Road salt plays an important role in keeping our roads safe while driving during the winter, but once that salt leaves the road surface it can impact our water resources. Many studies have found that salt levels in streams in the northern US increase not only after road salt application, but also show a long-term increasing trend in baseline levels from year-to-year. To reduce the amount of salt going into our surface waters, runoff from parking lots and roads can be diverted into stormwater management areas. There are many different types of stormwater management practices and infiltration basins are one option. At infiltration basins, stormwater infiltrates through surficial soil and recharges the water table. During the winter, this stormwater may contain high salt concentrations.
A recent publication in the Journal of Environmental Quality by DGS staff member Rachel McQuiggan and retired staff member Scott Andres finds that stormwater at an infiltration basin can impact groundwater in different ways, depending on the salt content. By installing clusters of monitoring wells with different depths, they observed salty groundwater moving through only a portion of the total thickness of the aquifer, and transport depended on geology and permeability. Two geochemistry mixing processes were identified from groundwater sampling. The first - salty stormwater mixing with existing groundwater during winter and early spring - led to sodium retention beneath the basin and mobilization of radionuclides (radium-226 and -228) to groundwater. The second - occurring from late spring through fall, as fresh or low-salt-content stormwater mixed with salty groundwater - resulted in sodium release to groundwater. Sodium retention can lead to a lag time between chloride and sodium movement in the aquifer. This is an important consideration when monitoring salt export from a watershed. While total radium concentrations at this site did not exceed any regulatory criteria, continued salt input into the aquifer may result in radionuclide mobilization in other areas, as salty water moves away from the infiltration areas.
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