The city of Cudahy, Wisc., is both an MS4 community and part of the Milwaukee River Basin TMDL., meaning it is required to remove suspended solids and phosphorus from storm water runoff that flows into Lake Michigan and the Kinnikinic River. The city is also committed to integrating green infrastructure practices into public works improvement projects that will include high-performance permeable pavement systems and vegetated biofilters, Project Manager Todd Weik of CBC Engineers said.
In 2016, the public works department was developing projects to replace an alley, called the Green Alley, the public Squires Avenue Parking Lot and the front of the city hall parking lots because of deterioration, all of which are in close proximity to redevelopment.
The city was introduced to air-cooled blast furnace slag (BFS) to use as a phosphorous control product. Based on past steel slag experiences, the city’s solid waste staff was hesitant to use BFS. Weik said there were concerns over pH levels and potential impact on freshwater fish. An ecotoxicity test showed a mortality rate for water fleas and fat head minnows of less than 5%. Based on these findings, department staff approved BFS as long as it was contained.
The city chose to use a permeable articulating concrete block to capture and transfer storm water under the alley and city hall parking lot. The subsurface aggregate storage gallery was made with semi-permeable geotextile membranes that allowed the water to pass through them at a rate of 175 gpm. High-performance biofilters were used to meet the water quality needs of the Squire Avenue lot.
Cudahy officials have implemented a three-year monitoring program of the discharge for both project areas. Specific storm water catch basins were identified as sampling locations, and an early sample from the Squire Avenue lot and Green Alley shows that the ortho phosphorus has been eliminated and the total phosphorus has been reduced by 90%.