Bioretention systems are commonly used to improve both quantity and quality (pollutant removal) aspects of urban runoff. A promising media modification process is developed to simultaneously achieve both high infiltration rate and improved P removal in bioretention systems, Bioretention soil media (BSM) and a sandy 'high flow medium' (HFM) were modified with three Al-based amendments: water treatment residual (WTR), alum, and partially hydrolyzed aluminum (PHA), 10% by dry mass, respectively. The sorption of adenosine monophosphate (AMP) and myo-Inositol hexakisphosphate (IHP), two forms of organic phosphorus, and inorganic phosphate by modified media mixtures were studied. Compared with unmodified BSM and HFM, the sorption of AMP was increased by a factor of 26 and 80 for media with alum and PHA, respectively. Similarly, the IHP removal capacity were increased by factors of 7 and 8, respectively. Alum and PHA modified media mixtures can reduce P Chapter 1. Introduction 1.1 Research Background Booming urbanization has increased the amount of impervious surfaces in developed areas. In fact, the rate of increase in impervious surfaces has exceeded the rate of population growth by 5 times over the last 40 years in the U.S. (Luoni et al., 2010). In undeveloped areas with natural ground covers (e.g., forest, meadow), the majority of rainfall can be transported through infiltration and evapotranspiration by soils and vegetation, resulting in a small fraction (~10%) of the incoming rainfall becoming surface runoff. However, surface runoff can be dramatically increased to more than 50% in urbanized lands with 75%-100% impervious areas, contributing to many problems in the water environment (Luoni et al., 2010). Impervious surfaces prevent rain from naturally infiltrating into the ground and as a result, large volumes of runoff form and run rapidly into storm drains, sewer systems, and drainage ditches, which can cause flooding problems. For those storm drains directly connected to open discharges, stormwater runoff can impair water quality in receiving water bodies as it concentrates hydrocarbon residues from household and lawn care chemicals, oil, gasoline, brake fluid, nutrients (N and P), and heavy metals (Luoni et al., 2010). In fact, urban stormwater runoff discharge is classified as a significant source of water quality impairment in receiving water bodies