Project overview
Bioretention practices (like rain gardens, bioinfiltration basins, and biofiltration basins) are shallow depressions in urban landscapes that capture and treat stormwater runoff. Stormwater passes through a media mix that is engineered to remove pollutants via filtration, chemical reactions, and biological transformations (plants or microbes growing in the media “digest” the pollutants). This means that bioretention designs often include plants growing on the surface of the engineered media mix. Compost is then added to supply these plants with the nutrients (carbon, nitrogen, and phosphorus) they need to grow.
However, research has shown that most composts in bioretention practices export the very nutrients the media mix is engineered to capture. Additionally, nutrient release from compost can double when exposed to road salt, greatly increasing the amount of pollution the filter media material is expected to absorb in cold climates.
Previous research identified amendments that increase the nutrient absorption capacity of the filter media. To read about those projects, please visit:
Previous research identified amendments that increase the nutrient absorption capacity of the filter media. However, instead of amending the engineered media mix so that it can absorb more pollution, an alternative solution might involve changing the compost so that it no longer exports nutrients, even when exposed to road salt.
This project is investigating compost alternatives that would be able to support plant growth on bioretention practices without exporting nutrients to the filter media. Materials such as vermiculite, perlite, zeolite, peat, and biochar will be tested to determine how much phosphate they export and how much moisture they retain. Materials that retain moisture without exporting significant amounts of phosphate will then be tested to see if they can support healthy plant growth and whether they leach any other pollutants besides phosphate.
Research questions
- Which media components and design parameters can support plant growth of typical bioretention plant species?
- Which media components and design parameters can limit nutrient export from bioretention media?
- What mixing ratios are optimal for supporting plant growth and limiting nutrient export?
- What is the relative cost comparison of any new media mixes developed through this research and compared to various forms of compost?
Key innovations/contributions
This project will identify filter media components that can support plant growth and microbial activity while also limiting nutrient export.
What does this mean for Minnesota?
The results of this research project will help stormwater designers, urban planners, landscape architects, and others working for consulting firms, watershed districts, or municipalities to design bioretention practices that are capable of supporting plant growth without exporting nutrients. These improved bioretention practices will more effectively capture and retain stormwater pollutants, preventing them from entering waterbodies. This will help municipalities and other local government units meet their stormwater management targets, which will lead to cleaner water, healthier aquatic and riparian habitats, and better recreational opportunities for Minnesota residents.