INTRODUCTION

Bioretention basins hold a volume of water that is filtered through sandy soil and allowed to infiltrate into the subsoil or drained to an outlet (Figure 1). Originally, bioretention basins were intended as a site-scale tool to improve the quality of urban stormwater runoff, but they have a demonstrated impact on reduction of runoff volume and time of concentration. Therefore, the design of basins are complicated by a range of possible goals. Recharging groundwater, improving runoff water quality to maintain or restore aquatic ecosystem health, reducing peak storm flows, extending time of concentration, and reducing runoff volume to prevent channel erosion and sedimentation are all possible goal options. Only a few states have guidelines or regulations for bioretention basins. Furthermore, some existing state design requirements do not reflect the range of goals or the research demonstrating the design and performance of bioretention basins. For example, the Idaho Department of Environmental Quality recommendations were published in 2005 but based information from 1993 (IDEQ, 2005). The first section below considers the original purpose—pollutant removal.