This study used nuclear magnetic resonance (NMR) relaxometry at different Larmor frequencies to investigate water dynamics in the pore space of natural porous media. Spin-lattice NMR relaxation times (T1) were determined in purified fine sand and two natural soils, Kaldenkirchen sandy loam and Merzenhausen silt loam, by means of fast field cycling relaxometry. This technique investigates relaxation processes as a function of the Larmor frequency ν in the range between 0.005 and 20 MHz, yielding so-called relaxation dispersion curves (1/T1 vs. log ν). The data were further analyzed by means of inverse Laplace transformation to calculate the T1 relaxation time distribution functions. Only the fine sand was characterized by monomodal distribution with T1 of about 1 s at 20 MHz, whereas the natural soil samples showed multimodal distribution functions in the range between 2 and 70 ms. With decreasing Larmor frequency, all distribution functions kept their shapes but were shifted to faster relaxation times. The corresponding relaxation dispersion curves indicate predominance of two-dimensional diffusion of water in the soils, whereas in the macroporous sand, diffusion behaved like unrestricted three-dimensional diffusion. In terms of the Brownstein–Tarr model, the decrease in the T1 relaxation times with increasing silt and clay content can be explained by an increase of the surface/volume ratios (S/V) of these porous media, i.e., by a decrease in the pore sizes. Finally, distribution functions of the pore size parameter V/S were obtained from the spin-lattice relaxation time distributions by normalizing on the specific surface area. They ranged from submicrometers in the Merzenhausen soil to micrometers and submillimeters in the Kaldenkirchen soil and fine sand, respectively.

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