The subject of this book is the discovery and quantification of the concept of groundwater storage in confined aquifers. All aquifers are water-bearing and permeable, but the nature of storage is distinctly different between confined and unconfined aquifers as discussed in the GW-Project books about Groundwater in Our Water Cycle by Poeter et al. (2020) and Hydrogeologic Properties of Earth Materials and Principles of Groundwater Flow by Woessner and Poeter (2020). The upper surface of an unconfined aquifer is the water table, as discussed in both of those GW-Project books. Pumping from an unconfined aquifer drains water from its saturated void space and lowers the water table. With caveats for films of water adhering to rock grains from surface tension, the storage parameter of an unconfined aquifer at the scale of a Representative Elementary Volume (REV) (the smallest volume at which properties are the same as the properties of the whole) is the ratio of its void volume to the volume of the REV, that is, porosity. REV and porosity are discussed in the GW-Project book by Woessner and Poeter (2020). The storage parameter for an unconfined aquifer is called the specific yield. The nature of the storage mechanism is relatively straightforward.
A confined aquifer, as its name implies, is a permeable rock unit sandwiched between impermeable layers. It is recharged where it outcrops and flow is constrained to remain within the unit. The head in any cross section of a confined aquifer is the same throughout its depth and elevated above the aquifer. Pumping from a confined aquifer removes water from its void space, but the void space remains saturated. There is no water table to lower. The commonality a confined aquifer has with an unconfined aquifer is that pumping lowers the head (Figure 1). The storage in a confined aquifer resides in the compressibility of rock and of water filling the pore space in response to groundwater head or pressure changes. An interesting tipping point occurs if the head is lowered below the top of the confined aquifer because the confined aquifer then becomes unconfined and the storage parameter changes orders of magnitude from its confined to its unconfined value.
When a REV in a confined aquifer adds or releases water in response to a change of hydraulic head  , this change in storage must be included in the mass balance equation for groundwater movement. In consequence, the response of the confined aquifer to pumping or other perturbations is transient and leads to the time-dependent groundwater flow equation. More detailed discussion of hydraulic head, mass balance, and the time-dependent groundwater flow equation is provided in the GW-Project book by Woessner and Poeter (2020). The property of storage is, therefore, fundamental to the understanding of groundwater availability and movement. Groundwater storage is arguably second in importance only to Darcy’s law in its centrality to hydrogeology.
This book takes a historical perspective of storage in confined aquifers. Benchmark papers, which span nearly half a century, weave together threads from hydrogeology, geomechanics, and petroleum engineering with binding stitches from mathematics and physics. The goal is to appreciate the concept of storage in a deeper sense than is obtained from its mere definition. The story begins with an examination of a paradox with regard to the origin of subsurface irrigation water in the Dakota Territory in the north-central portion of the continental United States. Important milestones were the field investigation of the hydrogeologic system (Darton, 1896, 1901, 1909), establishment of the connection between aquifer deformation and pore fluid withdrawal (Meinzer, 1928), mathematical solution of the head response to a pumping well by analogy to heat transport (Theis, 1935), and finally derivation of the time-dependent governing equation for groundwater movement in terms of aquifer and water compressibility “from scratch” (Jacob, 1940).