Evaluating the hydrologic and water quality performance of novel infiltrating wet retention ponds

Wet retention basins have typically been designed to capture and slowly release stormwater in order to mitigate peak flow rates and settle out particulatebound pollutants. However, in locations with Hydrologic Soil Group A soils, such as much of Fayetteville, North Carolina, many wet retention basins may provide additional stormwater benefits through infiltration and evaporation. These benefits are not currently captured in the State of North Carolina’s crediting system, wherein wet ponds are thought of as flow through systems. Engineering designers in the area must line these basins with bentonite clay to prevent infiltration and to comply with the state water quality requirements. Intuitively, a device that infiltrates and reduces effluent volume while still reducing the peak flow rate of infrequent return interval storms could be a desirable practice. However, currently there is a lack of data to assess how well these infiltrating ponds function and no guidance as to how much pollutant removal credit to award. Two infiltrating wet retention basins were monitored in Fayetteville in order to assess the hydrologic and water quality performance. Results show both ponds have substantial volume reductions. Both ponds had a volume reduction of 100% during October. Median peak flow reductions were 99.2% and 99.7%. Despite minimal data, it is clear that both ponds are effective in removing TSS with median percent reductions of 67% and 84%. Median percent reductions for TN were 20% and 38%, and those for TP were 15% and 63%. However, due to the substantial volume reductions, the performance of each pond should be assessed based on loading and not solely concentration reductions. Due to the substantial volume reductions, peak flow rate reductions, and potential for nutrient loading reduction these devices could be an acceptable stormwater control measure. 145 World Environmental and Water Resources Congress 2014: Water without Borders © ASCE 2014