How to Stop Ponds From Releasing Phosphorus

Authored by: Vinicius Taguchi, St. Anthony Falls Lab, University of Minnesota

Stormwater ponds are designed to leverage the natural relationship between phosphate, iron, and oxygen. In the presence of oxygen, iron maintains a 3+ charge, which binds it to the 3- charge of phosphate, allowing them to precipitate out of the water and remain in the sediment. However, in an anoxic environment with strong temperature stratification, the water does not maintain sufficient dissolved oxygen at the bottom, and iron changes to a 2+ charge. This no longer fits with the phosphate 3- charge so the iron and phosphate are released into the water. This webinar discusses the recent research on the impact of chloride introduced from winter road treatment, the relationship sheltering from trees has on stratification, and the importance of mixing the water column throughout the year.

KEY TAKEAWAYS FOR POND AERATION

• Add aeration
• Minimize sheltering that shields wind access to the pond
• Eliminate or discourage conditions that prevent mixing and cause temperature stratification
• In the Q&A section of the recording, a webinar participant mentions solar aeration systems being used in the Mid-Atlantic area. The author agrees that using solar or wind power would be preferred over the diesel generators that he sees used in Minnesota. (BRIEFS Editorial: At the time of the recording, the author is unaware PondHawk^® and the benefits it delivers.)

GENERAL TAKEAWAYS

• Load reduction: While this is cost-effective and long-lasting, it is the most difficult. As long as phosphorus is entering the pond, it is going to accumulate and dissolve in the water. Organic matter from leaves entering the pond can be another source of nutrients.
• Pre-treatment: Stop nutrients from entering the pond. Consider landscape management strategies that prevent trees from growing on the edge of the pond.
• Dredging may expose underlying soils that are full of phosphorus and organic matter and may not solve the nutrient issue.
• Treat internal phosphate loading directly with aluminum. Used in large lakes to bind phosphorus to aluminum which is no longer sensitive to oxygen as with iron. This treatment requires skilled staff and equipment, making it cost-prohibitive in small ponds. Too much aluminum in the water can be toxic for wildlife and plants.

Published: Webinar: Chesapeake Stormwater Network April 2019

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