient forms. Ongoing work with the US Army Corps of Engineers is further exploring this topic in Red Rock and Saylorville Reservoirs.

Project 4: Provide groundwater monitoring and geological services for Iowa State INRC projects and related nutrient related studies. We completed assistance for three Iowa State University studies as part of this project.

• IGS installed four shallow monitoring wells at an ISU research farm as part of a pothole study led by Dr. Antonio Arenas. IGS facilitated the purchase, installation and data management for continuous water levels in these wells.
• IGS installed six shallow monitoring wells on the floodplain of the Iowa River near Belle Plaines for Dr. Billy Beck and his graduate student. The wells were installed in grass and forest cover at three different floodplain flood elevations as part of an investigation of floodplain easement restoration and recovery after the 1993 floods.
• IGS collaborated with Dr. Tom Isenhart and a graduate student to install monitoring wells in the Rottinghouse saturated waterway project in Blackhawk County.

Related accomplishments and activities 

4 presentations

Publications

• Anderson, E. S. and Schilling, K. E. 2025. Quantifying the impact of Iowa’s flood-mitigation reservoirs on sediment and nutrient loss. Journal of the American Water Resources Association. 61:e70035.
• Anderson, E. S., and Schilling, K. E. 2024. The speciation of Iowa’s nutrient loads and the implications for midwestern nutrient reduction strategies. Journal of Soil and Water Conservation, 79(5), 233-246.

 2 graduate students were employed to assist with this project.