Seeking cost-effective approaches for protecting water quality

September 9, 2026
A stormwater pond covered in green algae

Minnesota’s more than 30,000 stormwater ponds are critical for protecting water quality downstream. They work by collecting and storing runoff and settling solids, including pollutants, to the bottom of the pond. One such pollutant — phosphorus — can spur excessive algae growth and deplete oxygen in our lakes, rivers, and streams.

Well-maintained stormwater ponds are generally expected to retain about 65 percent of incoming phosphorus — but University of Minnesota researchers found many older stormwater ponds are not functioning as designed, allowing excess phosphorus to escape to downstream water bodies.

A recent research project aimed to find ways to improve phosphorus retention in stormwater ponds and provide information to help state and local stormwater managers make cost-effective decisions. The project was funded by MnDOT and the Minnesota Local Road Research Board and led by John Gulliver, a CTS emeritus scholar, emeritus professor with the UMN Department of Civil, Environmental, and Geo- Engineering, and researcher with the St. Anthony Falls Laboratory.

In some ponds, water separates into distinct layers, known as stratification, based on temperature and road salt. As a result, the bottom of the pond has a low level of dissolved oxygen, which causes the sediment to start releasing the phosphorus that it's supposed to be storing. That released phosphorus could then be discharged from the pond during the next storm event. A stormwater pond’s performance can also be affected by wind sheltering (wind is more likely to mix the layers of a pond), conductivity (influenced by the presence of chloride from road salt), and floating surface vegetation.

A graphic showing the factors contributing to a faiing stormwater pond; details in caption.
In a failing stormwater pond, trapped phosphorus is released from the sediment, contributing to "pea soup" algae blooms and increased phosphous levels in pond outflows.
A graphic showing how a well-maintained pond works; details in caption.
A well-maintained stormwater pond retains trapped phosphorus, resulting in decreased phosphorus outflows and cleaner lakes and streams. 

Fieldwork and modeling

The researchers collected field data from six metro-area stormwater ponds to assess phosphorus levels, dissolved oxygen, temperature, water level, wind speed, and duck weed surface cover. The ponds were also instrumented for continuous monitoring.

The team then adapted an existing computer model used for lakes to simulate pond conditions and reviewed laboratory studies on pond sediments to support and verify the updated model. The pond model was then used to evaluate six potential methods for decreasing phosphorus release:

  • Treating water with chemicals
  • Aerating
  • Controlling water levels
  • Reducing wind sheltering
  • Reducing watershed pollutant load
  • Installing a clay lining

The researchers also analyzed the cost-effectiveness of these treatments, estimating the cost per pound of phosphorus reduction.

Results and recommendations

When phosphorus release is substantial, chemical treatment can be effective: Results showed that these treatments reduced phosphorus release by 10 to 13 percent for iron filings and 11 to 24 percent for aluminum sulfate (alum). However, field data suggests that these treatments, which were intermediate in cost, would need to be repeated every 5 to 10 years.

Adding oxygen through microbubble aeration in the spring could reduce phosphorus release if oxygen is maintained, but partial aeration may make the problem worse if layering is not fully addressed. Adaptive or “smart” water-level controls designed for flood mitigation and reducing wind sheltering did not offer much benefit in this study, the researchers say.

Watershed treatments included reducing the amount of stormwater runoff by allowing more water to soak into the ground (such as with rain gardens) or reducing the amount of phosphorus in the runoff before it reaches a pond (such as with street sweeping). The results showed that both approaches significantly decreased the amount of phosphorus leaving the pond, although the former was the most expensive option.

By removing debris from roads before runoff carries it into ponds and wetlands, street sweeping was found to be the most cost-effective method, reducing phosphorus export by 2 to 22 percent.

Gulliver notes that treating sediments with alum is an effective way to virtually eliminate phosphorus release from the sediments. “In addition, an intense street-sweeping regime is the most cost-effective means of reducing phosphorus export from a stormwater pond or historic wetland that serves as a stormwater practice by decreasing the load coming from the watershed,” he says.

The researchers recommend that agencies review the Minnesota Pollution Control Agency Stormwater Manual to determine an effective extensive street sweeping program, while noting that the cost effectiveness of chemical sediment treatment is the next best option.

Ross Bintner, engineering services manager with the City of Edina and the study’s technical liaison, says that by evaluating relationships impacting pond performance, such as phosphorous release from sediment and water oxygen levels, “This study demonstrated how modeling could value-test potential interventions.” 

—  Amy Friebe, CTS senior editorial manager

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Media contact

Michael McCarthy
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