Soil phosphorus in farmed potholes: assessing concentrations and testing export mitigation with steel media

Authors: BUSEMAN, ADAM - Iowa State University; Penn, Chad; CHARBONNET, JOSEPH - Iowa State University; KALEITA, AMY - Iowa State University; LONG, LEIGH ANN - Iowa State University; SOUPIR, MICHELLE - Iowa State University

Submitted to: Journal of the ASABE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/12/2023
Publication Date: 11/12/2023
Citation: Buseman, A., Penn, C.J., Charbonnet, J.A., Kaleita, A.L., Long, L.M., Soupir, M.L. 2023. Soil phosphorus in farmed potholes: assessing concentrations and testing export mitigation with steel media. Journal of the ASABE. https://doi.org/10.13031/ja.15759.
DOI: https://doi.org/10.13031/ja.15759

Interpretive Summary: Excess dissolved phosphorus (P) transported to surface waters through subsurface drainage has been identified as one of the main causes of harmful algal blooms and water quality degradation. Poorly drained depressions known as "prairie potholes" are often artificially drained through the use of a tile riser, providing a short-circuit for surface runoff to move into drainage ditches with no soil filtration. Replacement of tile risers with a blind-inlet allows for filtration of sediment, representing an improvement in water quality. However, traditional blind inlets will not remove dissolved P. The goal of this study was to construct a modified blind-inlet that utilizes a P filter media, and determine if it is capable of treating dissolved P. A modified blind-inlet was constructed on a prairie pothole located in Central IA, using a mixture of gravel and metal shavings, which replaced a tile riser inlet. Metal (steel) shavings have been previously shown to absorb large amounts of P from water. Through use of automated flow monitoring, it was determined that the new modified blind-inlet removed over 80% of dissolved P from runoff water that would have otherwise been allowed to leave the field unfiltered. This small modification to blind inlets represents a low-cost and practical method for dramatically improving water quality.

Technical Abstract: The Prairie Pothole Region, which extends from north-central Iowa, northward into Canada and west to Montana is typified by closed depressions that formed over 10,000 years ago during the most recent glacial retreat. Today, many of these potholes contain artificial drainage to lower the water table, allowing for agricultural production of the land. Even when drained, farmed potholes can become inundated with phosphorus (P) enriched surface waters after large rain events. The goal of this study was to characterize the spatial distribution of soil P in farmed potholes and evaluate a steel media-modified blind inlet as a tool to reduce P export from farmed potholes. Results from the soil sampling indicated that Bray-1 soil test P concentrations are nine times higher near the bottom of the pothole compared to locations outside of the farmed pothole. A pea gravel and steel shavings mixture was evaluated for P sorption in a flow-through column experiment and used as the media within the modified blind inlet that replaced a pothole’s surface intake. The modified blind inlet retained an average of 83% of the influent dissolved reactive P and 62% of the influent total P concentration. The results from this study demonstrate that farmed potholes contain a significant amount of legacy soil P, and that blind inlets containing steel shavings can replace surface intakes and reduce P export from farmed potholes. This research could inform future design and implementation of these P-removal structures.