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THE REPORTED EFFECTIVENESS OF COMMONLY USED BEST MANAGEMENT PRACTICES FOR THE CONTROL OF NUTRIENTS IN URBAN STORMWATER
Citation:
O'Shea*, M L. THE REPORTED EFFECTIVENESS OF COMMONLY USED BEST MANAGEMENT PRACTICES FOR THE CONTROL OF NUTRIENTS IN URBAN STORMWATER. Presented at National TMDL Science and Policy Conference, Phoenix, AZ, 11/13-16/2002.
Description:
Many structural BMPs, when appropriately designed and constructed, capture and treat urban runoff to remove particulate-associated pollutants. However, field monitoring programs show these same structures provide relatively little reduction in the loadings of dissolved constituents. For receiving waters impacted by eutrophication, these controls will provide minimal or even negative removal of the bioavailable nutrient species most responsible for the impairment of aquatic life and recreational usage. In overfertilized receiving waters, solids control in the absence of targeted nutrient reductions may additionally reduce productivity-limiting turbidity, or other limiting factors, potentially resulting in counterintuitive declines in water quality. Infiltration-based controls may satisfactorily capture particle-sorbed nutrients, but, for nonsorbed species, there is the potential for both groundwater contamination and eventual delivery to the receiving water through groundwater migration. Commonly employed vegetated systems combine biological uptake with sedimentation to control both particle-phase and dissolved nutrients. Yet, biologically-induced transformation can lead to negative removals of some species since nutrient uptake rates and ambient cycling vary seasonally. As productivity also varies seasonally, ecosystem response to loadings will depend not only on the magnitude but also the timing of inputs. Data from several BMP pollutant removal databases were examined to evaluate the nutrient capture potential of commonly-used stormwater BMPs. results indicated that there is significant variability in nutrient removal data amongst the commonly used structural stormwater BMPs, with no single BMP-type reporting consistent, positive removals for the bioavailable nutrient species, with data indicating on-average retention of the former, yet export of the latter. Key nutrient removal processes are strongly influenced by many temporal factors including: seasonal changes in vegetation, light and temperature; ambient and sediment layer conditions; and, sorption site availability resulting in significant variability in seasonal and/or long-term BMP removals. Many of these same factors influence the bioavilability or ecological-impairment potential of receiving water nutrient loads. Prior to the implementation of nonpoint and diffuse source management plans, watershed planners need to identify: which nutrient species need to be controlled; when is control most important; and which BMPs can retain the species of concern over the required period of control.