Soil Phosphorus in Farmed Potholes: Assessing Concentrations and Testing Export Mitigation with Steel Media

Abstract

Highlights Phosphorus concentrations were higher near the bottom of farmed potholes compared to soils outside of the potholes. Benchtop flow-through experiments demonstrated that steel shavings effectively adsorbed dissolved reactive phosphorus. A blind inlet amended with steel shavings reduced phosphorus export from a surface-drained pothole. Abstract. The Prairie Pothole Region, which extends from north-central Iowa northward into Canada and westward 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. Steel shavings were evaluated for P sorption in a laboratory flow-through cell evaluation where they removed between 60 and 70% (discrete P), even when cumulative P loading was over 6000 mg kg-1. In a field investigation, a modified blind inlet containing a mixture of steel shavings and pea gravel replaced a Hickenbottom surface intake. The modified blind inlet retained an average of 83% of the influent dissolved reactive P and 62% of the influent total P based on five paired samples. 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. Keywords: Drainage, Farmed wetland, French drain, Iron filings, Iron shavings, Iron tailings, Prairie pothole, Surface intake, Tile.