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Software packages that model geochemical speciation and complexation are useful for predicting how different materials such as heavy metals and organic matter interact with the environment. The East Fork Poplar Creek (EFPC) in Oak Ridge, Tennessee suffers from extensive mercury pollution as a result of post-WWII activities to develop thermonuclear weapons. A current model that predicts the speciation of mercury and methylmercury species treats dissolved organic matter (DOM) as a single entity instead of a multidimensional and multisite molecule. The Humic-Ion Binding Model VII is a discrete multisite model implemented by default in the WHAM7 software that represents binding behavior between protons, metal cations, and humic substances. Implementing Model VII into the current EFPC model using the PHREEQC speciation program can predict site interactions of organic matter with mercury and methylmercury. Adding surface complexation to the model shows a substantial increase in the amount of methylmercury bound to DOM compared to the original model. Thus, when appropriate, employing a surface complexation model in geochemical simulations should be considered.