University of Vermont Transportation Research Center

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Transportation infrastructure is a major source of stormwater runoff that can alter hydrology and contribute significant loading of nutrients, sediment, and other pollutants to surface waters. These increased loads can contribute to impairment of streams in developed areas and ultimately to Lake Champlain. In this study we selected six watersheds that represent a range of road types (gravel and paved) and road densities (rural, suburban, and urban) present in Chittenden County, one of the most developed areas in Vermont. The location and density of road networks were characterized and quantified for each watershed using GIS analysis. Monitoring stations in each watershed were constructed and instrumented to measure discharge and water quality parameters continuously from spring through early winter. Storm event composite samples and monthly water chemistry grab samples were collected and analyzed for total nutrients, chloride, and total suspended sediments. Results from this study show that road type and road density are closely linked with the level of impairment in each watershed. Total phosphorus and total nitrogen from storm event composite samples and monthly grab samples significantly increased along a gradient of increasing road network density. Chloride concentrations increased several orders of magnitude along this same gradient. With the exception of Alder brook where total suspended sediment (TSS) concentrations tended to be high, there were no significant differences in TSS concentrations between rural and developed watersheds. The event TSS concentrations in the rural streams were slightly higher than we expected and the event and base TSS concentrations in the developed streams were somewhat lower than we expected, suggesting that the unpaved roads in the rural watersheds might contribute to stormwater runoff loads and that sediment control, at least, in the developed watersheds might be fairly effective. The overall results from this study show that local roads are significant source of impairment for streams in the Chittenden County area. Most of these roads are municipal roads that are not under to management of the Vermont Agency of Transportation. Thus, local actions will be necessary to control runoff from these roads.