Stormwater associated with runoff in urbanized areas and transportation corridors introduces a wide variety of contaminants into water supplies. Best Management Practices (BMPs) are normally implemented to mitigate the impacts of these contaminant loads. However, the processes governing the effectiveness of these systems in treating stormwater are complex. Furthermore, the role of subsurface flow, a key feature of many of these systems, is difficult to quantify. An understanding of the processes governing the transport and transformations of these contaminants in BMPs is essential for protecting groundwater and surface water resources. The proposed research is a collaborative effort between Worcester Polytechnic Institute (WPI) and the Massachusetts Department of Conservation and Recreation (DCR) to address these processes. The project will characterize transport and transformations of contaminants within and adjacent to biofiltration systems, with specific consideration to the role subsurface processes in mitigating discharge of pollutants into water supplies. The approach combines a field sampling and monitoring program with field data analysis, reactive transport modeling, and design assessment. The field sampling program includes collection of field parameters (dissolved oxygen, specific conductance, pH, and temperature) and samples for nutrients (nitrogen and phosphorus compounds), additional cations and anions, suspended sediments, and bacteria. The laboratory analyses will include use of Atomic Absorption Spectrophotometry and ion chromatography to provide information on a broad range of cations and anions. Simulations will be developed using the PHREEQC software package to simulate geochemical transport and transformation experienced by stormwater in the infiltration bed and groundwater downstream of the basin. The reactive transport model will include consideration of surface exchange reactions, sorption processes and rate-limited oxidation processes (e.g. ammonium and carbon oxidation). The results are used to quantify the potential transformations that may affect groundwater quality, and the distance over which these transformations would be expected to occur. By comparing these results with conditions in an aquifer, the implications of transport in groundwater can be assessed with respect to the requirements for protecting water supplies.