Nov. 29, 2000 BLACKSBURG, VA. — The assumption, held for over half a century, that purely vertical strain used in the definition of the storage coefficient in aquifers is not valid, a study at Virginia Tech shows. Since the analytical work of Theis and Jacob over half a century ago, scientists have used only vertical strain to measure storage in aquifers, ignoring what is going on in the horizontal direction, said Thomas J. Burbey of Virginia Tech's Department of Geological Sciences in the College of Arts and Sciences. Water is released from aquifers in two ways, Burbey said—through aquifer compression or compaction and through the release of pressure that allows the water to expand, much as air expands in a tire when the valve is released.
Burbey said that the aquifer compression or compaction of water is occurring in a horizontal as well as vertical direction, leading to the release of water from storage. For his study, Burbey used numerical models that take into consideration only vertical strains, then wrote a model for three-dimensional strain, including horizontal, and compared the results with those using the vertical strain only.
He found that water release occurs due to horizontal compression, not just vertical. While his studies show that calculated hydraulic head values, or water levels, and the production of water in terms of volume strain are nearly identical for both models, he also discovered that, over time, the location of the maximum production, or place from which water is pumped, moves outward, horizontally, from the well. Burbey found, also, that when the model incorporates the horizontal strain, more than half of the water, and up to 70 percent, originates from horizontal strain and that the percentage of water pumped from horizontal strain increases over time.
Finally, Burbey found that producing the same quantity of water using just one dimension requires much more compaction, or land subsidence, to accommodate the volume of water pumped out.
"Results indicate that small changes in porosity resulting from horizontal strain can yield extremely large quantities of water to the pumping well," Burbey wrote. "This study suggests that the assumption of purely vertical strain used in the definition of the storage coefficient is not valid."
Burbey presented his findings at the Geological Society of American meeting in Reno in November.
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