Polluted subsurface soil and groundwater

The need for a new district courthouse parking lot and a Department of Environmental Protection regulation requiring the clean up of contaminated sites throughout the country recently merged. The result is a 2.4-acre soil and groundwater remediation and stormwater management project headed by the Massachusetts Division of Capital Asset Management (DCAM) and located in Taunton, Mass.

Previous uses of the site had seriously polluted subsurface soil and groundwater. Complicating the potential remediation was the extension of the site into an area of vegetated wetlands along the Mill River, making the site subject to the Wetlands Protection Act. As a result, both a soil remediation program and the installation of a stormwater management system to preserve the ecology and hydrology of the wetland and the river were required.

To move forward, the DCAM turned to GeoEnvironmental Inc. (GZA) to look for solutions to meet all regulatory and environmental requirements. GZA is a geotechnical, engineering and environmental remediation firm located in Norwood, Mass. A site assessment and a plan for remediation came first. It was determined that using onsite remediation technologies to treat groundwater and stabilize the soil would reduce the risk and cost of trucking materials, and this part of the project got underway.

As this was happening, GZA staff prepared the final design for the parking lot. Key to this design was an extensive stormwater management system to protect the Mill River. The stormwater system also needed to replicate pre-development infiltration rates while leaving the largest possible area for the future parking lot. Engineers decided on a system featuring separator structures and a large subsurface leaching system.

When the soil is used as a part of the treatment process for stormwater runoff, a large percentage of contaminates can be removed naturally and effectively. Subsurface stormwater management offers lower overall installed cost, optimum design flexibility, and enhanced performance.

The final design and installation of the stormwater management system in Taunton includes 220 StormTech SC-740 chambers to retain stormwater runoff from the parking area. The SC-740 chamber optimizes storage volumes in relatively small footprints. StormTech chambers feature an open bottom that allows them to recharge or infiltrate stormwater more effectively. They are injection molded, allowing precise production control and added overall strength not possible with thermoforming techniques. The chambers are also manufactured from polypropylene - a material inherently resistant to environmental stress cracking.

The separator structures used are a pretreatment device installed prior to inletting the chambers. They intercept 80 percent of Total Suspended Solids and oil, preventing significant sediments and hydrocarbons from accumulating within the chamber system.

StormTech chambers are designed to exceed the American Association of State Highway and Transportation Officials (AASHTO) Load Resistance Factor Design for Earth Loads and HS-20 Live Loads - important because they were being installed under a parking lot.

Because the chambers are designed to exceed AASHTO recommended design factors, we were confident that our structural requirements were going to be met, said Mike Conway, project engineer for GeoEnvironmental Inc.