David W. Ostendorf
The research site is located in Plymouth, Massachusetts, above the Plymouth Carver Aquifer. It is positioned along side State Route 25 which opened in the summer of 1987 in order to extend Interstate 495 to the Bourne Bridge. Snow/ice removing measures using salt, and an alternate deicing agent, CMA have been practiced since the winter of 1987. The highway runoff is collected into an infiltration basin, allowing salt and CMA to infiltrate the sandy aquifer below. The salt and CMA increase the specific conductivity of the groundwater while, the infiltrated acetate is biodegraded, leading to DO depletion within the aquifer. Once anoxic conditions are reached, any further acetate loaded into the aquifer is degraded anaerobically.
The objective of this research was to develop and calibrate, steady state, analytical models of specific conductivity and DO transport through the aquifer. The models feature advection, vertical dispersion and for the dissolved oxygen, first order decay. The vertical dispersivity parameter results would be used in the reactive model because it contributes to the mixing of the plume, therefore accounting for the microbial activity.
The data used to calibrate the model included monthly groundwater measurements of specific conductivity and dissolved oxygen collected and analyzed from October 1999 - October 2007. Groundwater quality data were obtained from four clusters of a total of 3 7 monitoring wells that allowed for a more detailed study of the vertical variation of parameters. Other data used in this research included monthly rain data and the Massachusetts Highway Department reports of deicing agent solids applied over seven complete road-deicing seasons.
The specific conductivity data calibrate the elevation of the plume bottom, the source concentration C~, and the vertical dispersivity. The bottom elevation decreases with distance from the basin as expected. The result for the second parameter searched in the transport model, C infinity was checked against the expected values based on the average amount of deicing agent applied yearly in the research area section ofSR-25. If the model is correct and the average C infinity predicted is as much as 1200 microsiemens / cm, then 56% of the deicing agent material drifted off the highway. The average vertical dispersivity result for all of the well clusters used in this project was 0.77 m. The first order decay rates results from the reactive transport model, suggest that the decay rate is zero order near the source and first order within time and distance away from the source. This behavior is exhibited in a Michaelis-Menten kinetics model.