Aquifer-system compaction, Tucson Basin and Avra Valley, Arizona

Abstract

Ground-water declines of several feet per year since the 1940's induced aquifer-system compaction and land subsidence of as much as 0.5 foot in the Tucson basin and 1.1 feet in Avra Valley. Aquifer-system compaction is affected by layering, hydraulic diffusivity, preconsolidation stress, and stress history of the aquifer system. Layering at extensometer sites can be categorized into three general groups that typify the fine- and coarse-grained layering within the Fort Lowell Formation and upper Tinaja beds. A one-dimensional compaction model was used to simulate aquifer-system compaction of less than 0.1 foot. Elastic and some inelastic specific storage values are comparable to previous estimates. Parts of the aquifer systems appear to be in transition from predominantly elastic to inelastic compaction. Water-level declines since 1940 at the six modeled extensometer sites are within an estimated preconsolidation-stress threshold of 50 to 150 feet. The simulations were most sensitive to reduction of initial preconsolidation stress and least sensitive to changes in aquifer specific storage.