Project overview
Stormwater ponds are designed to allow sediment and sediment-bound pollutants, including phosphorus, to settle out of the water column before runoff is discharged downstream, making ponds an important tool that stormwater managers use to reduce phosphorus loads. However, as ponds age and accumulate phosphorus-rich sediments, their ability to retain phosphorus can decline.
One factor contributing to this decline is the development of low-oxygen conditions, which facilitate phosphorus release from sediments. It has recently been discovered that shallow ponds can stratify, with warm surface water overlying colder, denser, chloride-rich bottom water. Stratification limits mixing, preventing oxygen from reaching bottom waters. Under anoxic conditions, phosphorus is released from pond sediments into the water column, reducing phosphorus retention and increasing phosphorus export downstream.
Previous modeling studies (Taguchi et al. 2022; Herb et al. 2026) identified three cost-effective remediation techniques that can improve phosphorus retention in ponds: alum treatment, iron amendment of pond sediments, and enhanced street sweeping throughout the watershed. However, the duration of these benefits has not been evaluated. This study therefore examined how effectively each remediation technique improves phosphorus retention and how long those improvements persist.
Research questions
- How long and under what conditions will alum treatments reduce sediment phosphate release?
- How long and under what conditions will iron filing treatments reduce sediment phosphate release?
- To what extent can street sweeping reduce sediment phosphate release and total phosphorus concentrations in ponds that treat stormwater?
Research findings
- Alum treatment was the most effective remediation strategy evaluated. When applied at rates sufficient to suppress internal loading, alum consistently reduced phosphorus release from pond sediments and maintained low total phosphorus concentrations in the study ponds for at least five years.
- Iron filings also reduced phosphorus release, but the benefits were shorter-lived compared to alum-treated ponds. Treatment effectiveness declined over time, with phosphorus release rates beginning to rebound within two years, and the iron-treated ponds maintained higher total phosphorus concentrations overall when compared to the alum-treated ponds.
- This study did not detect a measurable benefit from enhanced street sweeping on pond phosphorus retention. Although previous studies suggest that street sweeping can reduce phosphorus loading to stormwater ponds, the limited duration of this field study prevented its effects from being distinguished from the natural variability in pond processes. Because the benefits of reduced phosphorus loading may take years to manifest in pond sediments, longer-term studies are needed to determine whether enhanced street sweeping improves pond phosphorus retention.
What does this mean for Minnesota?
This study provides data that can help stormwater managers estimate how long different pond remediation techniques will effectively reduce phosphorus export from ponds. This information allows communities to select cost-effective maintenance strategies, avoid unnecessary treatment costs, and maximize phosphorus retention, reducing phosphorus inputs to Minnesota’s lakes, rivers, and streams.
Reports and Presentations
- Research brief 2026
- Final report 2026 .pdf
- Project update 2024 .pdf