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The MPRB and the Hiawatha Constructed Wetland

April 27, 2019

The MPRB proposal would build a "constructed wetland" on the Hiawatha Golf Course property, although, MPRB personnel say that they will be restoring this property back to its former self as a "natural wetland". The following information has been obtained from Environmental Protection Agency documents about Constructed Wetlands.

Artificial [constructed] wetlands are wetlands that have been built or extensively modified by humans, as opposed to natural wetlands which are existing wetlands that have had little or no modification by humans, such as filling, draining, or altering the flow patterns or physical properties of the wetland. (2)

SaveHiawatha18's statement: Based on the EPA definition of "natural wetlands", the Hiawatha Golf Course property has not been a natural wetland since it was modified in the 1930's, and it can never again be considered a "natural wetland" because it is a product of extensive modifications by human beings ,and extensive changes in the neighborhoods surrounding the property.

The following information about site selection for constructed wetlands comes from EPA documents:

  • The site should be above the water table and not be in a floodplain. (1)

    SaveHiawatha18's concern: Hiawatha Golf Course is in a floodplain.

  • The site should contain soils that can be sufficiently compacted to minimize seepage to groundwater. Although peats are common in natural wetlands, they are not preferred soil for establishing contructed wetlands. (1)

    SaveHiawatha18's concern: Hiawatha Golf Course is heavy in peat.

  • A large buffer zone should be placed between the wetland and neighboring property. The wetland should not be placed next to the edge of neighboring properties. (1)

    SaveHiawatha18's concern: The current plans put the wetland across the street from homes; i.e., within about 30 feet, and removes the large buffer zone that currently protects the low-lying homes.

  • The effectiveness of a constructed wetland in treating wastewater or stormwater is related to the retention time of the water in the wetland. High volumes of water through a wetland reduce the effectiveness of the wetland. (1)

    SaveHiawatha18's concern: Lake Hiawatha has one of the highest volumes of water running through it (lowest water residency rates) of any lake in the State of Minnesota.

  • Misconception - Constructed wetlands can remove significant amounts of phosphorus. Phosphorus removal in constructed wetlands is limited to seasonal uptake by the plants, which is not only minor compared to the phosphorus load in municipal wastewater, but is negated during the plants' senescence [condition or process of deterioration with age]. New plants growing in a freshly planted wetland will uptake more phosphorus than a mature wetland. Also, newly placed soils or media will have a greater phosphorus sorption capacity than a mature system which will have most sorption sites saturated. Within one or two years of start-up, removal of phosphorus will decline. (2)

    SaveHiawatha18's concern: The MPRB has not been diligent in maintaining the 18-hole golf course or the constructed wetlands at Lake Nokomis. If the MPRB does not maintain this new wetland, how will it serve the stated purpose of phosphorus removal over time, or will it just turn into a phosphorus saturated swamp?

  • Sources:
    (1) EPA - A Handbook of Constructed Wetlands
    (2) EPA Booklet - Constructed Wetlands Treatment of Municipal Wastewater