Standardizing the use of fast-field cycling NMR relaxometry for measuring hydrological connectivity inside the soil

Hydrological connectivity inside the soil (HCS) is applied to study the effects of heterogeneities in complex environmental systems. It refers to both the spatial patterns inside the soil (i.e., structural connectivity [SC]) and the physical-chemical processes at a molecular level (i.e., functional connectivity [FC]). NMR relaxometry has been already applied to assess both SC and FC components of the HCS by defining SC and FC indexes. Here, fast-field cycling NMR relaxometry has been applied on a water suspended soil and a sediment to optimize the conditions to standardize the technique. Proton Larmor frequencies (ω_L ) from 0.01 to 25 MHz were used on samples suspended in three different rates of Milli-Q grade water. The application of different magnetic fields revealed that the T₁ values of the sediment sample are always shorter than those measured for the soil sample. This difference was attributed to the soil erosion processes limiting FC by reducing the size of macropores. For the soil sample, analyses showed that both structural and functional connectivity indexes can be assumed ω_L independent. For the sediment sample, the connectivity indexes resulted ω_L independent only for ω_L  ≥ 0.1 MHz. This could be due either to instrumental problems for ω_L  < 0.1 MHz or to a real magnetic field effect when a finer textured sample is examined. Further research is required in this area. Finally, the experiments revealed that the optimal water amount to obtain reliable results was corresponding to the water holding capacity.