Sum Frequency Generation (SFG) is a powerful,, surface-specific vibrational probe ideally suited to studying buried interfaces; however, insight from theory is often necessary to explain the microscopic origins of the spectral features. To calculate the SFG spectrum at an insulating solid/liquid interface, we develop a flexible polarizability model that takes local dipole interactions into account, rather than assuming additive polarizabilities. We use this model to calculate bond dipoles and polarizabilities that reflect the local geometry of the interface. We apply our method to the Al2O3(0001)-H2O interface, where we reproduce the experimental spectrum and show the two H stretching peaks come from solvent and surface modes separately, not from H2O molecules with different coordination numbers as previously thought. Our work therefore emphasizes the importance of treating both surface and solvent at the same level of theory for accurate spectroscopic calculations.