Project overview
Stormwater ponds are designed to capture phosphorus pollution, storing it in sediments at the bottom of the pond, but recent research has shown that some ponds are releasing the phosphorus stored in their sediments, which could then pollute downstream water bodies.
This project aimed to quantify phosphorus export from different types of stormwater ponds to determine if pond characteristics influence whether ponds effectively store phosphorus or release it back into the water which then flows downstream. As excess phosphorus fuels the growth of harmful algae blooms (HABs), the second part of this project aimed to determine whether HABs are exported from stormwater ponds and if they survive after traveling through stormwater conveyance systems.
For this research project, three stormwater ponds in the City of Minneapolis were sampled during the summer growing seasons of 2023 and 2024. Prior to a significant storm event, an autosampler was used to measure flows and collect composite samples for laboratory analysis of selected pollutants. During or immediately following a significant storm event, samples were taken from the pond surface and bottom waters of the pond near the sediment surface. In addition, algae and microcystin were collected from the outlead and nearest downstream manhole.
Research questions
- Are stormwater ponds exporting phosphorus or cyanobacteria downstream? If so, how much?
- Can stormwater pond characteristics be used to predict phosphorus or cyanobacteria export?
- Are there any design improvements or maintenance actions that could reduce phosphorus or cyanobacteria export?
- Can cyanobacteria and cyanotoxins survive and persist throughout the storm sewer system?
Research findings
- All ponds in the study exported some phosphorus, but loads varied across storm events and ponds. The algal community in all three pond outlets and downstream were dominated by cyanobacteria species that can produce toxins.
- Total phosphorus concentrations in the ponds were partially explained by pond depth and the watershed area to pond surface ratio. The higher the ratio, the higher the likelihood that the pond is undersized relative to its drainage area. The watershed area to pond surface area ratio also explained most of the variability observed in total phosphorus export from the ponds.
- All ponds in the study were determined not to be meeting optimal design criteria based on the Minnesota Stormwater Manual. Due to the high number of confounding variables contributing to the nutrient loading and nutrient cycling dynamics within any pond, identification of retrofit opportunities requires site-specific consideration of individual pond conditions. Retrofits will enhance phosphorus retention capacity, although the dynamics across a range of storm event sizes needs to be accessed in future research.
- Cyanobacteria and cyanotoxins were observed downstream of the ponds, indicating that they can persist after being conveyed through the storm sewer system. This study did not identify any design guidance for mitigating cyanotoxins or destroying cyanobacteria transport from the ponds.
Key innovation/contributions
This project quantified the amount of phosphorus exported from stormwater ponds and documented cyanobacterial export.
What does this mean for Minnesota?
Stormwater pond managers can use the knowledge produced by this project to prioritize pond retrofits and maintenance actions to reduce phosphorus and cyanobacterial export from high-risk ponds. This will reduce the amount of pollution entering lakes, streams and other urban waterbodies, leading to healthier aquatic habitats and cleaner water for Minnesota residents.
“Stormwater ponds are dynamic systems that function differently depending on pond geometry, morphology, watershed area, pond surface area, pond depth, land use and variations in inlet and outlet control structures.”
Katie Hembre, Stantec
Reports and Presentations
- Research brief 2025 .pdf
- Final report 2025
- Project update 2024 .pdf