Retention Pond Performance: Examples from the International Stormwater BMP Database

Abstract

Comparison of the performance a various Best Management Practices (BMPs) for pollutant removal is often hampered by differences in facility design and watershed characteristics. For instance, retention ponds may have been designed with a range of permanent pool volumes, differ in the length to width ratio , and outlet design . Additionally, the influent concentrations and characteristics vary greatly between test sites and may strongly affect the observe d performance of these facilities. Similar issues apply to the performance characterization of all structural BMPs. This paper explores the factors affecting pollutant removal performance of retention (wet) ponds using data contained in the International Stormwater BMP Database. While the Database contains numerous studies with varying amounts of detail, this comparison is based on the performance of only those sites with reported basic design characteristics and water quality data (event mean concentratio ns) so that constituent concentrations can be accurately determined and related to the design of the individual BMP. The use of selected BMPs has a number of advantages. Some of the variability in performance observed for facilities of a specific type can be explained by differences in design and/or watershed characteristics; consequently, the expected performance for a given set of conditions can be predicted more accurately. Of the 33 retention ponds contained in the Database, 11 have adequate design dat a to determine normalized permanent pool volume, were implemented in typical urban watersheds, and were originally designed as retention ponds. These sites also show a striking correlation between influent and discharge concentrations. Large permanent pool volumes (greater than the mean storm runoff volume) appear to have little influence on discharge concentrations for most constituents except dissolved phosphorus. Variation in performance among the sites can be shown to be influenced mainly by influent concentrations, although seasonal effects are common in areas with extended freezing conditions. This analysis was funded by the Water Environment Research Foundation, American Water Works Association Research Foundation, and UK Water Industries Research. It has been conducted independently of the team which developed and maintains the Stormwater BMP Database and this interpretation should not be construed as approved or recommended by the Database team.