Pond Performance Evaluation During Wet Weather Conditions

Urban areas adjacent to Galveston Bay have been shown to contribute the greatest amount of non-point source pollutants to the bay. Many coastal communities, under Texas Pollutant Discharge Elimination System (TPDES) mandated stormwater management programs are addressing urbanized non-point source pollution through permanent storm water best management practices. Harris County Flood Control District, using partial funding from the Texas Coastal Management Program through the Texas General Land Office, conducted a pond performance evaluation during wet weather conditions to determine the effectiveness and pollutant removal capabilities of a typical dry pond detention facility in the Galveston Bay watershed. The detention facility known as B504-03-00 was located about 10 miles from Galveston Bay and was designed to restrict the post development peak flow arising from the 100-year storm event to the predevelopment peak flow and was not designed to provide water quality benefits. Quality-assured pollutant loading data for 21 water quality parameters were obtained from B504-03-00 to evaluate if any pollutant removal was occurring in the basin that would reduce loadings to the bay. Inflow and outflow from the detention facility was sampled multiple times on a flow-proportional basis, using automated sampling units, to obtain event mean concentration (EMC) data for 17 parameters from rainfall events that occurred from September 2005 to October 2006. Bacteria (E. coli, Fecal coliform and Enterococci) and oil and grease grab samples were also collected for these storm events. The EMC and grab sample inlet and outlet data were compared using statistical analyses. These analyses revealed that none of the 21 parameters monitored at the site showed a statistically significant reduction or increase between the mean inlet value and the mean outlet value. The levels of pollutants draining to the basin were also compared to mean values from the Nationwide Urban Runoff Program (NURP). The levels of pollutants draining to Basin B504-03-00 were found to be comparable to the NURP values, with the exception of suspended solids concentration, which were lower than the national data. The results of this study demonstrated that typical dry detention ponds designed for peak flow reduction may provide little water quality benefits when inlet suspended solids concentrations are low. This evaluation of the B504-03-00 detention facility, which does not include water quality features, will allow future comparison to the performance of basins with water quality features. These future monitoring studies are planned and underway by both HCFCD and Harris County Stormwater Quality Section.