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THE ECOLOGICAL EFFECTIVENESS OF PONDS AND WETLANDS AS "BEST MANAGEMENT PRACTICES (BMPS)" FOR STREAMS IN DEVELOPING LANDSCAPES
Citation:
Nietch**, C. AND M Borst*. THE ECOLOGICAL EFFECTIVENESS OF PONDS AND WETLANDS AS "BEST MANAGEMENT PRACTICES (BMPS)" FOR STREAMS IN DEVELOPING LANDSCAPES. Presented at Water Environment Federation TMDL 2003 Conference, Chicago, IL, 11/16-18/2003.
Description:
Ponds and constructed wetlands, also referred to as detention/retention basins, have a long history as best management practices (BMPs) used to mitigate the impacts of stormwater runoff from developed lands on receiving waters. Initially designed for flood control by peak flow attenuation, the design objective first evolved to include pollutant interception and storage for water quality control, and is currently under evaluation to account for effective discharge for sediment transport and habitat protection. The measure of design effectiveness has been routinely expressed as a pollutant mean percent mass removal. With respect to protecting stream ecology, the effectiveness of ponds and wetlands depends on the preservation of habitat and the temporal dynamics of allochthonous carbon and nutrient loads, including both quantity and quality. Existing data and preliminary observations from pond and wetland mesocosms were used to begin evaluating the relative differences between these two BMP types in terms of their ecological effectiveness and in relation to untreated stormwater runoff from urban areas. The results show minimal difference between ponds and wetlands in the percent removal of many ecologically important biogeochemicals including nutrient species and biological oxygen demand. While both BMP types remove significant percentages of suspended solid concentration, 66% on average, and with this a lower but significant fraction of total phosphorus, the removal of dissolved nutrient concentrations are low to insignificant and highly variable. Important considerations for watershed scale management, including total maximum daily load (TMDL) calculations, are that there is no significant difference between pond and wetland effluent concentrations for nutrient biogeochemicals, and for dissolved nutrient species the effluent from each is nearly indistinguishable fro mean untreated stormwater runoff.
With these observations, and under existing stormwater management guidelines, selection of either of these BMP types for stormwater nutrient load reductions in urban areas is not recommended and should only be attempted with a good understanding of the relative contribution of dissolved and particulate fractions in the stormwater influent. At best, nutrient load reductions can be approximated with high uncertainty that will vary dependent on nutrient species. Post selection, and as BMP design adjusts to take into account effective discharge, concomitant effects on the timing of the quantity and quality of carbon and nutrient loads carried by the BMP effluent will be required for ecological effectiveness assessment. To meet this need it is suggested that there be a switch in focus from storm to storm input/output type mass removal assessments to temporally-extended effluent monitoring. This will also help to better understand the effects of BMPs on annual load estimates and characteristics for TMDLs. The results from this analysis of biogeochemicals for pond and wetland effluent and untreated stormwater runoff can be combined with flow estimates and used currently to provide input to TMDL calculations, but will need reassessment as pond and wetland designs adjust to meet effective discharge guidelines for sediment transport.