The Illinois State Geological Survey is monitoring the quality of runoff from a Chicago-area tollway that is currently being expanded. As part of the expansion, bioswales are being installed to improve the quality of runoff into adjacent forest preserve lands. Highways have been shown previously to contribute to water-quality degradation in urban areas, especially related to metals, oils and greases, nutrients, chloride, and suspended sediment. Pre-construction runoff was sampled for two years at multiple locations directly from the tollway as well as from roadside ditches at future bioswale locations using automated samplers, passive tray samplers, and multi-parameter dataloggers. Flow-integrated runoff samples were collected and analyzed to calculate the total mass of contaminants being transported, determine loadings, and calculate the treatment effectiveness of the current runoff management system. Laboratory analyses confirm the presence of most expected tollway contaminants, such as metals, high suspended sediment loads, and very high chloride levels associated with de-icing activities. Existing ditches were shown to reduce contaminant levels, yet they also acted as sinks for later remobilization of contaminants. The bioswales are expected to increase residence time for sediment deposition and infiltration of runoff, and allow transformation of contaminants and removal of nutrients by plants. After bioswale installation, monitoring will focus on comparison of pre- and post-construction conditions to determine the effectiveness of runoff treatment.
|Published - 2010