Quantifying Compressible Groundwater Storage by Combining Cross‐Hole Seismic Surveys and Head Response to Atmospheric Tides

Abstract

Groundwater specific storage varies by orders of magnitude, is difficult to quantify, and prone to significant uncertainty. Estimating specific storage using aquifer testing is hampered by the nonuniqueness in the inversion of head data and the assumptions of the underlying conceptual model. We revisit confined poroelastic theory and reveal that the uniaxial specific storage can be calculated mainly from undrained poroelastic properties, namely, uniaxial bulk modulus, loading efficiency, and the Biot‐Willis coefficient. In addition, literature estimates of the solid grain compressibility enables quantification of subsurface poroelastic parameters using field techniques such as cross‐hole seismic surveys and loading efficiency from the groundwater responses to atmospheric tides. We quantify and compare specific storage depth profiles for two field sites, one with deep aeolian sands and another with smectitic clays. Our new results require bulk density and agree well when compared to previous approaches that rely on porosity estimates. While water in clays responds to stress, detailed sediment characterization from a core illustrates that the majority of water is adsorbed onto minerals leaving only a small fraction free to drain. This, in conjunction with a thorough analysis using our new method, demonstrates that specific storage has a physical upper limit of ≾1.3·10−5 m−1. Consequently, if larger values are derived using aquifer hydraulic testing, then the conceptual model that has been used needs reappraisal. Our method can be used to improve confined groundwater storage estimates and refine the conceptual models used to interpret hydraulic aquifer tests.

Comparing Methods of Barometric Efficiency Characterization for Specific Storage Estimation

“…All but two estimated specific storage values ranged from 7 × 10 −7 to 3 × 10 −6 /m, none of which exceeded the physically plausible upper limit of 1.3 × 10 −5 /m proposed by Rau et al (2018). Four dual-well historical pumping tests were previously undertaken in confined sandstone aquifers of the Leederville Formation.…”

“…These included that imposed stresses were uniaxial (i.e., occurring in the vertical plane only) and the existence of undrained conditions. The latter assumes that there is insufficient time for water to flow in response to an imposed pressure fluctuation (Rau et al ). The key assumption made when using the Jacob () solution (Equation 4) to calculate specific storage values from BE estimates is that grains are incompressible.…”

“…Conversely, two estimated BE values (0.627 and 0.656) corresponded to matrix compressibility values that exceeded the known range of sandstone grain compressibility. In the absence of poroelastic parameter characterization (e.g., Rau et al ) and site‐specific testing of sandstone grain compressibility, the assumption of grain incompressibility at these two wells cannot be excluded. Past studies of sandstone samples indicated that the failure to account for grain compressibility can lead to overestimation of specific storage values by 5‐12% (van der Kamp and Gale ).…”

“…For unconsolidated sedimentary aquifers, a comprehensive characterization of specific storage that does account for grain compressibility (van der Kamp and Gale ; Green and Wang ) should instead be used. This can involve the use of cross‐borehole seismic surveys to estimate the additional poroelastic parameters required (Rau et al ). Conversely, for aquifers hosted in consolidated rocks (such as those assessed in the present study), the assumption of grain incompressibility can be justified.…”

Author response to SC2

“…The second, mobile GNSS receiver (rover) uses the GNSS signals in conjunction with the error correction to calculate its distance from the base station. The error correction is determined from signal phase observations at both stations (Remondi, 1985). This can be achieved offline by post-processing the stored satellite signals in both receivers or in real time through a radio link between the rover and the base.…”