Preliminary Results of the Examination of Thermal Impacts from Stormwater BMPs

The preliminary results to examine the 4 years of monitoring of runoff temperature for a range of stormwater management BMPs and the affects on stormwater runoff temperatures in relation to established environmental indicators are presented for a study in Durham, NH. Research indicates that thermal impacts are not limited to summer months but include other times important for life-cycle considerations. Stormwater BMPs examined include conventional, Low Impact Development, and manufactured treatment designs. Surface systems that are exposed to direct sunlight have been shown to increase already elevated runoff temperatures, while other systems that provide treatment by infiltration and filtration can reduce runoff temperatures by thermal exchange with cool subsurface materials. Results indicate there is an increase in the thermal mass of runoff from a storm sewer system draining an area of asphalt pavement with a 97% impervious cover. For the storm sewer system, the mean EMT value of 53.5°F (± 12.7°F), which is greater than the mean groundwater temperature, which resembles the mean daily average annual air temperature, of 47°F that feed coldwater streams (NCDC 2005). The examination of stormwater BMPs indicates that the larger surface systems will see greater thermal variations and the larger subsurface systems will see greater thermal buffering. One large surface system, the Retention Pond, with a mean value of 61.8°F (± 11.8°F) is susceptible to greater thermal variations. One large subsurface system, the Gravel Wetland, whose mean value of 48.7°F (± 12.0°F), indicates a greater thermal buffering. The Runoff, the Retention Pond, and the Gravel Wetland calculated maximum and minimum values of event mean temperatures of 75.4°F and 33.3°F, 81.3°F and 32.7°F, 70.0°F and 33.0°F respectively. These temperatures are important to note when considering lethality indices of aquatic species. Another indicator of the health of a system that has been utilized by environmental monitoring programs is the mean July temperature of the stream under investigation