Reducing Phosphorus Export through Farmed Pothole Surface Inlets with P Filters and Blind Inlets

Aug 2020


Much of Iowa contains landscape features known as prairie potholes – landscape depressions with little natural surface drainage and soils that limit infiltration. Many of these features that occur in farmed areas have artificial subsurface drainage installed, with a surface inlet to drain away standing water. Past observations into the water quality of the standing water in these features indicate that phosphorus levels in this ponded water can be high, making the drainage system a source of phosphorus export from the field.


This project will study approaches for intercepting and treating phosphorus before it moves out of the field through the drainage network.


Specifically, investigators will research the potential for specially designed filter materials placed within the surface inlet to capture phosphorus while transmitting the drainage water.

Award Number: 

Project Updates

Note: Project reports published on the INRC website are often revised from researchers' original reports to increase consistency.

December 2022

Monitoring continues of the blind and standard inlets for a) ponded water level and inlet flow, b) dissolved phosphorus in the standing water and in the drained water. While the inlet material reduced the concentration of dissolved phosphorus as water moved through the blind inlet, there was not as much flow through the material as there should have been, and the duration of inundation was too long. There are several potential explanations which will be investigated in the spring and remediated so that the 2023 season will have more typical flow performance through the inlet and thus more believable P reduction data.

To complement the field work and because the very dry weather meant fewer flow events and fewer samples than expected, the team completed laboratory tests on the P sorbing properties of the gravel and the amended gravel. Tests showed that the gravel by itself had substantial P sorbing capacity. This result is likely very specific to the geochemistry of the particular gravel procured for the blind inlet. It does make it slightly more difficult to assess what the amendments would do for a different type of rock gravel. In 2023 more investigation of this will be conducted.

The graduate student on this project defended his master’s thesis and graduated in December, and we have recruited a new MS student to start in January to continue this work.

Activities included two presentations.

June 2022

Sample collection at the two pothole research sites commenced for the 2022 season on March 19th. To date, precipitation events have caused four inundation events in the control pothole (Bunny) and three inundation events in the phosphorus (P) removal blind inlet pothole (Walnut). During inundation events, water samples are collected daily from five locations, including the 1) surface water of Bunny, 2) surface water of Walnut, 3) subsurface tile line of Bunny, 4) subsurface tile line of Walnut, and 5) P-removal blind inlet effluent tile line. Depending on flow, weekly samples are collected from the two subsurface tile lines when the potholes are not holding water. Samples are analyzed for dissolved reactive phosphorus (DRP) and total phosphorus (TP) to help further characterize P dynamics within prairie potholes and to demonstrate the effectiveness of the P removal blind inlet.

Unfortunately, of the 13 days with ponding in Walnut, only five daily samples have been collected. During the two inundation events within Walnut lasting more than one day, the blind inlet stopped producing effluent flow after the second day of inundation. It is thought that the 12-inch sand cap on top of the blind inlet bed of gravel and steel shavings may be causing the blind inlet to clog. The sand may contain fine clay particles that redistribute during ponding and fill the pore spaces, causing clogging. Sand samples have been collected from the blind inlet to be analyzed for particle size distribution. If it is discovered that the sand contains a significant portion of fines, the sand cap from the blind inlet will be hand excavated to avoid damaging the geotextile. Clean gravel will replace the sand, allowing for more porous drainage. Though only a few blind inlet effluent samples have been collected, the samples collected show that the blind inlet is effectively removing an average of 82.6% of DRP from the ponded water. TP data is still being analyzed.

January 2022

BLIND INLET & CONTROL STRUCTURE INSTALLATION: Installation of the blind inlet occurred July 14 – 15, 2021 at the pothole referred to as “Walnut”. The pit was excavated and lined with geotextile fabric. A collection manifold was installed in the base of the structure which was then covered with a layer of gravel. The gravel will protect the structure and reduce the likelihood of fine particles entering and potentially clogging the collection manifold. Steel shavings were purchased for a local scrap yard and mixed with gravel using a backhoe. The mixture was then placed in the excavated pit, covered with geotextile, a sand cap and then covered with fill.

Additionally, two water control structures were installed for sample collection, one to collect samples from the blind inlet manifold (direct sampling of water that will flow through the blind inlet) and the second to monitor flow from the tile line. At a second pothole referred to as “Bunny,” a water control structure was installed to allow for sampling of the drainage below the pothole, and will serve as the control to the blind inlet for the purpose of this project.

SAMPLE COLLECTION: After completing installation, researchers anticipated that sample collection would occur throughout the summer. However, due to extremely low precipitation, only two samples could be collected through the summer, and those samples were collected from the drainage lines, as flow never occurred through the blind inlet structure.

SORPTION EXPERIMENTS: Sorption experiments were initiated in the Water Quality Research Lab (WQRL) in fall 2021 with the intent to develop isotherm models of the sorption of phosphorus to the gravel/steel shavings. These experiments were not in the original proposal but are a way to leverage the graduate student's time and engagement on the project despite the dry weather. Additionally, soil samples were collected from the bottom, middle, top and outside the potholes in an effort to characterize pothole soils and to understand phosphorus dynamics in pothole soils. These experiments will be completed in spring 2022.

July 2021

A master’s student was hired and onboarded to help oversee the project. Design of the blind inlet was completed and a contractor identified for the installation of the inlet and the monitoring access sites. Though the installation was delayed into summer, the lack of adequate precipitation means that the project did not miss out on substantial data collection. However, the lack of data from spring and summer impelled design of a companion lab experiment to explore P sorption and release from the soils and the inlet materials in a controlled setting.

December 2020

Due to difficulties recruiting graduate students on short notice during the pandemic, we did not bring on a student for this project in Fall 2020. Rather, we have a student who will begin in January, and we will initiate related project activities (installation of the blind inlet, most notably) in early spring 2020.