Probing water partitioning in unsaturated weather rock using nuclear magnetic resonance

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.