December 21, 2021

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.

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