The nature of the surface of crystalline water ice is investigated by monitoring isotope exchange in the first few bilayers. Near-monolayer amounts of D2O are deposited on thin films of H2O ice and isotope exchange at 145 K is monitored with Fourier-Transform infrared spectroscopy as a function of time. No exchange occurs on the surface of pure ice, however, exchange is readily observed on the surface of ice doped with small amounts of hydrogen chloride (HCl). The lack of exchange at the surface of pure ice stands in contrast to similar experiments performed of D2O embedded in the bulk. This suggests a depletion of mobile defects on the surface of pure crystalline ice at 145 K. This relative depletion may cause a significant difference between reactivity on the ice surface and in the ice bulk for other systems.