Probing water partitioning in unsaturated weathered rock using nuclear magnetic resonance

Abstract

The “Rock moisture” (exchangeable water stored in weathered bedrock beneath the soil) is a key and yet overlooked component in hydrologic cycles. It can be partitioned to free water and capillary-bound water. Determining dynamic partitioning of rock moisture is crucial for conceptualizing critical zone functions and climate and hydrologic modeling. However, the quantification of rock moisture partitioning is challenging, especially in rocks with complex pore structure and weathering patterns. To quantify the dynamics of rock moisture partitioning during the drying process, laboratory nuclear magnetic resonance (NMR) measurements are performed on heterogeneous bedrock samples from a merokarst aquifer. By fitting a multi-Gaussian function, NMR T2 relaxation time spectra are auto-decomposed into multiple T2 peaks representing different pore sizes and environments. This spectral analysis enables us to track the change of position, width, and area of peaks at any given saturation stage, shedding light on water depletion rates and patterns, water residence time, and partitioning and redistribution of the water in drying rocks. Combining with mineralogic information and T2- T2 measurements, it is shown that rock moisture depletion and redistribution are closely related to the pore structures. Limestone with well-connected macropores shows a sequential water loss from large to small pores, while limestone with poorly-connected macropores simultaneously loses water from all pore sizes. The T2 peak decomposition analysis is rigorous and can be extended to field-scale to discern nuanced rock moisture partitioning in complex pore structure and to harness the rich information provided by NMR during critical zone hydrogeological investigations. Valuable insights are gained from our work towards groundwater recharge and discharge, weathering, and dry season evapotranspiration.

Recovery of Rare Earth Elements from Ion-Adsorption Deposits Using Electrokinetic Technology: The Soil Conductivity Mechanism Study

“…Hence, obtaining water contents in distinct positions of the soil provides a strong basis for optimizing the addition of leaching agents. Various methods have been developed to measure the water content of soils in the field, such as seismic refraction [45], induced polarization [46], and nuclear geophysical [47] methods. Among these, measuring the soil conductivity by the four-electrode resistance method [48] to inverse moisture content of weathering crust soil by the extended Archie's equation is a convenient and economical way.…”

Water partitioning and migration in unsaturated bentonites by low‐field NMR characterization

“…The detailed fitting procedure is presented in Figure 1. To ensure the convergence of the optimization process, we set the maximum iteration number at 1500 (F. Zhang & Zhang, 2021). Our approach began with an initial value of K = 2, representing the number of Gaussian peaks, and proceeded to iteratively update K to find the optimal solution that provides a reasonable number of peaks.…”

A critical mini-review on the low-field nuclear magnetic resonance investigation of pore coupling effects in near-surface environments

“…2D T 2 − T 2 -maps (Figures 1E, F) are measurements to detect the chemical shift of protons and monitor the exchange of water among different pore environments during designed experimental mixing periods (Song, 2012). In pore coupled environment (Figure 1F), the magnetization can be exchanged between environments multiple times during the NMR measurement, leading to shifts in the peaks away from the diagonal and asymmetry in cross-peaks (Song, 2012;Zhang and Zhang, 2021).…”

“…Several versions of the Random Walk method have been developed deviating from the traditional fix-step-size technique: in the First Arrival or First Passage approach (e.g., Toumelin et al, 2003;Mitchell et al, 2019) the step-size of the walkers depends on its proximity to the pore wall, while, in the Variable-Step-Size approach (e.g., Zhang et al, 2011;Carneiro et al, 2013), each walker moves at changing step-sizes with its own internal clock. Furthermore, 1E, F), to detect and quantify pore coupling (e.g., Washburn and Callaghan, 2006;Mitchell et al, 2019;Zhang and Zhang, 2021). Although T 2 − T 2 -maps are great tools to identify a pore-coupled sample, they require more demanding experimental techniques than 1D approaches (Fleury and Soualem, 2009), with evidence suggesting that no additional pore couple strength information can be obtained from T 2 − T 2 -maps, in comparison to T 1 -or T 2 -distributions (Johnson and Schwartz, 2014).…”