Evaluation of Bioretention Nutrient Removal in a Rain Garden with an Internal Water Storage (IWS) Layer

Abstract

Stormwater runoff has been identified as a major source of pollution in urban and suburban streams. There are several innovative stormwater practices that integrate stormwater infiltration and storage to improve the quality of runoff, including bioretention areas such as rain gardens. This paper reports first year results comparing two different bioretention (rain garden) designs constructed at the Donald E. Davis Arboretum on the Auburn University campus, one using conventional, aerobic treatment and the other incorporating an internal water storage (IWS) layer. Initial evaluation of outflow from the newly installed rain garden revealed several trends directly related to the chemical and physical properties of the fill media. Results support what is known in the literature about the linkage between outflow rain garden water quality and the inherent soil properties of the fill media. In addition, it was observed that settlement and consolidation of in-place rain garden media immediately following construction resulted in gradually reduced outflow peaks during the 6-month study period from July through December 2006. Significant removal of particulate phosphorus and total phosphorus constituents was found under both conventional and IWS operation of the rain garden and some beneficial nitrogen removal was also detected from the IWS layer towards the end of the study. Beneficial hydrologic effects of the rain garden included significantly reduced outflow hydrograph peaks and reduced total outflow volumes, both effects which would act to reduce total contaminant load to receiving waterways. In addition, within six months after construction, the peak outflow from the rain garden was seen to decrease until a near steady-state outflow was achieved.