Stream and Retention Pond Thermal Response to Heated Summer Runoff From Urban Impervious Surfaces1

Abstract: Runoff from parking lots during summer storms injects surges of hot water into receiving water bodies. We present temperature data collected near urban storm sewer outfalls in Blacksburg, Virginia, using arrays of sensors in a stream and a stormwater pond. Surges occurred roughly a dozen times per month, ranging up to 8.1°C with average duration 2 h in the stream and up to 11.2°C with average duration 7 h in the pond. Surges were larger in the pond due to a larger contributing watershed, no dilution by upstream water, and cool background temperatures near the outfall. Surges began abruptly, warming at rates averaging 0.2°C/min for periods of 5‐20 min. Surges dissipated as they propagated into the water bodies, travelling further in the stream (>19 m) than the pond (～10 m) consistent with greater advection in the stream. Surges were largest and most frequent in the afternoon but occurred at all times of day and night. Stream surges exhibited two phases: an early high‐temperature low‐volume input from the storm sewer and a later low‐temperature high‐volume input from upstream. Surges at the pond did not exhibit two phases, consistent with inputs only from storm sewers. Surges are likely common in urban areas, and may cumulatively have consequences for aquatic organisms, biogeochemical process rates, and even human health. Such effects may be compounded by urban heat islands and climate change, so prevention or mitigation should be considered.