Steeling Phosphorus: An Update on Blind Inlet Conservation

A blind inlet is constructed by excavating soil from a depression, installing a layer of perforated drainage pipe connected to a tile drain, and back-filling it with highly permeable aggregate to the surface. Traditionally, limestone aggregates are used as the blind inlet bed material. However, limestone does not remove dissolved phosphorus, and the loss of excessive phosphorus is considered problematic to watersheds like the Western Lake Erie Basin.

Chad Penn, a research soil scientist with the USDA-ARS, is evaluating the water quality effects of using steel slag, a byproduct from steel manufacturing, as an alternative to limestone in blind inlets to remove dissolved phosphorus, nitrogen and herbicides. 

In a three-year study to evaluate a blind inlet constructed with steel slag and its ability to remove dissolved phosphorus, the “modified” blind inlet removed at least 45% of the dissolved phosphorus load and was still effective after three years.

“We’ve found that slag as a replacement for the limestone rock works well,” says Penn. “We now recommend constructing ‘modified’ blind inlets where the traditional limestone rock is either replaced with a mixed media that can remove dissolved phosphorus, adding an additional layer of a phosphorus sorption material, or mixing a phosphorus sorption material in with the rock.”

Penn states that one of the best types of media for use in a blind inlet is a pea-gravel and metal shavings mixture at about 8% metal shavings. 

Essentially, these “modified” blind inlets allow the blind inlet to become a phosphorus removal structure. An innovation that, in Penn’s experience, is by far the most economical type of phosphorus removal structure. 

For more information about blind inlets and their impact on water quality, please visit our resource library. Producers who are interested in installing a blind inlet should contact their local Natural Resources Conservation Service or Soil and Water Conservation District office for technical and financial assistance. 


Gonzalez JM, Penn CJ, Livingston SJ. Utilization of Steel Slag in Blind Inlets for Dissolved Phosphorus Removal. Water. 2020; 12(6):1593.

Blind Inlet Conservation

A blind inlet, also known as a French drain, is a structure that is placed in the lowest point of farmed depressions or potholes to minimize the amount of sediment, and potentially other contaminants, that would be transported to receiving ditches or streams. Using a blind inlet can remove at least 90% of the sediments from the drainage water.

Utilization of Steel Slag in Blind Inlets for Dissolved Phosphorus Removal

Blind inlets are implemented to promote obstruction-free surface drainage of field depressions as an alternative to tile risers for the removal of sediment and particulate phosphorus (P) through an aggregate bed. However, conventional limestone used in blind inlets does not remove dissolved P, which is a stronger eutrophication agent than particulate P. Steel slag has been suggested as an alternative to limestone in blind inlets for removing dissolved P. The objectives of this study were to construct a blind inlet with steel slag and evaluate its ability to remove dissolved P, nitrogen (N), and herbicides. A blind inlet was constructed with steel slag in late 2015; data from only 2018 are reported due to inflow sampling issues. The blind inlet removed at least 45% of the dissolved P load and was still effective after three years. The dissolved P removal efficiency was greater with higher inflow P concentrations. More than 70% of glyphosate and its metabolite, and dicamba were removed. Total N was removed in the form of organic N and ammonium, although N cycling processes within the blind inlet appeared to produce nitrate. Higher dissolved atrazine and organic carbon loads were measured in outflow than inflow, likely due to the deposition of sediment-bound particulate forms not measured in inflow, which then solubilized with time. At a cost similar to local aggregate, steel slag in blind inlets represents a simple update for improving dissolved P removal.

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The Past, Present and Future of Blind Inlets as a Surface Water Best Management Practice

Drainage of tile-riser inlets allow direct discharge of surface water into tile drainage systems, effectively bypassing soil filtration processes and negatively affecting water quality. Blind inlets have gained recent popularity in allowing for both depression drainage and removal of suspended particulate matter by filtration through a sand/gravel layer. This paper summarizes blind inlet development and all published studies, provides new data from dissection of the longest-operating blind inlet that was recently de-commissioned, and discusses new ideas for the future of blind inlets, given certain shortcomings. Previous studies, as well as current soil analysis of the 12-yr old blind inlet, confirmed the ability of blind inlets to reduce sediment and particulate phosphorus (P), with an overall removal efficiency of at least 40% for each. In addition to sediment and particulate P, soil sampling revealed the ability of the blind inlet to capture several pesticides: glyphosate, atrazine, S-metolachlor, and metabolites. Traditional blind inlet sand media are unable to remove appreciable amounts of dissolved P compared to alternative media such as steel slag. Enhanced removal of dissolved constituents could be easily achieved through the use of P sorption materials and organic materials such as biochar, as well as a combination with tile-drain filters.

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Livingston Blind Inlets

Blind inlets are an alternative to a tile riser to drain small impressions, but in reverse. Blind inlets reduce the direct connectivity to contaminants getting into streams. Interrupting the connectivity helps to reduce loads of sediment, nutrients and pesticides running into water systems. An ARS scientist has shown a Blind Inlet Study Site, sediment and nutrient loss data. 

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Delaying Drainage from Prairie Potholes Protects Water Quality

After a storm, a tile riser drains a pothole in a wheat field. Tile risers are perforated pipes extending a foot or more above the soil. Water flows into the holes in the pipe and then typically flows unimpeded and unfiltered into subsurface drains and eventually into streams, causing pollution. An ARS scientist has shown that a blind inlet provides a solution.

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