Storage coefficient revisited: Is purely vertical strain a good assumption?

The storage coefficient that is used ubiquitously today was first defined b y the analytical work of Theis and Jacob over a half-century ago. Inherent within this definition is the restriction of purely vertical compression of the aquifer during a reduction in pressure. The assumption is revisited an d quantitatively evaluated by comparing numerical results using both one- a nd three-dimensional strain models in the presence of three-dimensional flo w Results indicate that (1) calculated hydraulic head values are nearly ide ntical for both models; (2) the release of water from storage in terms of v olume strain is nearly identical for both models and that the location of m aximum production moves outward from the web as a function of time; (3) the vertical strain components are markedly different with at least 50% of the total volume of water pumped originating from horizontal strain (and incre asing to as much as 70%); and (4) for the one-dimensional strain model to y ield the necessary quantity of water to the pumped well, the resulting vert ical compaction (land subsidence) is as much as four times greater and vert ical strain is as much as 60% greater than the three-dimensional strain mod el. Results indicate that small changes in porosity resulting from horizont al strain can yield extremely large quantities of water to the pumping well . This study suggests that the assumption of purely vertical strain used in the definition of the storage coefficient is not valid.