State transportation agencies are a potential stakeholder in the total maximum daily load process and, in some cases, NPDES stormwater permits could be issued to the respective transportation departments. Issues facing the transportation industry include the incorporation of waste load allocation in the NPDES permit, and implementation of appropriate methods to develop waste load allocations and compliance strategies. Based on several stormwater and BMP monitoring projects completed over the last ten years, we have developed a better understanding and a database to characterize pollutant loads from roadway runoff. We continue to work toward the development of a TMDL implementation methodology to assist the transportation industry.
In this paper, we will present an analysis of incorporating the highway network as a separate land-use categories into a watershed model. This particular model defines watershed land-use into forest, shrub/grass, agriculture, developed, barren, wetland, and water. Each of these land-use categories is associated with a set of system coefficients including imperviousness, interception storage, dust fall collection efficiency, canopy characteristics and plant productivity. These coefficients must be modified for the highway land-use category. The model can then be executed to derive the respective pollutant loadings for each land-use category including highway land-use. We then compare the pollutant loadings from our database of highway runoff projects with that obtained from the watershed model. Several modeling scenarios have been attempted to assess the sensitivity of model coefficients. Results of our attempt will eventually provide guidelines for treating roadways as a separate, linear land-use category and developing the concept of a transportation separate stormwater sewer system (TS4).