Thermal pollution of streams by runoff from paved surfaces

Urban development dramatically alters a drainage system by landscaping, changes in surface cover, and addition of stormwater handling systems. Increased and warmer runoff from impervious surfaces into streams can lead to a degradation of habitat for coldwater fish. For the projection of thermal impacts of new land development projects, hydro-thermal simulation models have been developed. With measured climate as input, these deterministic models can be used to predict the thermal impact of individual storm events. Surface runoff and runoff temperature were simulated for an asphalt parking lot using 6 years of climate data from Minnesota, USA, with 282 individual rainfall events. The thermal loading from each rainfall event was quantified by three parameters: runoff temperature (°C), rate of heat export (W/m²) and total (integral) heat export (J/m²). Many storm events were found to have little thermal impact, but a few events per year have high thermal impact, either due to large rainfall amounts at a high dew point temperature or due to high land surface temperature combined with moderate rainfall amounts. The amount of heat added to the runoff is highly dependent on both the characteristics of the rainfall event and the weather conditions prior to the storm event. Runoff temperatures from asphalt were found to be well correlated to a linear combination of three parameters: average dew point temperature during the storm, air temperature prior to the storm, and solar radiation prior to the storm. The study results imply that stormwater thermal pollution is more severe when (1) atmospheric air and dew point temperatures are higher than stream temperature, e.g. for streams that are fed by groundwater that is colder than the ambient air, (2) rainfall events are short, intense and preceded by full or partial sun, and (3) watersheds have a high percentage of impervious, particular paved, surfaces.