Measuring hydrological connectivity inside a soil by low field nuclear magnetic resonance relaxometry

Abstract

Hydrological connectivity inside the soil is related to the spatial patterns inside the soil (i.e., the structural connectivity). This, in turn, is directly associated with the physical and the chemical processes at a molecular level (i.e., the functional connectivity). Nuclear magnetic resonance (NMR) relaxometry can be successfully applied to reveal both structural and functional components of soil hydrological connectivity. In the present study, the low field NMR relaxometry was applied on water suspended soils sampled at the upstream-and downstream-end of three different length plots. Also the sediments collected from the storage tanks at the end of each plot were water suspended and monitored by NMR relaxometry. The results from the NMR investigations were elaborated by using a mathematical approach in order to quantify both the functional and structural connectivity components. In particular, following integration of the T 1 distribution curve, an S-shaped curve was obtained. It revealed two plateaus corresponding to the shortest (low component) and the longest (high component) intervals of relaxation times, respectively. According to relaxometry theory, the two T 1 intervals, associated to the different plateaus, were attributed to micro and macro soil pores, respectively. The two T 1 intervals were used to define a functional connectivity index, while the central part of the S-shaped distribution was used to define a structural connectivity index. Here we provide the physical meaning of the our mathematical approach, thereby revealing that functional connectivity index increases with plot length, as a result of a selective eroded particle transport. Moreover, the relationship structural connectivity index versus plot length resulted quasi-independent of grainsize distribution, whereas the values of the structural connectivity index for the sediment samples resulted lower than those obtained for the corresponding soils.

Nuclear Magnetic Resonance with Fast Field-Cycling Setup: A Valid Tool for Soil Quality Investigation

“…However, 137 Cs analyses require special equipped labs as well as special precautions because of its radioactive properties. For this reason, the question on the possibility to use FFC NMR relaxometry to quantify soil erosion has risen [57,88,89]. The advantage of the technique is its low cost, the handling of non-radioactive nuclei, and its worldwide diffusion.…”

“…In order to understand how FFC NMR relaxometry can be applied to account for soil erosion, let us consider that the vertical motion described in Section 7.1 is not only related to water and nutrient leaching towards the lowest soil horizons, but also to the surface-water macroscopic flows. Both aforementioned water movements can be described by the concept of "connectivity" [57,88,89]. The latter refers to all the 'processes involving a transfer of matter, energy, and/or organisms within or between elements of a system such as landscapes, basins and soils' [90].…”

“…In fact, the shape of the relaxograms was associated to the structural connectivity, while the position of the bands in a relaxogram was related to the functional connectivity [88]. The quantification of both structural and functional connectivity can be achieved by a mathematical empirical model developed by Conte and Ferro [57,89]. In particular, the non-exceeding empirical cumulative frequency curve, F(T 1 ), obtained by the integration of any relaxogram (Figure 14), provides two different types of information.…”

“…Figure 14. The black curve is the relaxogram acquired at the proton Larmor frequency of 30 MHz for one of the soils described in Reference [89]. This relaxogram has never been published, and is only discussed in the aforementioned reference.…”

“…As a consequence, the FCI and SCI values also change, thereby allowing the monitoring of soil erosion (ongoing study) and permitting one to apply what is needed to restore soil productivity and functions. The black curve is the relaxogram acquired at the proton Larmor frequency of 30 MHz for one of the soils described in Reference [89]. This relaxogram has never been published, and is only discussed in the aforementioned reference.…”