Pond helps keep pollutants out of Tanners Lake

Have you ever noticed the pond just south of 10th Street/Minnehaha Avenue, on the Maplewood side of Century Avenue? You may have observed that it was looking a bit gunky this past summer. Some of you saw what looked like dredging this fall, and may have wondered just what was happening there.

Construction of the pond was a result of problems (and opportunities) related to nearby Tanners Lake. Back in the mid-1980s, the lake, located east of Geneva Avenue and north of I-94, was listed on the Minnesota Pollution Control Agency (PCA) Impaired Waters List for excess nutrients. Still, the Ramsey Washington Metro Watershed District (RWMWD) saw great potential for improving water quality, and they began to set goals and complete projects to support swimming, fishing and boating at Tanners. They targeted the nutrient phosphorus, which was polluting the lake and causing algae growth.

RWMWD built the pond you see on Century Avenue in 1998 as one of the key projects in its plan to try to stop pollutants before they could reach Tanner's Lake. The innovative system at this particular pond treats most of the stormwater from the watershed by injecting controlled amounts of alum into the creek as it flows past the alum injection facility next to the pond. The alum in the stormwater attaches to the phosphorus particles in the stormwater, causing the phosphorus particles sink to the bottom of the pond before the pollutant can flow into the lake.

Every few years, the pond needs to be cleaned, through dredging or other methods, to remove the alum floc (the "gunk") from the bottom of the pond. RWMWD normally waits until winter to clean the pond but decided to move faster this year because heavy spring rains and increased alum doses filled up the pond faster. This dredging activity is what you may have noticed this fall.

This facility has been very successful in improving water quality at Tanners Lake, with decreased algae and better water clarity. Although water quality in the lake varies each year due to rainfall and temperature, total phosphorus levels have been stable at around 30 ug/l (micrograms per liter), a great improvement from around 50 ug/l (peaking sometimes at 100 ug/l) before the alum plant was built.

The alum treatment facility and other watershed district projects are key to water quality, but you and I are still responsible for doing our part to keep our lakes and wetlands clean. The watershed website at rwmwd.org is a great resource for water quality best practices. I'd also encourage you to join us at Oakdale Environmental Commission's annual Tanners Lake cleanup next spring!

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