Publication Date

9-2007

Abstract

The site studied in this research project is owned by the Massachusetts Highway Department and has been used as a storage facility since the 1960's. The area has been characterized as a drumlin of glacial till composition. Improper storage practices occulTed during two decades (1960's - 1980's) by stockpiling deicing agents outdoors. Previous investigations verify groundwater contamination due to brine infiltration and establish several properties of the soil and the aquifer hydraulics. The chloride ion is unreactive and transported with groundwater molecules; however, field data and theory suggest that dissolved sodium partakes in cation exchange with sorbed cations on clay pmiicles. This work aims in describing and quantifying the distribution of dissolved and sorbed sodium frames as they are transported within the aquifer. The focal point of this study is the chemical profiling of the subsurface. This is accomplished by evaluating the cation exchange process and establishing the exchange equilibrium via a series of experiments. The significance of experimental results lies in the chemical homogeneity and uniformity of both glacial till layers. This conclusion is derived primarily from soil analyses that generated consistent cation exchange capacity at 5.7 meq/100 g, soil pH at approximately 7.60 and 0.5% organic matter content throughout depth. Furthermore, the exchange isotherms resulted in an average selectivity coefficient K equal to 0.00477 (lJg)1I2 at equilibrium, which conforms to the Gaines-Thomas convention and is in good agreement with reported values. Development of a one-dimensional steady state transport model that incorporates cation exchange reactions resulted in a system of differential equations that describes progression of dissolved species via advection and highly delayed migration of sorbed sodium reference frames. Field measurements of groundwater composition for three wells that experience rising source contributions are implemented in the calibration of the experimentally derived selectivity coefficient and provide a range of 0.004 to 0.0075 (L/g)I/2 which includes the experimental value. The method of characteristics is applied to uncouple the sodium and divalent cation concentrations and the forward finite difference method formulates a unique solution for predictions of the advective transport of dissolved and sorbed sodium frames. Model theory in conjunction with groundwater measurements from a ten-year period of data collection, and established equilibrium geochemistry, provides insight and describes qualitatively the general transport behavior of dissolved and sorbed sodium. Predictions on contamination in the subsurface are specified by three distinct leading regions. Primarily, ambient concentrations prior to contamination prevail. Once total dissolved concentrations anive at the water table they are advected with groundwater molecules however the corresponding sorbed cation frames remain at ambient concentrations due to high retardation factors. The last region is characterized by rising frames of all phases but occurs after prolonged periods of transport time. A finite difference method is employed to simulate this prediction. Sorbed sodium charactelistics for well cluster C follow the general prediction trend and provide a continuous solution for wells that experience rising total dissolved cation concentrations.

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