Stormwater infiltration BMP impacts on groundwater quality

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

During the first phase of this project GIS analysis and field visits to a number of stormwater management facilities were done. Field visits included water sampling. This work led to the selection of two stormwater management sites for site characterization and monitoring. Water sampling is building empirical relationships between inexpensive field measurements of electrical conductivity and more time consuming and expensive laboratory measured chloride concentrations.

A site near Summit Bridge has been selected 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 systems were installed in March 2019 to characterize properties of roadway runoff and salinity of shallow groundwater. A second site near Middletown is located in a stormwater basin. 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 systems to characterize infiltration and salt movement. Stormwater inlet pipes and the basin outflow weir are equipped with flow and salinity monitoring systems.

Data generated by operation of the automated monitoring systems and sampling of wells and runoff has led to development a conceptual model of the movement of water and salt from the roadway through the BMP 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 then horizontal and vertical movement of salty water through the aquifer where it is likely that this salty water is entering groundwater flow paths that extend hundreds to thousands of feet from the study sites. Maximum salinities in groundwater are more than 5500 mg/L at the Summit Bridge site and more than 4000 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. Additional sampling and assessment is being done to frame the range of impacts on groundwater.