Maplewood Mall rainwater runoff retrofit featured in symposium

The Maplewood Mall’s giant rainwater cistern (a very big watering can) is one of four special features used to catch, reuse, and clean rainwater. photos by Linda Baumeister/Review

Ornamental grasses and flowers and other flowers and shrubbery make up 55 rain gardens all around Maplewood Mall.

Informative signs explain features of the raingarden concept.

Gardens, trees, cisterns help reduce excess phosphorus

An urban forest is growing in a surprising area of Maplewood. Where one might expect to find a desert of paint lines, concrete and vehicles, an oasis of trees, gardens and art is popping up.

Last week, the Ramsey-Washington Metro Watershed District’s stormwater infiltration retrofit project at the Maplewood Mall parking lot was highlighted as part of the 2013 International Low Impact Development Symposium.

The symposium, which garners the attention of thousands of engineers, professors, environmentalists and other professionals each year, is hosted by the University of Minnesota at the St. Paul RiverCentre. The conference is a series of lectures and presentations about various programs across the Midwest that work to address urban water quality issues like stormwater runoff and sewer overflows.

This year, two presentations and a poster were dedicated to showcasing the Maplewood Mall Stormwater Infiltration Retrofit Project. RWMWD assistant administrator Tina Carstens and Erin Anderson Wenz from Barr Engineering tag-teamed one of the presentations, which explained the retrofit in detail.

Another presentation, entitled “Stormwater Art and Interpretation at Maplewood Mall,” explored the art features that were installed at the mall as a part of the retrofit project.

The retrofit was designed to treat watershed rainwater runoff before it reaches Kohlman Lake, which is situated about a mile west of the shopping center.

A watershed is a land area in which all the water that falls on it or runs under it flows to the same place. The RWMWD  is a 65-square mile area that contains bodies of water in St. Paul, Woodbury, Oakdale, Landfall, North St. Paul, Maplewood, Little Canada, White Bear Lake, Vadnais Heights, Gem Lake, Shoreview and Roseville, and eventually flows into the Mississippi River.

Excess nutrients a problem

A few years ago, the RWMWD took a look at Kohlman Lake, which is on the state’s “impaired waters” list. It identified excess nutrients as one of the lake’s main concerns, calling for a reduction in the total amount of nutrients, like phosphorus.

While phosphorus can be a beneficial nutrient for plant growth, it can also be detrimental to lakes if too much is introduced to the water, causing the rapid growth of aquatic weeds and algae. One easy way to tell if a lake has excess phosphorus is a large amount of algae present on the surface.

Phosphorus can enter a body of water in a number of ways, including grass clippings and yard waste that break down, releasing the nutrient, as well as surface pollutants like fertilizer and road runoff.

The RWMWD found that since Kohlman Lake is located in a mainly developed area with lots of roads and parking lots, much of the phosphorus entering the lake came from impervious surface runoff. After evaluating the lake’s watershed,

Maplewood Mall stood out as an area where a few small changes could make a big difference.

“The Watershed District took a look at Kohlman Lake, which is listed as ‘impaired’ by the state...because it has too much phosphorus,” Carstens said. “We had to find a way to treat the runoff in the watershed before it gets to the lake.”

The 35-acre Maplewood Mall parking lot was a clear setting for the retrofit project. RWMWD’s Kohlman Lake implementation plan had one basic goal: to reduce the amount of rainwater runoff entering the lake by at least one inch.

Multi-phase project

The retrofit began with phase one in 2009 after discussions with the Simon Property Group, which owns Maplewood Mall. The $600,000 initial phase was paid for with district funds and installed large rainwater gardens at the mall’s six road access points.

Phases two and three added infiltration tree trenches, rainwater garden and planter areas, porous pavers and a large cistern to capture roof runoff. The trenches are home to 375 trees, which are a mix of swamp white oak, honey locust, Amur corktree, Kentucky coffeetree, common hackberry and discovery elm—trees known for their heartiness.

These two phases, which were completed in 2011, were funded through a $500,000 grant through the Clean Water Fund and an additional $500,000 grant from the Minnesota Pollution Control Agency.

Also during this phase, a series of educational signs, exhibits and art were installed at the site to help inform the public about rainwater runoff in the area and what they can do to combat it. The cistern, which can hold about 1 percent of the mall’s roof runoff, also functions as an interactive pump and chime that also irrigates nearby rainwater gardens.

Finally, the last phase of the project was completed in fall 2012. Mall entrances were updated to include educational features and a grand opening was held in September 2012. Finishing touches including a mural and signs were completed earlier this summer.

The RWMWD says the four phases of the projects cost approximately $7 million.

A year later: progress

Now, nearly a year after its grand opening, the Maplewood Mall retrofit helps capture or infiltrate about 67 percent, or about 20 million gallons, of the parking lot’s runoff water, compared to a miniscule 3 percent before the project began.

The RWMWD estimates the 55 rainwater gardens filter about nine million gallons of runoff annually, with the cistern holding 5,700 gallons of roof runoff and tree trenches capturing about 11.2 million gallons. They estimate the 6,733 square feet of permeable pavers infiltrate nearly 260,000 gallons of water per year.

This rainwater mitigation is expected to reduce the parking lot’s phosphorus load by about 60 percent. The efficiency of the retrofit project is also projected to increase as the trees grow.

The project has gained attention across the state and nation, with word spreading even faster after the 2013 LID Symposium. The retrofit was recognized as the Minnesota Association of Watershed Districts’ “Project of the Year” last year.

With a little help from some rainwater gardens, trees and porous pavers, the newly-retrofitted Maplewood Mall is helping reduce the amount of pollutants in area lakes, one inch of rain at a time.

Johanna Holub can be reached at or 651-748-7822.

Sources of phosphorus

So where does excess phosphorus come from? There are several external and internal sources, Jennifer Koehler of Barr Engineering explained.

External sources include fertilizer, road runoff, leaking septic tanks, yard refuse like leaves and grass clippings, pet or animal waste, soil erosion and water treatment plants.

Internal sources of phosphorus include lake-bottom sediments and aquatic plants, especially the curly-leaf pond weed that grows over the winter and dies in the summer. As the plant breaks down, phosphorus is released into the water.

Luckily, a Minnesota law passed in 2007 does not allow the use of phosphorus-based fertilizers except in certain situations.

Koehler also explained that excess phosphorus does not pose a health risk to swimmers and others who use lakes, it just means they may have to be OK with getting a little green.

“Phosphorus isn’t necessarily a health concern to those who swim in an algal lake; it’s just a personal preference of ‘how green’ you are willing to go,” she said.

Residents can help curb the amount of phosphorus entering the lakes in a number of ways. One way is to limit the amount of leaves and grass that enters lakes and other bodies of water.

When you mow the lawn, dispose of mulch by composting it or using a yard cart.

If your home has a septic system, make sure it is inspected regularly to make sure it’s working properly and not leaking.


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