Tag Archives: #soil

Using Regional Materials to Manage Stormwater Runoff

Researchers determined that natural soil amended with locally sourced materials performed well in bioslopes and bioswales. This practice will allow MnDOT to avoid hauling in costly commercial materials for stormwater management installations.

What Was the Need?

Following the requirements of the Minnesota Pollution Control Agency, MnDOT designs and constructs roadways with means to contain and manage the first inch of stormwater runoff—the “first flush”—flowing from impervious pavements. The agency often accomplishes this with low-impact development (LID) practices such as bioslopes and bioswales (shallow ditches) along roadways that mimic the original landscape. Constructed of tested soil mixtures and vegetation, bioslopes and bioswales effectively absorb runoff and filter sediment, heavy metals, chemicals and other pollutants that wash from the roads, preventing entry into the watershed.

In previous MnDOT highway construction projects, soils that are unsuitable for supporting pavements, such as peat and muck from wetlands, are dug out and hauled away as waste. Commercial compost mixtures are transported to the site for bioslope and bioswale construction. These hauling operations are expensive. To reduce costs, MnDOT has been investigating the use of natural materials close to construction sites and previously discarded as waste as possible filtration media in LID stormwater management.

A cross section drawing of an engineered bioslope design shows the edge of the pavement, a filter strip of grass at the top of the slope and the section of filter material with an underdrain surrounded by small rock buried beneath it. Blue arrows show the flow of water.
This cross section of a common engineered bioslope shows the position of a vegetation filter strip, a section of biofiltration material and an underdrain pipe beneath it. The blue arrows indicate the flow of water.

Phase I of this project examined the infiltration capabilities of compost mixtures and naturally abundant peat and muck in northeastern Minnesota as alternatives to commercial soil and sand in bioslopes and bioswales. Researchers conducted laboratory tests and constructed pilot plots, monitoring their performance. The current project continued the lab and field investigations, and developed a new peat-based biofilter based on research findings.

What Was Our Goal?

The primary objective of this project was to apply the results of the first phase of study to the design and construction of bioswales and bioslopes using local alternative media. An essential corollary objective was to determine whether lab tests and procedures could accurately predict and monitor the performance of these media in the field. Further, researchers sought to determine the effects of aging on alternative media performance.

What Did We Do?

The multidisciplinary research team addressed all aspects of alternative filter media suitability, from water retention and infiltration capacity, to contaminant-filtering effectiveness, to the ability to grow and support vegetation. First, researchers conducted a comprehensive literature review of bioslope and bioswale designs and methods of monitoring filtration media properties on-site. They examined best management practices, including state and federal regulations. Next, they collected treatment media from across Minnesota, including salvaged peat and muck, and commercial peat from an approved source. Researchers evaluated the media and determined potential filtration mixtures based on the results of the previous project and MnDOT standards.

“Lab and field investigations showed the salvage and reuse benefits of muck and other organic materials for slope and ditch topdressing to retain the first flush of rain from roadways,” said Dwayne Stenlund, erosion control specialist, MnDOT Office of Erosion Control and Stormwater Management.

In situ and lab materials from nine established and newly constructed biofilter sites were tested. These sites were constructed between 1990 and 2014, and the main soil filtration medium was amended with compost, peat or muck. On-site materials were evaluated for compaction, conductivity and absorption while lab samples were analyzed for metal and other contaminant retention.

Researchers also monitored six pilot test sites from Phase I. Three plots were prepared with native soil mixed 50/50 with compost and three plots with native soil mixed 50/50 with peat. Plots were examined and data were retrieved from instruments previously installed to measure rainfall, soil absorption and ambient temperature.

The team also established and monitored a new peat-based biofilter stormwater system using selected filter materials and designs. The new biofilter was installed throughout 5.7 miles of new road construction at Eagles Nest, which included extensive bioslopes and a bioswale enhanced with an underdrain system to resist silt clogging.

What Did We Learn?

Most sites showed deficiencies in nutrient and organic matter to support plant growth, and were dominated by weedy plants. Additional fertilizer and organic matter along with appropriate seeding could assure a good cover crop of grasses.

“This project’s results allow MnDOT to use in situ soil to build bioslopes and bioswales to retain the first inch of roadway runoff and associated pollutants. Using in situ materials rather than transporting new materials to the site will save taxpayer dollars, said” David Saftner, department head, University of Minnesota Duluth Department of Civil Engineering.

Compost and peat showed comparable effective water absorption and infiltration performance in amended biofilters. Lab tests can conservatively predict field performance. Researchers noted that early trends should be reinforced by further monitoring.

Peat’s performance shows it is a good alternative to compost for removing metals and phosphates in biofilters. It removes metals as well as compost and leaches less phosphate. Capacities to retain pollutants diminish with age for both media.

What’s Next?

Alternative local filtration media show great promise in stormwater management bio-installations. The next phase of this project will gather additional data from all plots to more fully assess the alternative media’s capabilities over time.

This post pertains to Report 2019-31, “Development and Regionalization of In Situ Bioslopes and Bioswales,” published July 2019. Visit the MnDOT project page for more information.

Concrete Grinding Residue Doesn’t Appear to Negatively Affect Roadside Vegetation and Soil

A new MnDOT research study determined that depositing concrete grinding residue (CGR) slurry at specific rates on roadside vegetation and soil may not cause lasting harm to plant growth and soil quality; however, follow-up research is recommended.

Study results showed that CGR did not appear to hinder vegetation growth or soil quality, but did change soil chemistry. At some roadside areas, the increase in soil pH enhanced plant growth. Results cannot be generalized for all soil types, plant communities, concrete residues or water sources in Minnesota. Access to real-time slurry disposal activity is needed for a thorough investigation.

Study background

Construction crews use diamond grinders to level newly cured concrete with adjacent slabs of older pavement and to smooth new pavement surfaces for improved friction and tire traction. Diamond grinders are fitted with hoses for rinsing grinding burrs with water to keep the burrs clean and prevent overheating. Vacuum lines then collect the residual dust and rinsing fluids, generating a slurry of concrete grinding residue (CGR) that is frequently discarded on roadside slopes and vegetation. 

When slurry dries, it leaves pale gray patches on roadside vegetation and other features, lightening the soil surface for a season or more. The effect of this slurry on vegetation, soil and drainage was unknown. Engineers and researchers presumed that the concrete dust temporarily coats roadside turf and plants, raises the soil pH, clogs soil pores and inhibits water drainage, invites invasive species to take root, and may infiltrate storm drains and waterways. 

What Was Our Goal?

MnDOT needed to study the impact of CGR on roadside vegetation and soil. Research would evaluate sites where residue has been deposited and determine its impact on vegetation and soils common to state roadsides. 

What Did We Do?

A literature review indicated that related research has been limited and that vegetation samples of only one or two species have been examined. Researchers developed two approaches for investigating the impact of CGR on plant density, plant growth and soil properties. 

First, researchers collected CGR slurry from a slurry tank at a Minnesota construction site to replicate residue application at the Kelly Farm, an Iowa State University research site near Ames, Iowa, that features prairie vegetation similar to that found along Minnesota roadsides. They applied slurry at application rates of zero, 10, 20 and 40 dry tons per acre. Plant cover, soil chemistry and soil structure properties, such as plant biomass, density, hydraulic conductivity, infiltration and pH, were measured before the slurry was applied and again at one-, six- and 12-month intervals after application. 

Second, researchers visited two roadside locations along Interstate 90 near Austin, Minnesota, where CGR had been applied. The research team evaluated vegetation content and cover, took soil samples and compared survey results to neighboring roadside environments that had not received CGR slurry.

The infiltrometer system setup at the Kelly Farm site in Ames, Iowa.
This water infiltrometer measured infiltration of water at the roadside environment test site.

What Did We Learn?

Statistical analyses established that at the Kelly Farm, CGR did not significantly impact soil physical properties and plant biomass, but did alter soil chemistry. Levels of soil pH, electrical conductivity, metals content and other properties rose significantly after CGR application. These effects increased with increases in application rate and decreased at increased soil depths. These changes did not reduce soil quality, and higher pH levels did not persist after one month. For certain warm-season grasses and legumes, increased pH improved plant growth. Some nutrients such as calcium and magnesium leached from CGR could benefit plant growth as well.

“Concrete grinding residue or slurry can, under certain conditions, be a benefit. It can act as a liming agent, changing soil pH in a positive manner.” —David Hanson, Integrated Roadside Vegetation Manager, MnDOT Roadside Vegetation Management

The two roadside environments yielded differing results. Slurries had been deposited in 2009 at the first site and in 2013 at the second. At the first site, soil bulk density and hydraulic conductivity in the slurried areas did not differ significantly from measures at the nonslurried areas; at the second site, the levels differed significantly. At both sites, electrical conductivity, calcium content and base saturation values were higher at the areas with CGR than the areas without CGR. 

Researchers concluded that at the Kelly Farm and at the roadside locations, slurry applications at a rate of up to 40 tons per acre did not reduce soil quality and vegetation growth for longer than three years. 

What’s Next?

Efforts to access grinding operations and CGR deposits in real time were not embraced by Minnesota’s concrete industry, and researchers were unable to properly assess residue composition and rates, and volumes of slurry deposition on roadway environments. A thorough investigation of residue impact will require such access and follow-up on site conditions after established periods of time. 

Researchers noted that findings cannot be easily generalized since CGR compositions may vary depending on source and water quality, influencing soil and vegetation differently, and soil and plant communities may differ in response to comparable CGR applications. Investigators recommended that MnDOT develop quick field measures of slurry pH, electrical conductivity and alkalinity to use in adjusting slurry spreading rates at grinding sites.

“This study was a great start to this topic. Follow-up research is recommended to evaluate live projects, field demonstrations and data collection.” —Halil Ceylan, Professor, Iowa State University Department of Civil, Construction and Environmental Engineering

This technical summary pertains to Report 2019-06, “Concrete Grinding Residue: Its Effect on Roadside Vegetation and Soil Properties,” published January 2019. Visit the MnDOT research project page for more information.