Project overview
There is growing concern that aging stormwater retention ponds may become net exporters of phosphorus to receiving waters. This two-year project investigated phosphorus dynamics in three stormwater ponds in the Twin Cities metropolitan area using intensive monitoring and sediment sampling to:
- Identify potential causes of elevated phosphorus concentrations in stormwater pond water
- Investigate drivers of pond phosphorus retention
If phosphorus is not retained within stormwater ponds, this poses a risk to downstream water bodies.
Research findings
This project produced new knowledge regarding water column phosphorus concentrations and pond phosphorus retention.
Two factors were observed that might be driving water column phosphorus concentration:
Though these three ponds are shallow (<6 feet deep), they were persistently stratified during the warm season and generally anoxic. These conditions should promote redox phosphorus release from the sediment into the water column.
Pond sediments were dominated by labile organic phosphorus, which may also contribute to internal loading.
We also observed that pond phosphorus retention during rainfall events was associated with lower antecedent (pre-rainfall) water levels. This finding highlights the importance of non-outflow water losses, such as evaporation or exfiltration, as contributors to pond storage capacity.
What does this mean for Minnesota?
The overall goal of the project was to reveal factors impacting the capacity of stormwater ponds to retain phosphorus. This knowledge can be used to develop methods to rapidly identify ponds that are likely to export phosphorus, allowing stormwater pond managers to strategically prioritize the maintenance of existing ponds. Quickly identifying high-risk stormwater ponds can increase the efficiency and cost-effectiveness of pond maintenance programs, while also protecting the quality of downstream water bodies.
Reports and presentations
- Final report 2021