Tag Archives: MPCA

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.

Evaluating Iron-Enhanced Swale Ditch Checks for Phosphorus Removal

Researchers documented performance of an iron-enhanced ditch check filter to remove phosphorus from stormwater over three years. The filter was effective, but its performance decreased over time, and it will require relatively frequent maintenance. Several design changes may be considered.

What Was the Need?

The Minnesota Pollution Control Agency (MPCA) requires that developments adding more than an acre of impervious land must try to include methods to infiltrate the first inch of stormwater runoff. This “first flush” of stormwater from impervious road pavement contains pollutants that could contaminate watersheds.

To meet its MPCA permit requirements, MnDOT might consider constructing strategically designed ditch blocks and swales: wide, shallow ditches with mounds of engineered soil and sand mixtures and vegetation along roadways that manage stormwater flow.

“Our success in removing phosphorus from stormwater runoff using iron particles in filtering ditch checks showed strong proof of concept. Modification of the filters’ orientation in the ditches could result in a device that is both more effective and low maintenance for MnDOT,” said John Gulliver, department head, University of Minnesota Department of Civil, Environmental and Geo-Engineering.

Previous research has shown that MnDOT’s linear swales are effective filters of contaminated highway runoff. That study also conducted laboratory investigations into enhanced ditch checks—low permeable mounds placed in a swale to reduce water velocity. Researchers tested the ability of various media in an experimental ditch check filter to remove more pollutants. A subsequent MPCA study examined the capability of an iron-enhanced sand trench to remove phosphorus.

In September 2014, MnDOT constructed two iron-enhanced ditch checks in a swale along CR15 (TH5) in Washington County to test two design versions. One failed to function effectively; the other was monitored for four months in 2015, showing effective phosphorus and metals retention.

What Was Our Goal?

The goals of this project were to investigate the long-term effectiveness of iron-enhanced ditch checks in retaining pollutants and to develop recommendations for maintenance actions needed to support effective filter performance.

What Did We Do?

Researchers monitored the performance of the CR15 (TH5) iron-enhanced ditch check from 2016 through 2018 while temperatures were above freezing. A tipping-bucket rain gauge was connected to a data logger to record rainfall at the site. Water samples were collected automatically through tubing at four points: the inflow and outflow of the check dam and filter insert. (Two monitoring wells had been built into the filter.) An automated sampling instrument was triggered by flow through the filter insert. The data logger recorded water levels, flow rate, cumulative flow volume and rainfall depth information continuously at five-minute intervals.

Near the end of the study, workers used a boring machine to take core samples of the ditch check and the filter insert.

Three workers in safety vests and hard hats stand near a tall boring machine taking a core sample of the ditch check filter.
Near the end of the study, workers used a boring machine to take core samples of the ditch check and the filter insert.

Pressure transducers installed inside the monitor wells measured upstream and downstream water levels at the filter insert section. Flow rate through the filter was calculated. Researchers retrieved inflow and outflow samples within 24 hours of the end of a rain event. The University of Minnesota St. Anthony Falls Laboratory tested these samples for phosphorus and metals.

In July 2018 researchers collected three core samples from the ditch check sides and the filter to test for retained phosphorus and signs of diminished performance. 

What Did We Learn?

The iron-enhanced ditch check filter successfully removed phosphorus during the majority of the 40 rain events, reducing the phosphate mass loads between 22 percent and 54 percent. However, the cumulative phosphorus retention in the filter decreased from 42 percent in 2015 to 30 percent in 2016, 25 percent in 2017 and 23 percent in 2018. The core tests confirmed that the bottom 3.9 inches of the filter media filtered most of the inflow volumes of the runoff. This heavy runoff load reduced its sorption capacity over three years while the upper part of the filter was active only infrequently.  

“Enhancing ditch checks with iron filings will aid in the removal of phosphorus in stormwater. However, since the bottom of the filter receives the most flow, more frequent mixing of the iron filings is required than originally anticipated,” said Beth Neuendorf, water resources engineer, MnDOT Metro District.

The ditch check itself showed a somewhat lower phosphorus retention performance than the filter insert, though performance varied some years. Researchers considered that the soil and sod covering may have leached phosphates into the ditch check and filter insert, affecting its overall performance.

Neither the ditch check nor the filter insert were very effective in copper and zinc retention, although the metal concentrations in the inflow and treated runoff were generally lower than in typical highway runoff.

Regarding maintenance, researchers recommended the filter insert medium be mixed up every other year to redistribute the filter media at the bottom. They also recommended that the entire filter insert be replaced after six years.

What’s Next?

Researchers presented possible design changes that could improve performance, such as using peat instead of soil and sod to cover the ditch check. They also suggested installing ditch checks in series and re-engineering the filter to address the findings revealing the heavy runoff load taken by the filter’s bottom 3.9 inches. Reducing the depth of the filter berm could avoid the excessive inundation of the bottom media while the upper portion remains unused. Maintenance frequency could then be reduced.

This post pertains to Report 2019-27, “Iron-Enhanced Swale Ditch Checks for Phosphorus Retention,” published July 2019. For more information, visit the MnDOT project page.

How roadside drainage ditches reduce pollution

Stormwater can pick up chemicals and sediments that pollute rivers and streams. Roadside drainage ditches, also known as swales, lessen this effect by absorbing water. But until recently, MnDOT didn’t know how to quantify this effect and incorporate it into pollution control mitigation measures.

In a recently completed study, researchers evaluated five Minnesota swales, measuring how well water flows through soil at up to 20 locations within each swale.

“There’s a big push in Minnesota, and probably everywhere, to do more infiltration,” Barbara Loida, MS4 Coordinator Engineer, MnDOT Metro District, said. “We know that our ditches are doing some of that, but we wanted to look at how much infiltration these ditches are providing.”

A key finding: grassed swales are significantly better at absorbing water than expected, which may reduce the need for other, more expensive stormwater management practices, such as ponds or infiltration basins.

This could save MnDOT and counties significant right-of-way and construction costs currently expended on more expensive stormwater management techniques. While swales were recognized in the Minnesota Pollution Control Agency’s new Minimal Impact Design Standards, there was a need to quantify the amount of water a swale can absorb so it could receive the appropriate MIDS credits.

Researchers also tested the ability of carbon, iron chips, steel wool and other materials to remove pollutants as ditch check filters—material put into swales to enhance removal of pollutants.

Gradations on a Modified Philip Dunne infiltrometer allow the measurement of stormwater infiltration.
Gradations on a Modified Philip Dunne infiltrometer allow the measurement of stormwater infiltration.

What’s Next?

A follow-up project, which the MPCA is participating in, will seek to clarify the impact of swale roughness on infiltration rates. The goal is a calculator for real-world infiltration rates that MnDOT and local agencies would be able to implement.

MPCA, MnDOT and the city of Roseville are also partnering on a project to install and test the effectiveness of ditch check filters in real-world locations.

Maintenance recommendations should help MnDOT and local agencies ensure that swales operate at maximum efficiency. These recommendations should continue to be revised as knowledge evolves.

Related Resources

*Editor’s note: This article was adapted from our upcoming edition of the Accelerator. Read the newsletter online, or sign up to receive by mail.