Poornima Natarajan, St. Anthony Falls Laboratory, University of Minnesota
Ben Janke, St. Anthony Falls Laboratory, University of Minnesota
John Gulliver, Civil, Environmental and Geo-Engineering, University of Minnesota
Jacques Finlay, Ecology, Evolution and Behavior , University of Minnesota
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The Problem and Symptoms
Stormwater ponds across a wide variety of characteristics (i.e., watershed history, age, size, depth, aquatic vegetation) are used to treat runoff to remove a significant portion of sediment and associated pollutants such as phosphorus (P). Our field and lab-scale studies on ponds started in 2016. We discovered that in summer, many ponds:
- are poorly mixed and stratified despite their shallow depths
- have canopy and topographical sheltering from wind mixing
- have chemical stratification due to road salt accumulation
- have low dissolved oxygen (DO <2 mg/L) at the bottom, but sometimes over entire water column depth
- are covered by free-floating plants that further sustain low DO conditions (anoxia)
- release P from anoxic bottom sediments at high rates
A graph labelled “Alameda - Dissolved Oxygen” is shown with a photograph of Alameda Pond. The photograph’s caption reads “Pond with heavy sheltering and dense floating plant cover”. The graph displays dates on the x-axis, beginning with April of 2001 and ending with November of 2001, and depth on the y-axis. Anoxic water is confined to the bottom of the pond in May, but anoxia spreads through the entire water column by August.
A graph labelled “Camden - Dissolved Oxygen” is shown with a photograph of Camden Central Pond. The photograph’s caption reads “Pond with low sheltering and no floating plants”.The graph displays dates on the x-axis, beginning with April of 2001 and ending with November of 2001, and depth on the y-axis. Anoxia develops at the bottom of the pond at the end of June and remains at the bottom, never spreading throughout the entire column. By the end of September, the entire water column is oxic again.
These graphs are captioned: “Figure 1. Pond DO conditions impacted by sheltering and floating plants”.
These findings have led to key research questions such as:
- Are aging ponds still providing P removal?
- What are the main factors impacting P retention (or release)?
- How to identify problem ponds? How to evaluate if ponds are working?
- What can be done to improve treatment performance?
Study Approach
Data collection in the 10+ pond projects done so far has involved intensive field sampling, comprehensive field survey and directed laboratory studies.
A photograph of a person in the front of a canoe on a plant-covered pond is captioned “Data collection using a water quality sonde”.
A photograph of the bow of a kayak on plant-covered water, surrounded by cattails, is captioned “Pond survey from a boat”.
A photograph of a tripod, holding data collection equipment, in a pond, is captioned “Monitoring station installed in the pond for continuous data logging”.
A photograph of a human hand reaching over the gunnel of a canoe to collect a water sample is captioned “Surface water sample collection”.
An image of a cylindrical tool with a length of cordage is captioned “A vanDorn sampler to collect water samples at depth”.
These images are captioned “Figure 2: Field monitoring and sampling methods”.
- Vertical profiles of DO, temperature, conductivity
- Water samples to analyze for P concentrations (surface and bottom water; total P and orthophosphate)
- Vegetation type and coverage, and free-floating plant biomass
A photograph of three people in a canoe is captioned “Sediment core collection from a boat”.
A photograph of a person boring a hole through the surface of a frozen pond is captioned “Sediment coring through ice.”
A photograph of sediment and water in test tubes is captioned “Sediment cores set up in the lab.”
A photograph of dark material in a white dish is captioned “Preparation for chemical analysis.”
These four images are together captioned “Figure 3: Laboratory experiments and chemical analysis.”
- Sediment cores set up under controlled conditions in the lab to measure oxic and anoxic sediment P flux
- Sediments analyzed for organic matter content, total P, bioavailable P, unavailable P, and metals that bind P
- Combined with field DO data, the internal P load can be calculated (P flux X duration of anoxia X pond area under anoxia)
Indicators of Pond Functionality
Many pond and landscape features directly or indirectly drive P cycling mechanisms and thus P retention in ponds. The main risk indicators are strongly associated with pond P concentrations, pond anoxia, and sediment P release. These indicators are related to basic pond characteristics or are parameters derived from field, lab and spatial data analysis.
A table, entitled “Risk Indicators and Effects on P Cycling Mechanisms” lists indicators and their associated phosphorus cycling mechanisms:
- Pond age and origin has an effect on sediment P release, organic matter accrual, and litter inputs.
- Pond surface area has an effect on oxygen dynamics, vertical transport, and hydrology.
- Pond mean and max depths have an effect on oxygen dynamics.
- Surrounding land use/land cover affects phosphorus and sediment inputs to ponds.
- Shoreline canopy cover and height affect wind sheltering/oxygen dynamics and litter inputs.
- Aquatic vegetation (submerged, floating, and emergent) has an effect on wind sheltering/oxygen dynamics and organic matter inputs.
- Anoxic factor (AF) affects oxygen dynamics and sediment P release.
- Pond sediment chemistry (TP, bioavailable P, organic matter, and chemicals) affect sediment P release potential.
Four charts are captioned “Figure 4: Relationships between key risk indicators and pond TP”. One is a scatter plot with mean summer WC DO on the x-axis and mean summer TP on the y-axis. The second chart is a scatter plot with mean summer DW cover on the x-axis and mean summer WC DO on the y-axis. The third chart is a scatter plot with AF-weighted anoxic P flux on the x-axis and mean summer TP on the y-axis. The fourth chart is a bar chart displaying the fraction of tree canopy (shown on the x-axis in three categories: 1-20%, 20%-60%, and over 60%), with mean water TP on the y-axis.
- Low DO results in high sediment P release and high pond TP
- Vegetation in and around ponds is important for pond mixing, oxygen status, and TP concentrations
Assessment and Management
The Pond Assessment Tool was developed by the SAFL researchers to easily and rapidly assess a large number of ponds for risk of poor phosphorus water quality. The Tool uses risk indicators to evaluate pond P water quality.
- Screening tool - rank and prioritize ponds at risk and in need of management
- Prediction tools - estimate water column TP concentrations, DO status and sediment P release rate
- Rapid assessment method for vegetation - identify main vegetation groups and associated impact on water quality
An image of a bathymetric chart of a pond.
An aerial photograph of a pond and surrounding tree canopy cover
A Google Earth image of a pond with duckweed
An image of a portion of a box and whisker plot
Download the Pond Assessment Tool and a tutorial
Effectiveness of various pond remediation methods (aeration, chemical treatment of sediments, dredging, vegetation management, watershed P load reduction) are currently under investigation by field research and modeling.
Acknowledgements
Many SAFL staff and students contributed to field data collection and lab analyses. Supporters of our research include the Minnesota Stormwater Research Council and MnDOT based upon recommendation from the Local Road Research Board. We also thank all the individuals who advised and reviewed the projects and the cities that allowed access to their ponds.
References
Finlay, J.C., Janke, B.D., Natarajan, P., Larkin, D.J., Distel, J. 2026. Managing urban pond vegetation to enhance water quality benefits. Final report to the Minnesota Stormwater Research Council. https://wrc.umn.edu/projects/managing-veg
Janke, B., Natarajan, P., Gulliver, J.S., and Finlay, J.C. 2023. Stormwater pond maintenance, and wetland management for phosphorus retention, MnDOT Report No. 2023-25, Minnesota Department of Transportation. https://hdl.handle.net/11299/256430
Natarajan, P.N., Janke, B.D., Finlay, J.C. 2025. Enhancement and validation of a stormwater Pond Assessment Tool. Final report to the Minnesota Stormwater Research Council. https://hdl.handle.net/11299/273518.
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