Estimating the water holding capacity of the critical zone using near‐surface geophysics

Abstract

In high-mountain watersheds, the critical zone holds crucial life-sustaining water stores in the form of shallow groundwater aquifers. To better understand the role that the critical zone plays in moderating hydrologic response to fluxes at the surface and in the subsurface, the hydrologic properties must be characterized over large scales (i.e., that of the watershed). In this study, we estimate porosity from geophysical measurements across a 58-ha area to depths of~80 m. Our observations include velocities from seismic refraction, downhole nuclear magnetic resonance logs, downhole sonic logs, and samples acquired by push coring. We use a petrophysical approach by combining two rock physics models, a porous medium for the saprolite and a differential effective medium for the fractured rock, into a Bayesian inversion. The inverted geophysical porosities show a positive correlation with measured values (R 2 = 0.93). We extrapolate the porosity estimates from 30 individual seismic refraction lines to a 3D volume below our study area using ordinary kriging to quantify the water holding capacity of our study area. Our results reveal that the critical zone in our study area holds 2.9 × 10 6 ± 9.6 × 10 5 m 3 of water, where 34% of this storage is in the saprolite, 55% is in the fractured rock, and the remaining 11% is in the bedrock.

Characterizing the Critical Zone Using Borehole and Surface Nuclear Magnetic Resonance

“…We interpret the region between ∼14 and 17 m as saturated saprolite instead of a large fracture zone for three reasons. First, the maximum water content measured by the borehole NMR (0.34 m 3 m −3 ) is consistent with the average measured porosity of saprolite (0.34 ± 0.06 m 3 /m 3 ) based on 25 measurements collected along the ridge in the study area (Flinchum et al, 2018a, 2018b). Second, it occurs just above the casing depth (Fig.…”

“…8c). The water table measured by hand and reported by Flinchum et al (2018a) is associated with increased water content observed in six of the seven soundings (Fig. 8c).…”

“…Conceptually, saprolite is expected to have high porosity but will only produce an NMR response if there is water in the pore space. For our site, the saprolite porosity has been measured on laboratory samples up to depths of 9 m and ranges between 0.28 and 0.4 m 3 m −3 (Flinchum et al, 2018a). The weathered bedrock is expected to have water in fractures that could be either open or filled with weathering products, and these fractures would be bound by a rock matrix with significantly lower porosity than the saprolite.…”

“…1b; Novitsky et al, 2018). Underneath the ridge at our study site, the water table is between 12 and 13 m below the ground surface (Flinchum et al, 2018a). An ephemeral stream with numerous beaver ponds in the southern drainage reaches peak runoff between April and June.…”

Digging deeper: what the critical zone perspective adds to the study of plant ecophysiology

“…New methodological advances, particularly using geophysical methods (such as seismic refraction and nuclear magnetic resonance) can be used to test these models and characterise the resulting subsurface porosity structure that determines the water‐holding capacity of weathered bedrock (e.g. Flinchum et al , ; Rempe et al ., ).…”