Effect of Bioretention on Runoff Temperature in Trout Sensitive Regions

Although it has been established that thermal pollution from urban stormwater can negatively impact the aquatic ecosystem, there are few mechanisms available to reduce the temperature or urban stormwater runoff. A monitoring study was conducted at 4 bioretention areas in western North Carolina, located along the southeastern extent of United States trout populations, to examine the effect these systems have on runoff temperature and identify any design criteria pertinent to temperature reduction. Median runoff temperatures leaving the pavement surfaces, based on storm events, exceeded the 21°C upper avoidance temperature of trout at all sites during the months of June through September. Of the bioretention areas studied, the two systems that covered an area smaller than 10% of their contributing watershed were able to significantly (p<0.01) reduce both maximum and median storm temperatures between the inlet and outlet. At the two bioretention areas that were larger than 10% of their contributing watershed area, maximum effluent temperatures were significantly (p<0.01) cooler than influent temperatures; however, there was no significant (p<0.05) difference between median influent and effluent temperatures. Despite reductions in temperature, effluent temperatures were not significantly (p<0.05) cooler than the upper avoidance temperature for trout at any of the monitoring locations. There was evidence that substantial reductions in runoff volume occurred at all bioretention areas, especially the systems that were larger than 10% of their contributing watershed. Overall, monitoring results indicated that bioretention areas served as effective treatment mechanisms for reducing, but not eliminating, the thermal impacts associated with urban stormwater runoff.