Emma Squires - St. Anthony Falls Laboratory, University of Minnesota
Poornima Natarajan - St. Anthony Falls Laboratory, University of Minnesota
John Gulliver - Civil, Environmental, and Geoengineering, University of Minnesota
Jake Newhall – WSB
Bill Alms - WSB
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Introduction
Stormwater pond provide:
- Runoff storage and flood control
- Water quality benefits
- Allow time for solids to settle out
- Remove phosphorus from stormwater
Stratification & Sediment Phosphorus Release
Ponds stratify more easily than lakes do due to their small surface area and shallower depth. Ponds stratified by their banks and surrounding tree are often stratify.
When stratified, oxygen can be depleted in the bottom of the pond, resulting in anoxia (DO <2 mg/L). Under anoxic conditions, phosphorus can be released from the sediments, generating an additional phosphorus load to the pond. This may result in ponds exporting phosphorus to downstream waters.
Dredging stormwater ponds
Ponds fill up with sediment over time. To maintain their holding capacity, they must be dredged every ~15-20 years, depending on the rate of sediment accumulation.
Surface sediment is often rich in organic matter and phosphorus. Because dredging removes this surface layer, dredging is expected to reduce sediment phosphorus levels.
Four photographs show a pond being dredged. Caption reads: “Dredging of Ponds P-128 (top) and BNW15-H (bottom) (Photo courtesy WSB)
Research Questions
- Is sediment removal an effective pond maintenance measure to reduce the release of sediment-bound phosphorus and thus improve water quality in stormwater ponds?
- Should sediment treatment with phosphorus-binding chemicals be considered to further reduce sediment release of phosphorus at the time of dredging?
Methods
This study compared pond conditions before and after dredging in five ponds.
Field monitoring:
Phosphorus water quality, pond DO, and mixing status were monitored for one season pre and post dredging.
Lab study:
Sediment cores were taken before and after dredging and used analyzed for:
- Sediment P release (oxic and anoxic conditions)
- Sediment P composition (bioavailable and unavailable P)
- Sediment Characteristics (organic matter, sediment oxygen demand)
A photo shows three people in a canoe on a pond. Caption reads: “Staff collecting sediment cores from a pond. Photo courtesy: poornima Natarajan”
A photo shows test tubes containing sediment and pond waters. Caption reads: “Pond sediment cores set up for phosphorus release study in the lab.”
Results
A bar chart titled “A. Surface TP Concentrations displays pre dredging and post dredging surface phosphorus.” Another bar chart titled “B. Bottom TP Concentrations displays pre and post dredging bottom phosphorus.” Caption reads: “Figure 1. Field data is averaged across the monitoring season. Green indicates pre dredge data and blue indicates post dredge data. A. Surface water TP concentrations. 4/5 sites saw a decrease in TP concentrations after dredging, except for Pond 175. B. Bottom water TP concentrations. 1 site was not deep enough for bottom water sampling prior to dredging. 2/4 sites saw a decrease in TP concentrations after dredging. 2/4 sites saw an increase instead.”
Five box-and-whisker plots, together titled “Mean Phosphorus Release Rate Pre and Post dredging by pond.” Caption reads: “Figure 3. Mean phosphorus release from all cores for each pond pre and post dredging under anoxic conditions (DO <2 mg/L). 3 ponds saw a decrease in sediment P release after dredging. 2 ponds (NW15-H and 95) saw a slight increase in P release after dredging.”
A table, titled “Pond Results Comparison” displays increase or decrease of bioavailable P, unavailable P, release, surface TP, and bottom TP. Caption reads: “Figure 4. Table summarizing results as a comparison of pre to post dredge (i.e, decrease indicates a decrease after dredging). Bioavailable P and Unavailable P are results from the Sediment P Composition analysis. Anoxic P release is results of lab P release study in sediment cores. Surface TP and Bottom TP are field monitoring results. All ponds saw a decrease or no change in Sediment P concentrations. However, some ponds saw an increase in sediment P release or field TP concentrations.”
Four box-and-whisker plots, together titled “Mean P (mg/g) Pre and Post dredging by pond” display P fraction of bioavailable P and unavailable P. Caption reads: “Figure 2. Mean phosphorus concentrations in the top 10 cm depth of sediment from all cores for each pond pre and post dredging. Bioavailable P is releasable to the water column while Unavailable P is more likely to stay in the sediments. All ponds saw a decrease for both forms of P, except for W175 which saw only a minor change in Bioavailable P.”
Discussion
Dredging reduces sediment phosphorus concentrations. Dredging also often reduces release of phosphorus from the sediment under anoxic conditions and TP concentrations in field water quality.
Future study will examine when additional phosphorus remediation measures (alum addition) are needed in combination with dredging.
Acknowledgements
Funding for this project came from the Local Road Research Board and Minnesota Department of Transportation. WSB is conducting field monitoring for this project. Thanks to the cities who allowed access to their ponds and all the staff and students who worked on the projects.
References
Gulliver et al. 2024-2027. Effectiveness of pond sediment removal for phosphorus management in stormwater ponds, ongoing project funded by the Local Road Research Board (LRRB). https://researchprojects.dot.state.mn.us/projectpages/pages/projectDetails.jsf?id=29689&type=CONTRACT
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