Carrier trapping and recombination is the primary fate of conduction band electrons. Interfaces are known to play an important role in these processes. However, probing the interface is challenging as the interface represents but a small fraction of the total number of atoms in a macroscopic sample. Distinguishing between photoprocesses due to surface states and those from excitation of states in the bulk, resulting from crystal impurities or defects, is difficult especially if one is monitoring a bulk property such as photocurrent. Second Harmonic Generation (SHG) is known to be a versatile in-situ, real time probe of buried interfaces. In this study SHG was used to probe carrier relaxation at various Si interfaces and to asses the passivating nature of the surface. Carriers were created by optical excitation via an interband transition induced by an 800 nm, 4 psec pump pulse. Subsequently, the nonlinear optical response of the interface was probed by SHG of 800 nm light. The SHG dynamics depend strongly on the surface chemistry. Native oxide covered surfaces exhibit an initial increase in SHG response followed by a linear decay on a nanosecond timescale. Surface modification changes the dynamics. RCA cleaned, oxide covered samples also show an initial increase in SHG response, but the subsequent decay appears slower. H terminated surfaces show distinctively different behavior: initially the SHG signal decreases then recovers faster then the oxide covered silicon. The different dynamics probably reflect the degree of interface passivation. Defect sites deplete the conduction band carrier population by trapping or recombination.