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MONITORING OF A RETENTION POND BEFORE AND AFTER MAINTENANCE
Citation:
OCONNOR, T. AND J. ROSSI. MONITORING OF A RETENTION POND BEFORE AND AFTER MAINTENANCE. Presented at ASCE/EWRI conference, omaha, NE, May 21 - 25, 2006.
Impact/Purpose:
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Description:
The USEPA’s Urban Watershed Management Branch has monitored stormwater drainage and best management practices (BMP) as part of its research program. One BMP being monitored is a retention pond with wetland plantings in the Richmond Creek (RC) watershed. This BMP, designated RC-5, is owned and operated by the New York City Department of Environmental Protection (DEP) as part of the Bluebelt program. The Bluebelt incorporates BMPs on a watershed basis. Dedicated funding to the DEP programs allows for maintenance activity at Bluebelt BMPs. During monitoring performed in 2003 and 2004, BMP-RC-5 removed suspended solids (SS), but not chemical oxygen demand (COD). Calculated SS loads demonstrated a 61% removal, exceeding anticipated removals of 55-57%. Of three events monitored for COD, one had no removal, the second had a negative 146%, and the third had a 37% removal. The increase in COD during the second event may be due to biological activity in BMP-RC-5 and seasonal considerations, i.e., decaying leaves and wetland vegetation going into dormancy. The largest SS and COD discharges occurred on 12/11/03, which was the largest rain (>1 in.) and flow event monitored. Results indicated that the number of storms monitored was adequate to describe SS removal using a paired t-test on composite influent and effluent data after applying log transformations. Directly calculating the significance, using a statistical test for randomly comparing two data sets of non-paired results, indicated a 0.94 significance level in the log transformed SS results at a power of 0.5, just below 95% confidence. Maintenance activities, including cleaning of the forebay and stilling basin, were performed in the spring of 2005. Almost four truck loads of sediment and leaf litter, estimated between 46 to 48 yd3, were removed from the site. Monitoring during the fall of 2005 measured the effects of this maintenance activity, to better quantify seasonal effects of leaf decay and to assess scheduling of maintenance activity. Indications are that the maintenance activity significantly reduced the COD effluent.