Biofilms can be simply defined as communities of microbes attached to a surface. There are various types of biofilm, which can be either beneficial or harmful to an ecosystem. Good biofilm offers valuable services to society or in the function of natural ecosystems such as those that contribute to controlled bioremediation of ground water and soils in Low Impact Development approaches called bioretention cell. This thesis researched ways to model biofilm activity at the field-scale and used experimental data (BOD₅ and NO₃⁻) to verify these models. Two mathematical models are presented in this work. The first model provides and tests the solution of substrate and biomass concentration while the second model modified the expression for the substrate flux into the biofilm. They are analyzed using a sensitivity analysis and their performance is compared using field-scale data. The solution for concentration is computed with some selected values of dimensionless biofilm thickness (0.0375 and 3.75) and dimensionless substrate concentration outside of the biofilm (0.005 to 0.5), which shows these two variables significantly affect model results. The simulations illustrate that biofilm activity mostly occurs in the summer while the substrate flux is normally stable at similar levels in the same season.