Urban Flow-Through Facilities' Soil Media Compositions for Stormwater Quality and Quantity Improvements

Author(s): Overbey, E.; Gallo, E.; Kröger, R.

An emerging practice for reducing the amount of nutrients and pollutants entering receiving waters is to filter urban stormwater runoff with infiltration based best management practices (BMPs). Small-scale BMPs for urban environments such as rain gardens, bioretention facilities, flow-through planters, and green roofs have been shown to slow peak flows and reduce the amount of nutrients that are washed from impervious surfaces during storm events. These BMP technologies are relatively new in design practice and all aspects of their structural design, potential for nutrient removal, and peak flow reduction is yet to be fully examined. The objective of this study is to provide understanding of different soil compositions in flow-through planters currently used in practice and determine their potential for water quality and quantity improvements. Eighteen 30"x18"x12" aquaria were modified to model flow-through facilities in a practical application using synthetic runoff which contained a 2ppm mixture of nitrogen and phosphorus. Soil mixtures varied across treatments and were composed of different percentages of sand, topsoil and compost. A hydrograph was used to simulate the most intense 4.5 hours of a 2-inch, Type II 24-hour rainfall event and was applied to the aquaria using manually controlled flow rate pumps. The outflow hydrograph was recorded to determine if peak flow was reduced and water quality samples were collected and analyzed to determine if nitrate and phosphate retention differed between soil treatments. Water quality data analyses indicate phosphate retention values ranged from (33-81%) and poor retention of nitrate (4- 23%). Nitrate values showed increases in concentration for some treatments. Preliminary results indicate the need for modification of the study design as higher infiltration rates in soil treatments reduced the residence time expected for the stormwater runoff in these facilities and as a result did not allow for the desired reduced peak flow. Further research is needed to test structural design modifications of flow-through facilities in order to increase their quantity reduction performance.

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