Steady groundwater transport of highway deicing agent constituents from an infiltration basin

The highway deicing agent groundwater plume from an infiltration basin in the Plymouth-Carver Aquifer near State Route 25 in southeastern Massachusetts is modeled and measured in order to assess the impact of the basin on groundwater transport processes. The advective transport model superimposes the existing steady models of axisymmetric basin hydraulics of Zlotnik and Ledder in 1992 and the two-dimensional ambient flow of Gelhar and Wilson in 1974. The basin component incorporates a surface source of finite-radius into a Hankel transform model for unconfined aquifers, whereas the ambient component varies linearly in the horizontal and vertical directions. Contaminant streamlines describe the resulting groundwater plume, and highway deicing agent constituents provide useful tracers. A simple vertical dispersion model quantifies the spread of contaminants across the bottom of the plume. Deicing agent constituent data calibrate a 17 m basin radius, a 45 m plume width, a bottom streamline 6-10 m below the water table, and a vertical dispersivity of 34 cm. The latter value is comparable to the amplitude of vertical excursions caused by storm scale fluctuations of hydraulic head, as measured by Ostendorf in 2007. Infiltration basins alter ambient advection by displacing streamlines downward and augment vertical mixing by imposing aperiodic vertical fluctuations on the ambient flow field.