Groundwater extraction is increasing rapidly in many areas of the world, causing serious impacts such as falling water tables, ground surface subsidence, water quality degradation, and reduction of stream baseflow on which many ecosystems depend. Methods for understanding and predicting the impacts of groundwater extraction generally lack detailed spatial and temporal knowledge of the subsurface hydrogeomechanical properties. This review provides a comprehensive understanding of Earth and atmospheric tides and their impact on subsurface pore pressure. First, we evaluate the global occurrence of Earth and atmospheric tides. Then, we illustrate their impact on the groundwater response and connect this with the theory of poroelasticity, which underpins quantitative analyses. Finally, we review methods that utilize these impacts to characterize groundwater systems and to quantify their hydrogeomechanical properties. We conclude by highlighting their potential as passive and low‐cost investigation techniques and by outlining the research and developments required to progress and make analyses readily available. Thus, hydrogeomechanical properties of subsurface systems could be obtained at unprecedented spatial and temporal resolution, adding additional value to commonly acquired groundwater and atmospheric pressure data.

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.

Accurate determination of groundwater state of confinement and compressible storage properties at vertical resolution over depth is notoriously difficult. We use the hydraulic head response to atmospheric tides at 2 cpd frequency as a tracer to quantify barometric efficiency (BE) and specific storage (Ss) over depth. Records of synthesized Earth tides, atmospheric pressure, and hydraulic heads measured in nine piezometers completed at depths between 5 and 55 m into unconsolidated smectitic clay and silt, sand and gravel were examined in the frequency domain. The barometric efficiency increased over depth from ∼0.05 in silty clay to ∼0.15 in sands and gravels. BE for silty clay was confirmed by calculating the loading efficiency as 0.95 using rainfall at the surface. Specific storage was calculated using effective rather than total moisture. The differences in phase between atmospheric pressure and hydraulic heads at 2 cpd were ∼180° below 10 m indicating confined conditions despite the low BE. Heads in the sediment above a fine sand and silt layer at 12 m exhibited a time variable phase difference between 0° and 180° indicating varying confinement. Our results illustrate that the atmospheric tide at 2 cpd is a powerful natural tracer for quantifying groundwater state of confinement and compressible storage properties in layered formations from hydraulic heads and atmospheric pressure records without the need for externally induced hydraulic stress. This approach could significantly improve the development of conceptual hydrogeological model used for groundwater resource development and management.

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Degradation of the quality of groundwater due to salinization processes is one of the key issues limiting the global dependence on groundwater in aquifers. As the salinization of shallow aquifers is closely related to root-zone salinization, the two must be considered together. This chapter initially describes the physical and chemical processes causing salinization of the root-zone and shallow aquifers, highlighting the dynamics of these processes and how they can be influenced by irrigation and drainage practices, thus illustrating the connectivity between soil and groundwater salinization. The processes leading to aquifer salinization in both inland and coastal areas are discussed. The roles of extractive resource industries, such as mining and coal bed methane operations, in causing aquifer salinization are also outlined. Hydrogeochemical changes occurring during salinization of aquifers are examined with the aid of Piper and Mixing Diagrams. The chapter then illustrates the extent of the problem of groundwater salinization as influenced by management and policy using two case studies. The first is representative of a developing country and explores management of salt-affected soils in the Indus Valley, Pakistan, while the second looks at a developed country, and illustrates how through monitoring we can deduce

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