After road construction, soil in revegetation areas must retain adequate stormwater and provide stable embankments along the road. Revegetation can be challenging due to thin topsoil, and it often requires importing healthy organic soil and exporting excess construction soil generally unsuitable for plant growth.
In a recent study, researchers identified and tested regionally available industrial byproducts to use as soil amendments and found sustainable solutions with multiple financial and environmental benefits.
Stormwater permits for road construction require revegetation areas to retain the first inch of stormwater runoff—which contains most of the pollutants deposited from the air—to protect the state’s waterways. In northeastern Minnesota, thin topsoil often necessitates adding soil to support plant growth as much of the excess soil from road construction is unsuitable for revegetation.
“This project confirmed that a wide range of granular and organic materials can be used in practical and implementable stormwater solutions along the roadways,” said David Saftner, associate professor, University of Minnesota Duluth Department of Civil Engineering.
Successful revegetation to provide functional integrity for the road and hold adequate stormwater requires amending the soil to ensure sufficient organic matter and infiltration capacity. MnDOT and county road engineers saw a potential synergy with regional mineral, forestry and other industries that produce byproducts that are otherwise stored or hauled away and discarded. In addition to the costs of these practices, the byproducts can release excess carbon and have other environmental impacts. Reusing these materials in revegetation areas after road construction could solve restoration and stormwater challenges, reduce solid waste, sequester carbon and create financial advantage for the generating industry.
What Was Our Goal?
The goal of this project was to identify regionally available byproducts to amend otherwise unsuitable soil for restoration and revegetation after road construction.
What Did We Do?
After reviewing state stormwater permitting requirements and biofiltration systems used by other states, researchers focused a search in MnDOT District 1 for suitable byproducts from regional businesses and agencies. They explored 23 materials from 14 organizations and ultimately chose 10 to analyze, including:
- Tree bark from wood companies.
- Fine and coarse tailings from iron mining.
- Peat screenings and scrapings from peat mining.
- Fine-grained sediment from harbor dredging.
- Street sweepings from a public works department.
Researchers first analyzed the byproducts’ chemical properties looking for organic matter, nutrients, metals and toxic chemicals. They tested biological characteristics of the individual materials and various blends of materials through greenhouse tests for germination and vegetation growth rates.
Environmental attributes analyzed included chemical release and removal capabilities. From a civil engineering perspective, researchers measured moisture content, particle size and gradation, specific gravity—a measure of density important to soil stability—and other measures indicative of whether a soil containing this material would be suitable for roadside revegetation.
Hydraulic conductivity tests suggested how the material would perform in biofiltration and stormwater retention. Researchers also performed these tests on blends of the peat byproducts (which are highly organic) with inorganic materials, including tailings, street sweepings and dredge sediment.
Lastly, researchers developed life cycle analyses to determine environmental and economic impacts of using these byproducts for blended soil. They compared 16 materials—10 individual byproducts and six byproduct mixes—to using straight topsoil. The analyses included transportation to the job site, mixing and using the material.
What Did We Learn?
Researchers found that most of the byproduct materials tested were suitable for amending soil for revegetation after road construction. The soil mixtures designed with organic and inorganic materials retained sufficient water, filtered pollutants and supported plant growth.
“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. We’d like to explore similar opportunities throughout the state,” said Dwayne Stenlund, erosion control specialist, MnDOT Office of Environmental Stewardship.
Absent vegetation, all materials removed more than 50% of phosphorus, which, in excess, is harmful to ecosystems and human health. Fine iron mining tailings were the least supportive of plant growth, likely due to the low permeability and infiltration. Street sweepings and dredge sediment had the largest absorption of copper—which bioaccumulates in plants and animals—among all the materials, however, had low organic content and did not support abundant plant growth on their own. The tree bark had the highest hydraulic conductivity of the materials. The most beneficial balance of nutrient supply, chemical absorption and water retention was achieved in a blend of peat material and inorganic materials—tailings, street sweepings and/or dredge sediment.
Through life cycle analyses, researchers confirmed that the further material is transported, the greater the environmental impact although these analyses are project-dependent. Finally, through this project and building on previous work, researchers developed test methods to evaluate byproduct materials for use as soil amendments.
Blending these byproducts with otherwise unsuitable soil at construction sites has multiple benefits. Counties can also use designed soils for projects such as revegetating gravel pits. Future research will identify suitable byproducts in other MnDOT districts to design sustainable soils and will develop a more in-depth life cycle analysis to identify the environmental impacts on and benefits to air, water and ecosystems of using regional materials for revegetation after construction. A statewide implementation guide for designing sustainably engineered soil mixes will be produced to aid MnDOT and county engineers in choosing the right material for the right place.