Capturing and quantifying coarse organic matter in urban stormwater

Scientific abstract

We are missing the phosphorus in stormwater. Traditional stormwater samples will often miss or intentionally discard large organic solids, also known as gross solids. These gross solids, consisting of eroded soil, tree leaves, tree seeds, grass clippings and other organic or inorganic material, are a major problem for stormwater management and are often discarded in sample processing due to the size and water content, or when small and granular they are mistaken for mineral sediments. Large solids clog stormwater conveyances and quickly fill stormwater ponds requiring expensive clean out. These solids also convey large amounts of nutrients such as nitrogen (N) and phosphorus (P), which may mean that true nutrient loadings to surface waters may be considerably larger than measured by conventional monitoring programs.

Final report

Research results

Organic material is present in urban stormwater runoff in surprisingly large amounts and contains a significant amount of phosphorus that needs to be accounted for and managed. This study estimates dry mass at 210 - 250 mg/ha/yr.
Automated samplers are missing phosphorus contributions - this study identified a need for new sampling methods, sampler designs, and models. Find more information on the implications for the P8 model on the MN Stormwater Manual

Large, full trash bags sit next to a collection device made from tarp and tubes installed on a curb cut leading into a dry pond — The study’s sampling methods were designed to capture gross solids such as leaves (note the volume of collected materials in the yellow circle). Typical samplers filter out this material, which leads to the underestimation of phosphorus. Photo credit: John Chapman.

What does this mean for Minnesota?

Properly accounting for gross solids can help us better predict maintenance frequency and costs and design better control measures. The results of this study show that we might be currently underestimating or overestimating performance of best management practices. Other considerations:

What is the role of snow melt and its composition on runoff?
How can pre-treatment measures help address P?
How might sampling techniques and models be adapted to capture gross solids?

"We are likely underestimating nutrient loads in urban watersheds, but there are good opportunities to capture some of this material. Encouraging enhanced street sweeping with smart timing, especially in the spring, can do a great job of reducing this material.”
Aaron DeRusha, Washington Conservation District

A screenshot of a computer program that calculates phosphorus loads in stormwater — The researchers have developed a tool for practitioners (in beta) to estimate gross solids present in BMPs. Access the tool below and contact us with questions. Access tool.

Project outputs

Project team

Principal investigator

John A. Chapman
Department of Bioproducts and Biosystems Engineering, University of Minnesota

Co-investigators

Lawrence A. Baker,
Department of Bioproducts and Biosystems Engineering, University of Minnesota
Jacques Finlay,
Department of Ecology, Evolution and Behavior, University of Minnesota

Collaborators

Grace L. Wilson,
Department of Soil, Water, and Climate, University of Minnesota
Kathryn Hoffman,
Department of Ecology, Evolution and Behavior, University of Minnesota

Graduate students

Aaron Pietsch,
Department of Bioproducts and Biosystems Engineering, University of Minnesota

Undergraduate students

Tristan Blechinger
Freda Cozza
Rose Coyne
Lucy DeBoer
Isabella Fischer
Natalie Narváez
Rachel Sanborn
Amanda Van Pelt

Partners

Mississippi Watershed Management Organization
Capitol Region Watershed District
City of Saint Paul
City of Minneapolis
City of Roseville
City of Shoreview

Capturing and quantifying coarse organic matter in urban stormwater

Research results

What does this mean for Minnesota?

Project outputs

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