1. Microbial Characterization At A Site Exposed To Highway Runoff And Deicing Agents 2. Chloride Dispersion Across Silt Deposits In A Glaciated Bedrock Valley

Highway surfaces accumulate a wide range of pollutants (suspended solids, heavy metals, inorganic salts, aromatic hydrocarbons, oil and grease, rust and rubber particles, deicing agents, etc), which are washed off during rain or snow storm events into nearby waters or lands. This dissertation presents the impact of highway runoff to soil and groundwater. The dissertation is composed of two major distinct parts. First part assessed the bacterial community diversity in highway runoff contaminated sediment from an infiltration basin located in Plymouth, MA that had undergone nineteen years of acetate-based de-icing agents addition followed by three years of acetate-free de-icing agents. Sediment sample from four drilled soil cores collected in two field events documented the change in bacterial community structure with depth and distance from the infiltration basin origin. To meet the objective, a variety of multi-disciplinary techniques were employed. Soil samples were collected, using drilling and core barrel / freeze-shoe coring methods, from different locations and depths to obtain vertical profiles of bacterial distribution along groundwater plume. Analyses of groundwater dissolved oxygen and pH, physical and chemical properties of the soil complemented molecular phylogenetic determinations to distinguish ambient and contaminated plume zones. Analysis of sediment samples from four drilled soil cores by means of 16S rDNA PCR indicated an overall high bacterial diversity both into the plume and into the ambient aquifer, with no prominent members within the communities. Sequence analyses provided evidences that each sediment sample displayed a specific structure bacterial community. Proteobacteria -affiliated clones (50%) predominated in all samples, followed byActinobacteria (14%), Firmicutes (10%) and Chloroflexi (9%). Statistical analysis revealed that the levels of Fe(II) and dissolved oxygen were strongly correlated with bacterial communities. The plume in the infiltration basin was anaerobic and iron reducing. As iron levels declined as oxygen levels increased below the plume bottom, there was a gradual shift in the community structure toward the increase of aerobic bacteria. These data indicate shifts in microbial communities in correlation with depth, substrate and oxygen availability in a deicing agent impacted subsurface. The second part of the dissertation investigated the subsurface fate and transport of deicing materials on groundwater quality across silt deposits in Dedham, Massachusetts. The deposit analyzed consists of two layers: a silty sand floodplain that is underlain by an aquitard, comprised mainly of silt. The aquitard overlies a confined aquifer, that is used as a public water supply. Soil and groundwater from the Neponset River aquitard and overlying floodplain deposit that receive high concentrations of deicing agents from nearby highways were collected and analyzed. Soil and groundwater samples collected over a period of four years from boreholes and 10 wells grouped in two well clusters were analyzed for dissolved chloride concentration. These data calibrate a one-dimensional, vertical dispersive transport model across the deposits. The flow and contaminant transport in floodplain is controlled by the aquitard presence and its low permeability. Soil and groundwater quality data confirmed a high chloride concentration at the floodplain surface near the highway runoff drainage outlets. These data confirmed the aquitard\u27s capacity to contain deicing agents over decadal time frame, protecting the underlying aquifer from contamination