This paper reports the development and application of a three-dimensional multi-component reactive transport model (BGTK) to simulate a wide range of biogeochemical processes in subsurface environments. The model can handle both equilibrium and kinetically controlled reactions and is based on the well tested modular models RT3D <Clement et al., 1998> and PHREEQC-2 <Parkhurst and Appelo, 1999>. Here we describe the details of the new coupled model and demonstrate its capabilities using test problems involving microbial transport in a laboratory column and redox zonation in a contaminated aquifer.