Evaluation of hydraulic retention time on BMP water quality performance, An

2011 Fall.Includes bibliographical references.Urban stormwater contains elevated concentrations of pollutants that are carried to receiving waters as runoff travels over roads, rooftops, and other hard surfaces. Structural best management practices (BMPs) are used to mitigate the negative impacts of urbanization by improving the water quality of stormwater runoff. Volume-based BMPs attenuate the peak flow of runoff and increase the hydraulic retention time (HRT) of runoff allowing pollutants to be removed through settling, adsorption, and other physiochemical processes. When BMPs provide longer HRTs for runoff events, the capacity for pollutant removal is increased because there is greater opportunity for pollutants to settle out of the water column and more time for plant and biological uptake. However, increasing the HRT that a BMP provides requires more storage volume, costs more to construct, and takes away land that could be developed for other uses. There is a tradeoff between the size of a BMP, the cost to build a BMP, and the capacity for pollutant removal. Two regional BMPs that serve the downtown area of Fort Collins, CO, were investigated in an effort to relate the HRT of a BMP to its water quality performance. The Udall Natural Area (Udall WP) is a wet extended detention basin that provided storm HRTs of over 80 hours. Contrastingly, the Howes St. BMP has an unregulated outlet and provided storm HRTs less than 20 hours. Stormwater quality data was collected from 2009-2011 at the inlet and outlet of each facility. The pollutant removal at each BMP was quantified for various runoff constituents including heavy metals, total suspended solids (TSS), bacteria, and nutrients. The Udall WP consistently had cleaner TSS effluent than the Howes St. BMP had and also removed significant amounts of heavy metals. The cleaner effluent at the Udall WP can be attributed to the longer HRT that the BMP provided. If the Howes St. BMP were modified to have a water quality outlet, it is believed that the BMP could enhance water quality more consistently and that it would actually be more cost-effective than the Udall WP. Furthermore, the degree of pollutant removal from the undersized and unregulated outlet at the Howes St. BMP was enough to warrant the suggestion that the Udall WP was constructed larger than necessary for significant pollutant removal. To further develop the relationship between HRT and water quality enhancement, additional stormwater studies for wet ponds and extended detention basins were investigated from the International BMP Database. A lognormal approximation was used to estimate the average HRT provided by a BMP based on the volume of runoff recorded at the BMP inlet during a storm event. The computed storm HRTs were matched with effluent water quality results for TSS, total recoverable zinc, total recoverable copper, and total phosphorous. Results were binned into HRT groups and a statistical analysis was conducted to determine whether longer HRTs enhanced the water quality at the BMP outlet. The analysis did not focus on water quality enhancement from inlet to outlet, but was aimed at determining whether additional treatment occurred from longer HRTs at the outlet. The results indicated that additional pollutant removal was not achieved in wet ponds when HRTs longer than 12 hours were provided. The only exception was total phosphorous, which was statistically lower in concentration when extremely long HRTs were provided. For dry extended detention basins, better pollutant removal was achieved when longer HRTs were provided, and longer HRTs (greater than 60 hours) may be required if total phosphorous or heavy metal reduction is desired. The findings could be used to refine BMP design criteria for the optimal HRT that will provide significant enhancements in water quality