Measurement of vadose zone water content with direct-push nuclear magnetic resonance logging

Authors: Osterman, Gordon; KNIGHT, ROSEMARY - University Of Stanford

Submitted to: Environmental Research Communications
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/7/2025
Publication Date: 1/15/2025
Citation: Osterman, G.K., Knight, R. 2025. Measurement of vadose zone water content with direct-push nuclear magnetic resonance logging. Environmental Research Communications. DOI 10.1088/2515-7620/ada733.
DOI: https://doi.org/10.1088/2515-7620/ada733

Interpretive Summary: Manged aquifer recharge involves inundating surficial or near-surface sediments with water that will travel down through the unsaturated zone into the underlying aquifer, and has been explored as a tool for achieving groundwater sustainability. To predict the efficiency of recharge, i.e. how much water used for recharge actually reaches the underlying aquifer, we need to know a) if there are permeable flow-paths leading from the surface to the aquifer and b) how much water is already in the ground. In the past, researchers have used direct-push tools such as cone penetrometer testing (CPT) to understand the architecture of the subsurface. We expanded on this methodology using a direct push logging tool called borehole nuclear magnetic resonance (bNMR). By acquiring bNMR logs co-located with the CPT logs, we can quantify the water content as a function of sediment type. We demonstrate the viability of this approach at an active almond orchard in California's Central Valley. Our results show that, in spite of very low infiltration at the site in the preceeding years, the fine-grained sediments (e.g., clays, silts) can retain large amounts of water (up to 40%), while the coarse-grained sediments (e.g., sands) tend to retain much less water, although they can sometimes hold onto higher amounts. These results have implications for efforts to model managed aquifer recharge efficiency, as highly saturated clays and silts will not absorb and trap as much recharge water. The methodology we demonstrate is minimally invasive and can be readily extended to other proposed managed aquifer recharge sites.

Technical Abstract: Predicting the efficiency of managed aquifer recharge (MAR) operation requires understanding the distribution of sediment type and water content in the vadose zone. In this study we assess the use of a direct-push nuclear magnetic resonance logging technique which can rapidly image water content profiles without the need for permanent boreholes. By combining nuclear magnetic resonance logging with direct-push sediment texture logging using cone penetrometer testing, it is possible to estimate the water content in the vadose zone and relate differences in water content to sediment type. We demonstrate this approach using four co-located nuclear magnetic resonance and cone penetrometer testing logs acquired to depths of approximately fifteen meters at an active almond orchard in California’s Central Valley. The results show water contents typically greater than 25% in clay- and silt-rich sediments, and up to 10% in sand-rich sediments. The ability to rapidly assess sediment type and water content using two direct-push tools opens up many exciting opportuntities to study complex groundwater systems, even active orchards and other agricultural environments.