The magnetic relaxation dispersion from the water deuteron resonance in aqueous colloidal silica sols has been measured with the field-cycling technique in the frequency range from 1.5 kHz to 7.7 MHz. Dispersion profiles were recorded as a function of pL, L stands for a general hydrogen atom, in the range 2-11 for two sols with different particle size and at two temperatures. The profiles were well described by a Lorentzian dispersion function with an amplitude of beta and an apparent correlation time of tau. The near invariance of t with particle size, temperature, and pL demonstrates that the usual motional narrowing theory of spin relaxation is not applicable. A more general, nonperturbative theory, however, can quantitatively rationalize the data and yields, through a global fit, physically meaningful values of the microscopic parameters in the model. The analysis shows that the dispersion is partly due to long-lived water molecules and partly to silanol deuterons in rapid exchange with water. The silanol contribution is about 50% at pL 5, increasing to 90% at pL 8-10. Over most of the pL range, tau is essentially a measure of the residual quadrupole frequency of water and silanol deuterons and, hence, is not directly related to a motion in the system. The long-lived water molecules contributing to the dispersion have a residence time distribution spanning the microsecond range and are presumably trapped in micropores at the silica surface. The surface density of such trapped water molecules is found to be higher for silica particles of the more porous Stober variety. The relaxation data also yield the surface density and orientational order parameter of silanol deuterons, as well as the rate constants for acid- and base-catalyzed silanol hydrogen exchange.