Thermal reduction by an underground storm-water detention system

Increases in stream temperatures by heated storm-water runoff from impervious surfaces are a serious environmental problem. An underground detention with slow-release facility is a versatile storm-water best management practice (BMP) for buffering high flows. Temperature reductions in underground storm-water storage BMPs, however, have not been quantified. A field study on an underground detention BMP located in Maryland was undertaken to characterize its effect on storm-water runoff temperatures. In colder months, when the runoff temperature ranged from 5 to 15°C, small or no temperature change was observed. Runoff produced during summer storm events, however, with event mean temperatures over 20°C, exhibited mean temperature reductions of 1.6°C through the BMP. While statistically significant, the reductions were not sufficient to cool the summer runoff discharges below the Maryland Class III temperature standard (20°C) 100% of the time. The results indicate that underground facilities can moderate high runoff temperatures, but that more efficient designs are needed for heat transfer.