Stormwater quality documentation of roadside shoulders borrow ditches




Kearfott, Pamela Jane

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Nonpoint source pollution is an environmental problem that is a concern among regulatory agencies and water quality professionals. A portion of this pollution is conveyed to receiving waters by stormwater drainage from highways, often via vegetated roadside shoulders, also called borrow ditches. Vegetated filter strips are recognized by many regulatory agencies as a Best Management Practice for the control and treatment of stormwater; however design parameters such as length, width, and vegetative cover are not specified. Therefore, it is important to evaluate and document the extent to which these vegetated areas may reduce pollutant loads in runoff and to mitigate the effects of discharging untreated highway runoff directly into receiving bodies of water. The primary objective of this study is the documentation of the stormwater quality benefits of these vegetated sideslopes typical of common rural highway cross sections in Texas. The scope of this project included the selection of three sampling sites in the Austin area that met a predetermined set of site criteria; the installation of 12 passive stormwater samplers and collection systems; monitoring of the sites and collection of runoff samples from storm events over a 14 month period; laboratory analyses of each of the runoff samples; compilation of the results into a database; statistical and graphical analyses of the results to determine differences between sites; and the evaluation of the performance of each of the vegetated filters and recommendations of site conditions conducive to maximum pollutant removal. Results from this study indicate that significant removal of some pollutants occurs over the width of vegetated filter strips, often within the first four meters of the edge of pavement. The results also indicate that vegetation density has a direct effect on the performance of vegetated filter strips. Dense vegetative cover within close proximity to the road surface and vegetative covers of at least 90% are recommended to allow for maximum pollutant removal. The effects of a permeable friction course on the quality of runoff leaving the road surface were also examined. These results indicate that the porous surface appears to have a substantial impact on the quality of runoff leaving a road surface. These improvements in water quality are as great, if not greater, than the improvements gained from the vegetated filter. Overall, the results from this study indicate that vegetated filter strips should be utilized by TxDOT as a best management practice for controlling and treating stormwater runoff from Texas’s highways. These filter strips demonstrate consistently high removal efficiencies for many of the pollutants of concern in stormwater runoff. In addition to providing water quality benefits, these vegetated areas are inexpensive and easy to implement, are easy to manage, and provide aesthetic benefits to the surrounding environment