David W. Ostendorf
Deicing materials that include calcium magnesium acetate (CMA), road salt, and premix are applied over a 1.2 mile section of the highway on State Route 25 (SR25) in Plymouth, Massachusetts. This part of the highway was designed to route runoff to an infiltration basin. The main goal of this research was to evaluate when and where deicer contamination infiltrated to the groundwater table at the site, with a focus on the infiltration basin. This project evaluated the steady-state hydraulics of the site, as well as the fate and transport of the deicing materials. There are many sources of data available for modeling the Plymouth-Carver . Aquifer at the site. Monthly groundwater monitoring provided hydraulic head and specific conductivity measurements and ion concentrations in groundwater sampled from wells located in and around the infiltration basin. Groundwater specific conductivity measurements were available from nine permanent conductivity points (PCPs) located in and around the basin. Slug test data collected from one deep well (BO) and one shallow well (BP) were used to confirm the calibration of the permeability of the aquifer material. A rain gauge located in the infiltration basin established the amount of precipitation that fell on the subject site area, and was also compared to data collected by the UMassCranberry Station located a few miles west of the basin. Surface water measurements of hydraulic head and specific conductivity collected at five to ten minute intervals at the main exit weir (West Weir) in the infiltration basin were used to characterize the volume and quality of water entering the infiltration basin. Once the hydraulic model was calibrated, it was first used to recover a constant permeability for the aquifer. A permeability of 1.5 x 10.10 d was found, a value which fell within the range indicated by the slug test data. Next, the transverse and vertical limits of deicer contamination were predicted from the modeL The plume was determined by the model to be approximately 20 m wide in the east-west direction, and as deep as 12 m below the water table. This result agreed well with past studies. The model was then used to hindcast travel times and source locations of deicing constituents in the groundwater. As expected, results confirmed that the infiltration basin is the largest source of deicer contamination at the site, and source isopleths created from hindcasted data showed deicer infiltration peaking in the winter and in the spnng. Monthly average specific conductivities of the groundwater in the basin were hindcasted with the model for a nineteen month period (which included the deicing seasons of2002 - 2003 and 2003 - 2004) and were compared to what was observed in discharge over the exit weir. The hind casts for the first deicing season compared well to what was observed over the weir, while the second deicing season was underestimated by the illndcasted results. However, these results may be attributed to the basin being deeply frozen over the course of that winter, thus not allowing the water discharging over the weir to infiltrate the groundwater. The mass fluxes of each conservative deicer ion were compared to the specific conductivity fluxes as ion flux ratios for an entire year after deicing activities began (November 2002 - November 2003). The ion flux ratios found over the course ofa year for cr suggested a decreasing trend, which was also indicated for the water discharging over the weir by a recent study. Tills showed the high mobility and solubility of cr compared to the other ions. Finally, the mass ratios for each deicer ion was hindcasted for the same year and compared to the composition of the deicers applied. The ratios hindcasted compared favorably to what was recorded. This steady-state particle hindcasting analysis may be a useful tool in determining relative orders of magnitude of deicer contaminant.