Development and evaluation of a biphasic rain garden for stormwater runoff management

Rain gardens are bioretention systems that have the potential to reduce peak runoff flow and improve water quality in a natural and aesthetically pleasing manner. In spite of their popularity, results from column and field-scale studies show that level of pollutant removal in rain gardens varies and is not always positive. To date, research has often focused on a limited number of runoff pollutants. This study was conducted to develop and evaluate a new rain garden design for flow management and pollutant removal from stormwater runoff. Both column and field-scale biphasic rain gardens were designed and constructed to increase retention time and maximize removal efficiency of multiple runoff pollutants by creating a sequence of anaerobic to aerobic conditions. To evaluate hydraulic performance and pollutant removal capacity of the biphasic rain gardens, studies were conducted under actual and simulated runoff conditions with spiked concentrations of nutrients (nitrate-N and phosphate-P), and herbicides (i.e. atrazine (6- chloro-N-ethyl-N'-(1-methylethyl)-1,3,5-triazine-2,4-diamine), glyphosate (N- (phosphonomethyl)glycine), dicamba (3,6-dichloro-2-methoxybenzoic acid), and 2,4-D (2,4-dichlorophenoxyacetic acid)). Both column and field-scale studies showed that the biphasic rain gardens have the potential to be an effective best management practice for reducing stormwater flow and pollutant loads. Peak flow and runoff volume were effectively reduced five-fold in the biphasic rain gardens for actual and simulated runoff events by holding runoff in the rain gardens (mainly in the anaerobic zone) until a subsequent runoff event. The field-scale biphasic rain gardens were highly effective in removing nitrate-N (&sim91%), phosphate-P (&sim99%), atrazine (&sim90%), dicamba (&sim92%), glyphosate (&sim99%), and 2,4-D (&sim90%) under high levels of pollution loading conditions simulated in both agricultural and urban runoff events. The column studies demonstrated that the biphasic rain garden, compared to the monophasic (conventional) rain garden, was better able to reduce peak flow, runoff volume, and pollutant loads by employing the sequence of anaerobic to aerobic (biphasic) conditions. In addition, higher organic matter content in soil media increased removal efficiency of nitrate-N through denitrification in the anaerobic zone. Overall, this study showed that increased retention time of runoff pollutants, as determined by design configuration and rainfall size, intensity, and interval, significantly increased overall nutrient and herbicide removal in the biphasic rain gardens. Demonstrating more efficient hydraulic and pollutant removal capacity by the biphasic rain gardens, compared to the conventional monophasic rain gardens, is expected to stimulate the implementation of rain gardens as an effective stormwater best management practice.