Stormwater infiltration BMP impacts on groundwater quality

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In cooperation with DelDOT, DGS is monitoring groundwater at stormwater infiltration best management practice sites (BMPs) to characterize the fate and transport of chloride. The goal is to characterize the potential risks to groundwater quality from de-icing practices at both DelDOT-managed and shared-use (receiving runoff from roadways, businesses, schools, etc.) stormwater management sites. This project arose because increasing chloride concentrations were observed in a number of groundwater-supplied public water systems in New Castle County. Increased concentrations of radionuclides were also observed in some of the impacted water sources.

During the first phase of this project, staff visited a number of stormwater management facilities and performed GIS analyses to evaluate hydrologic conditions. This led to the selection of two stormwater management sites for characterization and monitoring. While visiting the sites, staff collected stormwater water samples. Water samples tested in the lab for chloride were compared to field-measured electrical conductivity to develop an empirical relationship between the two parameters. This allowed us to use automated sensor measurements as a proxy for chloride.

One of the two field sites is near Summit Bridge and is co-located with a pavement runoff and soil amendment project established by Dr. Paul Imhoff of the UD Department of Civil and Environmental Engineering. Four shallow monitoring wells and two runoff collectors with automated water level, temperature, and salinity monitoring sensors were installed in March 2019 to measure the salinity of roadway runoff and shallow groundwater. The second field site is a state-managed stormwater infiltration basin in Middletown. Here, twenty monitoring wells, and stormwater inlet and outlet monitoring systems have been installed. The monitoring wells are installed in clusters finished at different depths and equipped with automated water level, temperature, and salinity monitoring sensors to characterize infiltration and salt movement. Stormwater inlet pipes and the basin outflow weir are equipped with flow and salinity monitoring systems.

Using data collected from our monitoring network and stormwater and groundwater sampling, we developed a conceptual model of the movement of water and salt from the roadway through the management areas and into groundwater, as well as statistical tools to compute water and salt budgets from the sensor and grab sample data. The high temporal resolution sensor data show clear evidence of salty water from roadway drainage systems recharging the shallow aquifer and horizontal and vertical movement of salty water through the aquifer. It is likely that this salty water is impacting groundwater extending hundreds to thousands of feet from the study sites. Maximum chloride in groundwater is more than 11,000 mg/L at the Summit Bridge site and nearly 4,000 mg/L at the Middletown site.

Groundwater chemistry data show that infiltration of salty water has significant impacts on the major ion and trace element composition of groundwater. Read more about the impacts on groundwater quality in our publication: