Evaluating Phosphorus Retention in Saturated Riparian Buffers
- Wenjuan Huang
- Tom Isenhart
- Gabriel Johnson
- Bill Crumpton
Issue
A saturated riparian buffer (SRB) is an established edge-of-field conservation practice to reduce nutrient transport from tile-drained agricultural fields to surface waters. In an SRB, a field drainage tile is intercepted, and a fraction of the flow is diverted as shallow groundwater through a riparian buffer. Although SRBs have been shown to be effective for nitrate-nitrogen (N) removal in tile-drained landscapes, their impacts on phosphorus (P) retention remain unclear, hindering inference of the broader benefits of SRBs beyond N reduction.
Objective
The overarching goal of this project is to provide a more comprehensive understanding of the performance of SRBs on P retention beyond their effectiveness in N removal. Specific objectives of this project are to:
- accurately quantify P loads to SRBs;
- evaluate P retention within SRBs; and
- investigate relationships between P retention and N removal in SRBs.
Approach
This research will leverage three already-established SRBs in central Iowa: SH-1 in Boone County in the headwaters of the Big Creek watershed, BCR-2 in Hamilton County on Bear Creek, and LT-5 in Polk County on Alleman Creek. We will conduct high-frequency measurements of drainage flow and the concentrations of total N, nitrate, total P and reactive P in water samples collected from water control structures and monitoring wells at the three SRBs. We will evaluate the co-benefits or trade-offs of SRBs in water pollutant reduction from tile drainage.
Project Update
December 2025
The overarching goal of this project is to provide a more comprehensive understanding of the performance of saturated riparian buffers (SRBs) on phosphorus (P) retention beyond their effectiveness in nitrogen (N) removal. Specific objectives of this project are to:
- accurately quantify P loads to SRBs;
- evaluate P retention within SRBs; and
- investigate relationships between P retention and N removal in SRBs.
To achieve these objectives, we conduct high-frequency measurements of flow and P concentrations at the water control structure at three long-term monitored SRBs, including SH-1 in Boone County in the headwaters of Big Creek watershed; BCR-2 in Hamilton County on Bear Creek; and LT-5 in Polk County on Alleman Creek. We have continuously measured the total and diverted flow of drainage water in the control structure at the three SRBs in 2025. We have collected daily water samples from the control structure using automated samplers to measure total N, nitrate, total P and total reactive P. We have manually collected groundwater from multiple wells within transects spanning the width of the buffer to measure total N, nitrate, total P and total reactive P. We have also measured redox-sensitive Fe pools (reduced Fe(II) and oxidized Fe(III)) in water samples from the monitoring wells as indicators of redox conditions, which can help provide some mechanistic insight into relationships between N removal and P retention.
As planned, we will continue to conduct high-frequency measurements of flow and N and P concentrations during the ice-free sampling season over the course of this project. We will compile and analyze the data collected over the past year. We plan to submit an abstract to the Iowa Water Conference to present results from this project and to prepare a manuscript based on the findings.
Related Activities and Accomplishments
Building on this project, we have trained three undergraduate students.
2 presentations: Master’s student Mia Waid presented a poster at the American Geophysical Union (AGU) conference in December 2025, and the project director was invited to give a seminar talk in the Department of Civil, Construction and Environmental Engineering at Iowa State on Dec. 2, 2025.