Impact of Microbial Partitioning on Wet Retention Pond Effectiveness

Abstract

Wet detention basins are among the most common best management practices (BMPs) being implemented as means of complying with United States Phase II storm-water rules and impending Total Maximum Daily Load limits. The effectiveness of these basins for removal of microbial contaminants, one of the most frequent causes of water quality impairment, may be significantly affected by the degree to which microbes associate with particles in storm water. Little is known with regard to where microbial-particle associations are initiated within the storm-water transport chain as flow travels from upland sources (e.g., lawns, parking lots) through storm sewer systems and BMPs and finally on to receiving waters. A similar lack of information exists on the relative concentrations of microbes at each point in the transport chain. Both of these factors have important implications for the location of wet detention basins within a watershed, as well as their anticipated effectiveness. This study tracked the concentrations and partitioning behavior of three indicator organisms (fecal coliform, E. coli, and enterococci) at several locations in the transport chain and also explored the impacts of partitioning on wet pond removal efficiency. Results suggest that microbial-particle association is primarily initiated at upland sources and the degree of microbial partitioning does not vary greatly throughout the transport chain; therefore, treatment ponds will likely be most effective if located near upland contaminant sources. The overall reduction in microbial concentration brought about by the ponds was less than that assumed by most regulatory agencies, but the ponds did show some evidence of preferentially removing particle-associated fecal coliform and E. coli, suggesting that sedimentation is a key removal process. These findings should provide insights useful in the design and implementation of storm-water management strategies.