Assessment of the Impacts of a Biofiltration Best Management Practice (BMP) and Associated Groundwater Flow on Water Quality

• Main Author: Tupper, Jacquelyn E
• Other Authors: Paul P. Mathisen, Advisor
• Format: Others
• Published: Digital WPI 2015
• Subjects: BMP; HydroCAD; Stormwater; Groundwater; Water Quality
• Online Access: https://digitalcommons.wpi.edu/etd-theses/588; https://digitalcommons.wpi.edu/cgi/viewcontent.cgi?article=1587&context=etd-theses

Stormwater runoff from urbanized areas can have detrimental impacts on groundwater and surface water supplies by mobilizing contaminants such as bacteria and nutrients from surrounding areas. Best Management Practices (BMPs) are commonly designed to mitigate these impacts, but the processes governing the effectiveness of these BMPs are often not well understood. Biofiltration BMPs, which include storage, sediment removal, and infiltration processes, are particularly challenging to quantify. This research involved an investigation of the processes associated with a biofiltration BMP located in West Boylston, MA adjacent to the Wachusett Reservoir. The basin treats runoff from an 8-acre watershed with two roadways (Routes 12 and 110) and surrounding residential and commercial land uses. Water exits the basin by either seepage directly to groundwater or by seepage through a two-foot filtration bed to an outfall pipe on one side of the basin. A field sampling program was conducted in collaboration with the Massachusetts Department of Conservation and Recreation to characterize the various flow paths of contaminants upstream, within, and downstream of the biofiltration facility. The program included collection of volumetric flow information, field parameters (dissolved oxygen, specific conductance, pH, and temperature), and water quality samples. Samples were tested for alkalinity, bacteria, dissolved organic carbon, nutrients, additional anions and cations, and suspended sediments. Stormwater samples were collected for storm events that included substantial rainfall and illustrated seasonal variability. A set of seven monitoring wells installed for this project provided information on groundwater flow and quality at the site. The field program provided quantitative data on the flows and transformations that occur within and in the groundwater downstream of the biofiltration basin. The results demonstrated that stormwater infiltration to groundwater is an important component to consider for BMP design. The flow path through the outfall was effective in removing sediments, but was found to have limited capacity for water quality treatment, since only small changes in stormwater quality occurred between the culvert inflow, basin, and outfall samples. However, analysis of the flow data showed that infiltration to groundwater was comparable to discharge through the outfall. Furthermore, the signatures of stormwater infiltration could still be seen in the wells, indicating that the infiltration from the stormwater basin can impact groundwater quality. The groundwater pathway was found to impact the chemistry of the constituents, and was particularly effective in removing bacteria and phosphorus. The results demonstrate the value of groundwater recharge as a component of BMP design, and provide a basis for a number of specific design recommendations related to biofiltration basins.