When Space Is Tight, Take Stormwater Treatment Underground

Step back in time to 1962, and you’d find Stillwater’s Lily Lake beach crowded with swimmers. Though small, Lily Lake was considered to be one of the deepest and clearest lakes in the Twin Cities area, much like Square and Christmas Lakes, and folks came here from around the region to swim, relax, and enjoy sunny summer days. Things began to change during the 70s and 80s, however, when new homes, roads and businesses were built on what was then the west side of town. To avoid flooding, road engineers directed stormwater runoff from these new developments into Lily Lake and, over time, water quality began to decline.

A black and white photo from 1962 shows Stillwater’s Lily Lake bustling with sunbathers.

By 1998, local citizens had launched the Save Lily Lake campaign to keep stormwater runoff – and the sediment and nutrients it carries – out of Lily Lake. During the next twenty years, Stillwater and the Middle St. Croix Watershed Management Organization (MSCWMO) developed a lake management plan and implementing dozens of lake-improvement projects.  Projects included stabilizing eroding ravines, redesigning the city-owned boat launch with porous pavers and native plants along the shoreline, installing raingardens in surrounding neighborhoods, and retrofitting parking lots and stormwater ponds in commercial areas.

Residents and local government have worked hard over the past 15 years to restore water quality in Lily Lake with on-the-ground projects to reduce runoff pollution.

To date, we’ve cut phosphorus flowing into Lily Lake by 100 pounds per year and water monitoring data shows that the lake is gradually improving as a result. However, we know that phosphorus levels in Lily Lake are still higher than they should be. A 2006 study conducted by the City of Stillwater found that phosphorus needed to be reduced by 145 pounds per year to reduce algae, meaning we have 45 pounds to go to achieve that goal. Unfortunately, finding ways to reduce stormwater runoff in a fully-developed community can be challenging.

In St. Paul, Capitol Region Watershed District (CRWD) faced a similar challenge in 2007 when they began looking for ways to reduce stormwater runoff to Como Lake. Eventually, the watershed district installed underground treatment systems in addition to raingardens and a new stormwater pond. The Arlington-Pascal Stormwater Improvement Project features eight underground infiltration trenches, built beneath Arlington and Nebraska Avenues, and a large underground stormwater storage and infiltration facility built beneath parkland. Runoff from the neighborhood now flows into perforated pipes beneath the streets where it is filtered and soaks into the ground, instead of flowing into Como Lake.

Here in Stillwater, the Brown’s Creek Watershed District installed an underground treatment system at Brown’s Creek Park in 2016 to cool-down parking lot runoff that was making the creek too-hot for trout. Now, the City and MSCWMO are considering an underground infiltration system to protect Lily Lake from stormwater pollution. According to Mike Isensee, MSCWMO’s administrator, an engineering feasibility study shows that a 10,000 square foot underground system at Washington Square Park could keep 18-25 pounds per year of phosphorus out of Lily Lake – nearly half of what is needed to meet the lake’s water quality goal. The infiltration chamber could be installed between the trees and the ball field at the park, with little visual impact other than during construction, and would be capable of treating runoff from 33.7 acres of land for up to a 1.1 inch rainstorm.

Photos from the Capitol Region Watershed District show what the Arlington-Hamline stormwater treatment system looks like below and above ground.

Over the past ten years since Capitol Region Watershed District installed its underground infiltration system in the Arlington-Pascal neighborhood, the district has continued to monitor the project’s effectiveness and results show that the system is functioning properly and capturing approximately 20 pounds of phosphorus per year. Water monitoring data and engineering models suggest that a similar project in Stillwater could go a long way towards nursing Lily Lake back to good health. The WMO will be meeting with city officials and local residents this spring to discuss this and other potential lake improvement projects and, with public support, hopes to apply for state funding to begin work in 2019.

To learn more about efforts to improve water quality in Lily Lake, go to www.mscwmo.org or attend the upcoming public meeting on Wed., March 28, 6-8pm at the Stillwater Library.