Phosphorus pollution research provides guidance to stormwater pond management

By John Chapman

Minnesota has amazing surface water resources, from Lake Superior to the Boundary Waters to the local lakes such as lake Phalen. Unfortunately, not all of these resources are fully usable because of pollutants that flow into the lakes and rivers each times it rains. Wet ponds are common stormwater management practices and have been used for decades to trap pollutants to help protect surface water resources however, there are still many questions about how well stormwater ponds function and how they can be managed to best protect the lakes and rivers.

Recently completed research from multi-disciplinary teams at the University of Minnesota is providing answers to some of the questions about how stormwater ponds are working. Common pollutants trapped in wet ponds include sediments and phosphorous. The belief has been that phosphorous is attached to the sediments, which settle to the bottom of the pond and stay there because of gravity. The research found that complex water chemistry similar to the complex chemistry cycles found in deeper natural water bodies are also occurring in these shallow stormwater ponds. The water chemistry unfortunately results in the phosophorous detaching from the soils and flushing out of the ponds and back into the lakes and rivers. The research suggests that even excavating the sediments from the pond can result in the phosphorous pollution washing out of the pond. With this new understanding, researchers plan to create new guidance for better operations and maintenance of these systems.

This research also looked at a carcinogenic pollutant, polycyclic aromatic hydrocarbons (PAHs), not originally considred when many of these wet ponds were designed and built. While the ponds were not specifically designed to trap the carcinogenic pollutants known by the acronym PAH, these PAH's are being trapped and kept out of our lakes and rivers, which is the good news. The bad news for the cities who own these ponds, is they now own these PAH's in the pond sediments. The research found that about half of the ponds evaluated has PAHs in the sediments, but did not determine a single source of these PAHs. The research did look at how likely these PAH's would be absorbed into people or animal tissue, and the results suggest that these pollutants are unlikely to move out of the pond or become absorbed into our body tissue.

There is a lot more to know about stormwater management, including some of the practices that have been used for many years. While the phosphorous has been found to be more mobile or less predictable if it stays put, other new pollutants are being found and trapped as well. Part of this research also looked at additional research questions that cities and practitioners have across Minnesota in attempt to keep the research aligned with the state needs.

UMN departments contributing to this research project include: the College of Science & Engineering, School of Public Health, St. Anthony Falls Laboratory, Ecology, Evolution and Behavior, Civil, Environmental, and Geo-Engineering and the Water Resources Center.