Monitoring Bio-retention Efficacy on Interstate 20 in Atlanta to Assess Water Quality
At the heart of the City of Atlanta and at the headwaters of Intrenchment Creek are the neighborhoods of Peoplestown, Summerhill, Mechanicsville, and Pittsburgh, which surround the Georgia State University Stadium. Over the years, interstate highways and other development in the Intrenchment-Custer basin have transformed the headwaters of the watershed into a landscape that consists of 90 percent impervious surface. The resulting high volumes of stormwater runoff coupled with significant quantities of pollutants are washed from the parking lots and highways into the combined sewer system and then to the Creek, causing flooding in streets and residents’ homes. In July 2012, heavy rainstorms within the Intrenchment Creek watershed’s combined sewer system, the Custer Avenue Combined Sewer Overflow (CSO), flooded many homes in the neighborhood of Peoplestown and Summerhill with sewage and stormwater. This event prompted the City of Atlanta to implement the Southeast Atlanta Green Infrastructure Initiative, which established green and gray stormwater practices in targeted locations in the watershed. However, this initiative did not address the significant volumes of stormwater runoff from the 87 acres of interstate highways at the headwaters of the Intrenchment Custer Basin. American Rivers conducted an assessment of the potential for green stormwater infrastructure (GSI) to provide stormwater runoff reduction to both alleviate flooding downstream and provide additional quality of life benefits associated with GSI. This assessment demonstrates the feasibility of using GSI to manage the 95th percentile storm event from portions of the interstates which are bounded by the overlap of the Turner Field Stadium Neighborhoods Livable Centers Initiative (LCI) and the Custer Combined Sewer basin. The assessment concludes that with retrofits such as curb-cuts and bioretention, the 1.8” storm (1.8 inches of rainfall in 24 hours) could be captured and infiltrated from 31.85 acres of the site, removing 1,275,243 gallons of stormwater from the combined sewer system (36% of the 3.5 million gallons generated) per event containing significant quantities of fecal coliform and sediment. The assessment also states that retrofitting the highways in the Intrenchment Custer basin will help reduce the burden of recurring floods on downstream communities, while simultaneously improving the overall livability of the community. To implement the LCI recommendations, in 2017 American Rivers and GDOT District 7 agreed to pursue a pilot green infrastructure project (the first of its kind in Georgia) to remove significant quantities of stormwater runoff from the Intrenchment-Custer combined sewer system. This project is currently in initial phases of construction, and the monitoring of the influent and effluent from the built bio-retention cells will determine how effective the bio-retention cells are to remove pollutants (nutrients, sediment, bacteria, and limited metals) from the stormwater prior to it entering the combined sewer system or creek. In addition, the monitoring will provide information on the volume of stored stormwater that can be accomplished prior to the stormwater from the Interstate entering the combined sewer or Creek. Monitoring prior to construction and after construction of the bio-retention cells will also determine how the flow changes, thereby spreading the release times of the stormwater out over time and reducing the potential of additional flooding contributed by the stormwater runoff from the Interstate.