Recognition that molecular-scale chemistry plays a significant role in the environmental fate and transport of contaminants has led to the use of numerous spectroscopic techniques to understand chemical reactions in nature. Interpretation of various spectra can be difficult and ambiguous, however, so researchers have turned towards computational chemistry techniques to help clarify the relationships between observed spectra and chemical speciation. Quantum mechanical calculations are useful in interpreting IR/Raman, NMR, and EXAFS spectra. This talk includes examples where quantum mechanical calculations have been used to understand reactions at the mineral-water interface among various Al- and Fe-oxyhydroxides and siderophores, herbicides, antibiotics, and arsenic. In each case, model results are compared against observed spectra and used to generate a more complete picture of mineral surface chemistry. In addition to spectroscopic parameters, the model results can predict thermodynamic properties such as Gibbs free energies of adsorption as well as kinetic properties such as rate constants. Constraining surface speciation via spectroscopy and computational chemistry is a necessary first step in order to create realistic surface complexation models that are used in macroscopic transport equations. An example study that will be presented is the prediction of equilibrium Fe isotope fractionation factors for complexation of Fe with various organic ligands.