A review of specific storage values from pore pressure response to passive in situ stresses: Implications for sustainable groundwater management

Specific storage (S_s) values are important for analyzing the quantity of stored groundwater and for predicting drawdown to ensure sustainable pumping. This research compiled S_s values from multiple available studies based on pore pressure responses to passive stresses, for comparison and discussion with relevant poroelastic theory and groundwater applications. We find that S_s values from pore pressure responses to passive in situ stresses ranged from 1.3x10^-7 to 3.7x10^-5 m^-1 (geomean 2.0x10^-6 m-1, n=64 from 24 studies). This large S_s dataset for confined aquifers included both consolidated and unconsolidated strata by extending two recent literature reviews. The dataset included several passive methods: Individual strains from Earth tides and atmospheric loading, their combined effect, and values derived from soil moisture loading due to rainfall events. The range of S_s values spans approx. 2 orders of magnitude, far less than for hydraulic conductivity, a finding that has important implications for sustainable groundwater management. Both the range of values and maximum S_s values in this large dataset were significantly smaller than S_s values commonly applied including laboratory testing of cores, aquifer pump testing and numerical groundwater modelling. 

Results confirm that S_s is overestimated by assuming incompressible grains, particularly for consolidated rocks. It was also evident that Ss that commonly assumes uniaxial conditions underestimate S_s that accounts for areal or volumetric conditions.  Further research is required to ensure that S_s is not underestimated by assuming instantaneous pore pressure response to strains, particularly in low permeability strata. However, in low permeability strata S_s could also be overestimated if based on total porosity (or moisture content) rather than a smaller free water content, due to water adsorbed by clay minerals. Further evaluation is also required for influences on S_s from monitoring bore construction (ie. screen and casing or grouting), and S_s derived from tidal stresses (undrained or constant mass conditions) that could underestimate S_s applicable to groundwater pumping (drained or changing mass conditions). In summary, poroelastic effects that are often neglected in groundwater studies are clearly important for quantifying water flow and storage in strata with changing hydraulic stress and loading conditions.