Biphasic Rain Garden Design for Urban/suburban Storm Water Runoff Management.

Urban/suburban storm water runoff has been identified as a significant nonpoint source of pollution for surface water bodies due to increases in runoff volumes and pollutant loads from impervious surface. One potential approach to treat storm water runoff is the development of rain gardens. Rain gardens are small, landscaped storm water bioretention areas that reduce peak runoff and improve water quality in a natural, aesthetically pleasing manner. In spite of their popularity, there is little information on how to integrate hydrological, soil, plant, and microbial components to optimize water flow, groundwater recharge, and pollutant removal in the rain gardens. For many organic pollutants, sequence of anaerobic to aerobic conditions is required for their efficient removal. In order to evaluate this novel concept for improving retention and removal of pollutants in rain gardens, three replicate biphasic field-scale rain gardens have been constructed to handle a 10-year return storm (1.76 inches for 1 hour rainfall duration). The biphasic rain garden is designed to increase retention time and treatment efficiency of runoff pollutants by creating both anaerobic and aerobic zones. During selected precipitation events, inflow rates of storm water runoff into the rain garden and outflow from each zone will be measured and analyzed. Some experiments will also involve spiking various concentrations of multiple simulated storm water pollutants to inflow storm water to get additional data on treatment capacity of the rain gardens. A water quality analysis will be conducted to estimate load reductions for total BTEX (benzene, toluene, ethylbenzene, xylenes), phosphorous, nitrate, and heavy metals. This presentation will include a discussion of the design and construction process, and results of treatment performance monitoring.