Evaluation of ¹H NMR relaxometry for the assessment of pore‐size distribution in soil samples

Abstract

1 H NMR relaxometry is used in earth science as a non-destructive and time-saving method to determine pore size distributions (PSD) in porous media with pore sizes ranging from nm to mm. This is a broader range than generally reported for results from X-ray computed tomography (X-ray CT) scanning, which is a slower method. For successful application of 1 H NMR relaxometry in soil science, it is necessary to compare PSD results with those determined from conventional methods. The PSD of six disturbed soil samples with various textures and soil organic matter (SOM) content were determined by conventional soil water retention at matric potentials between −3 and −390 kPa (pF 1.5-3.6). These PSD were compared with those estimated from transverse relaxation time (T 2 ) distributions of water in soil samples at pF 1.5 using two different approaches. In the first, pore sizes were estimated using a mean surface relaxivity of each soil sample determined from the specific surface area. In the second and new approach, two surface relaxivities for each soil sample, determined from the T 2 distributions of the soil samples at different matric potentials, were used. The T 2 distributions of water in the samples changed with increasing soil matric potential and consisted of two peaks at pF 1.5 and one at pF 3.6. The shape of the T 2 distributions at pF 1.5 was strongly affected by soil texture and SOM content (R 2 = 0.51 − 0.95). The second approach (R 2 = 0.98) resulted in good consistency between PSD, determined by soil water retention, and 1 H NMR relaxometry, whereas the first approach resulted in poor consistency. Pore sizes calculated from the NMR data ranged from 100 μm to 10 nm. Therefore, the new approach allows 1 H NMR relaxometry to be applied for the determination of PSD in soil samples and for studying swelling of SOM and clay and its effects on pore size in a fast and non-destructive way. This is not, or only partly, possible by conventional soil water retention or X-ray CT.

Understanding NMR relaxometry of partially water-saturated rocks

“…The relaxationtime distribution functions derived from NMR measurements for such partially saturated rocks are frequently found to be shifted towards shorter relaxation times outside the original envelope observed for a fully saturated sample (Fig. 2b) (e.g., Applied Reservoir Technology Ltd., 1996;Bird et al, 2005;Jaeger et al, 2009;Stingaciu, 2010;Stingaciu et al, 2010;Costabel, 2011).…”

Proton NMR Relaxometry as a Useful Tool to Evaluate Swelling Processes in Peat Soils~!2009-07-26~!2009-12-04~!2010-06-15~!

“…Various rock core samples, e.g. sandstones [28,36], were assigned to the fast-diffusion regime [31], whereas some sandy soil samples [37] were in the transition from the intermediate-to the slow-diffusion regime [31]. However, it has been shown that Eq.…”

“…However, it has been shown that Eq. 2 can be also used for the calculation of pore sizes in soils by assuming the fast-diffusion regime in these soil samples and by using two surface relaxivities for each soil, one for micro-and one for mesopores, [37].…”

“…After every centrifugation step, the gravimetric water contents and the T 2 and T 1 distributions of water in the peat samples at 19°C were determined. Instead of the largest T 2 and T 1 in the relaxation time distribution, T 2 and T 1 at 90 % of total sum of amplitudes were determined to reduce variation between replicates [37]. After the last centrifugation step, T 1B and T 2B were measured in the extracted soil solution, which was gained during centrifugation, to determine the contribution of the soil solution to the total proton relaxation in the wet peat samples.…”

Effects of olive mill wastewater disposal on soil: Interaction mechanisms during different seasons

“…A CarrPurcell-Meiboom-Gill (CPMG) pulse sequence was used to obtain T2 and the corresponding relaxation rates of the water protons in the samples (Jaeger et al, 2009;Meiboom and Gill, 1958). Pore size distribution was determined following the procedure described in Meyer (2015).…”