Measuring hydrological connectivity inside soils with different texture by fast field cycling nuclear magnetic resonance relaxometry

Abstract

The locution “hydrological connectivity inside the soil” is generally used to disclose how the spatial patterns inside the soil affect the physical–chemical processes at a molecular level to influence water transfer into the soil, the surface runoff and related sediment transport. Fast Field Cycling (FFC) Nuclear Magnetic Resonance (NMR) relaxometry has been used to measure both structural and functional connectivity by two indexes indicated as structural (SCI) and functional (FCI) connectivity index. Here, FFC-NMR relaxometry has been applied to analyze three samples: two non-degraded soils, having different grain-size distribution, and a degraded soil sampled in a badland area. Proton Larmor frequencies (νL) ranging in the 0.015–35 MHz interval were used on water suspended samples. The relaxograms (i.e., the distributions of the longitudinal relaxation times, T1) obtained by the FFC-NMR investigations were integrated. The resulting S-shaped curves were handled to obtain the aforementioned FCI and SCI parameters. Results showed that the empirical frequency distribution of the SCI index is related to soil texture and structure, while soils having similar SCI revealed different functional connectivity values. This approach allowed (i) to distinguish SCI values of degraded soils from those of non-degraded ones and (ii) to verify whether the structural connectivity index is related to soil texture and structure. The analysis also showed that the variation coefficient of SCI and FCI, achieved from the measurements done at different νL values, is minimum in the measurement range 0.015–1 MHz. The frequency of 1 MHz can be considered the optimal value to have accurate measured connectivity indexes. For the investigated samples, the obtained relationship between FCI and SCI allowed to conclude that high structural connectivity values correspond to a better functional connectivity. Finally, the hydrological connectivity inside a soil was defined as the sum of the two components representing structural and functional perspective.