Big parking lots collect large amounts of rainwater, washing pollutants into storm drains and then to nearby lakes or streams.
For years, the water quality in Kohlman Lake in Maplewood has suffered from too much phosphorus washed into the lake by stormwater runoff. The phosphorus caused the lake to turn green with algae, and as a result, was put on the state's impaired waters list by the Minnesota Pollution Control Agency.
Nearby Maplewood Mall and its parking lot cover over 70 acres of land. This area presented a unique opportunity to reduce or filter stormwater running off the mall's large roof and paved parking lots.
The primary goal of the Maplewood Mall Stormwater Retrofit project was to reduce phosphorus going into the lake from the mall by 60 to 80%. But the Ramsey-Washington Metro Watershed District wanted go two steps further — educate the public on stormwater quality by putting up highly visible and informative signs around the mall’s entrances and inspire nearby commercial area owners to implement similar projects in the watershed by demonstrating how these aesthetically pleasing features can be retrofit into the urban environment.
What was done
The Maplewood stormwater project included designing a chain of interconnected infiltration rain gardens and planter areas around the parking lot, as well as areas of porous pavement and sand filters.
In addition, stormwater is filtered through rock-filled trenches dug beneath the surface of the lot. The trenches support the growth of trees. Under the system, the trees are rooted in a rock medium that allows room for their roots to grow. A series of swales, catch basins, and drains direct runoff water into the trenches, where the phosphorus is absorbed by the trees. The rock layers also help clean the water before it reaches an under-drain that connects to the storm-sewer system and ultimately to the lake. In all, the project installed:
- 55 raingardens
- 6,700 square feet of permeable pavers
- 1 mile of tree trenches
- 375 trees
- 5,700 gallon cistern to catch roof runoff for irrigation
This project sets a strong example that demonstrates how stormwater retrofit projects can be implemented in commercial areas in an aesthetically pleasing way.
The system, funded in part by the Clean Water Land and Legacy Amendment, is designed to capture the runoff from a one-inch rainfall that falls on the parking lot, keeping pollutants out of nearby Kohlman Lake, which is impaired.
Prior to the project, only about 3% of the stormwater runoff from the mall’s 35-acre parking lots was captured, infiltrated, or filtered before running off to Kohlman Lake. Now that the project is constructed, 67% (2 million gallons) of the parking lot’s annual runoff is intercepted and made available to plants and tree roots for uptake, infiltrated, or filtered before it leaves the site. Total reduction in phosphorus is estimated to be about 13 to 18 pounds per year.
Find out more about other water quality efforts on our water stories page.