Treating stormwater with local by-products reduces road construction costs, minimizes waste

Tray of soil samples with small green plants growing out of them
Tests of the individual and blended biofilter materials were conducted on greenhouse-grown plants.

Ongoing research is looking into the possibility of using local industrial waste for roadside stormwater construction projects. This would help reduce material and transportation costs and put otherwise wasted materials to use.

Managing stormwater runoff and the pollutants it can carry is a huge priority for highway departments. The Minnesota Pollution Control Agency requires stormwater installations for new construction projects, and these installations need to be capable of capturing the first inch of rainfall in a given storm event.

Researchers from the University of Minnesota Duluth (UMD) and the Natural Resources Research Institute (NRRI) have been partnering with MnDOT in a series of projects that aim to find viable construction materials that are closer to construction sites. Their most recent work tested a wider range of materials.

Bioslopes and bioswales (shallow ditches along roads that catch and filter runoff) are a recommended tactic. However, these biofilters have traditionally involved hauling materials such as compost, peat, and sand from other areas, which costs fuel and money.

“By-products and waste materials generated by mineral, forestry, agricultural, and industrial sectors have the potential to be recycled, reused, or combined with traditional materials to create a sustainable source of value-added soil or soil amendments,” says David Saftner, principal investigator for the 2022 project and associate professor in the UMD Department of Civil Engineering.

Previous studies published in 2019 and 2021 laid the groundwork; researchers identified properties that a successful biofilter material needs, such as good water retention, high infiltration capacity, contaminant-filtering effectiveness, and an ability to grow and support vegetation.

Preliminary studies at test plots found that peat and compost mixed with taconite tailings show promise. Long-term monitoring found that these experimental biofilters are capable of capturing the required first inch of rainfall.

Pile of dirt from street sweeping
Street sweepings, such as this stockpile in Duluth, can work well in soil blends. Photos: Meijun Cai

Continued research in 2022 tested the viability of additional by-products and waste materials. Researchers found and tested 11 potential candidates local to northeastern Minnesota, including:

  • Peat by-products (often generated during land use and industry processes)
  • Tree bark (timber/forestry residuals)
  • Harbor dredge sediment (from Duluth’s shipping channel)
  • Coarse and fine taconite tailings (by-product of taconite mining)
  • Street sweepings (from public works departments)

Analysis of the environmental and civil engineering characteristics of the different materials found, overall, that peat by-products continue to show a lot of promise, says NRRI environmental engineer Meijun Cai. Peat by-products proved the most effective at removing metals from runoff, and they provide phosphorous for plant growth. Since peat by-products tend to turn stormwater acidic, however, the researchers recommend mixing them with inorganic materials such as coarse taconite tailings, street sweepings, or dredge sediment.

Biological and chemical tests found that none of the proposed materials are hazardous, and life-cycle studies confirmed that locally sourced materials have lower environmental impacts.

“This conclusion reinforces the importance of the current research,” Cai says.

Future work will involve testing the in-situ performance of the different materials in biofilter systems (the 2022 study was done purely in a lab). The effectiveness of the materials will also be studied over a longer period of time, Saftner says.

In addition, a wider variety of materials will also be studied on a statewide basis. The researchers plan to produce an implementation guide to help MnDOT and county engineers with factors such as mixing ratios and technology, design and construction specifications, and monitoring requirements.

“The sustainable soil design practices identified in this project have multiple benefits: Financial savings for us, the counties, and the material producers; water quality protection; a reduction in solid waste and carbon into the atmosphere; and aesthetics,” says Dwayne Stenlund, erosion control specialist with the MnDOT Office of Environmental Stewardship. “We’d like to explore similar opportunities throughout the state.”

Writer: Sophie Koch


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