Internal Water Storage Enhances Exfiltration and Thermal Load Reduction from Permeable Pavement in the North Carolina Mountains

Thermally-enriched stormwater runoff can negatively impact coldwater streams and their associated ecosystem services. The introduction of local and federal guidelines specifically targeting urban runoff temperature in the United States and Canada promulgate the use of practices that reduce thermal pollutant load from a catchment. Several stormwater control measures (SCM) have been shown to buffer thermal impacts to waterways by passing water through cooler subsurface media or by reducing overall discharge volume. Permeable pavement exposes stormwater to these mechanisms, which makes it a promising practice for thermal mitigation. A newly constructed permeable interlocking concrete paver (PICP) parking lot was monitored for 1 year in the mountain region of North Carolina to quantify runoff reduction, temperature buffering, and thermal load export. The effects on hydrology caused by internal water storage (IWS) in the pavement profile were also investigated by dividing the $239\text{-}{\mathrm{m}}^2$ lot into three cells with varying aggregate depths and drainage configurations. Results showed substantial stormwater volume reductions from all permeable pavement configurations, with the most pronounced reductions (99.6 and 100%) experienced in the shallow and deep IWS cells, respectively. Thermal loads from the conventionally drained cell, deep IWS cell, and shallow IWS cell were consequently reduced proportionally to rates of runoff reduction. Median stormwater effluent temperature from the conventionally drained cell (78% runoff reduction) exceeded the critical trout threshold temperature of 21°C; however, outflow only occurred during eight storm events (out of the 54 recorded) and exceeded 21°C for 10.5 h of the stream temperature monitoring period (4.7 h more than ambient stream conditions). Effluent from the two cells with IWS never exceeded trout threshold temperatures because of retention and exfiltration of virtually all incoming stormwater. Temperature differentials between the top and bottom of the PICP profiles suggest that buffering of extreme runoff temperatures can occur. The subsoil interface never reached freezing temperatures during the unseasonably cold winter months, suggesting that frost heaving of permeable pavement systems is not expected in similar climes. These results merit the use of PICP and perhaps other forms of permeable pavement, particularly with the incorporation of IWS, where thermal impact to streams is of concern.