Potential for aquifer compaction, land subsidence, and earth fissures in Avra Valley, Pima and Pinal counties, Arizona

Open-File Report 87-685




Avra Valley is a large north-trending alluvial basin about 15 mi west of Tucson in Pima and Pinal Counties, Arizona. The climate of Avra Valley is semiarid, and the average annual precipitation ranges from 8 to 12 in. Two major ephemeral streams, the Santa Cruz River and Brawley Wash, drain the area. These streams and their tributaries provide a source of recharge to an extensive alluvial aquifer that underlies the valley floor. The aquifer consists of interbedded gravel, sand, silt, and clay and contains a vast quantity of groundwater. The physiography, fertile soil, and mild climate of Avra Valley make it an ideal environment for agriculture and urban development. Although the aquifer is replenished by natural recharge and underflow, rates of recharge and underflow have not kept pace with the rate of pumping. Pumping has exceeded recharge for several decades, resulting in a lowering of groundwater levels throughout most of the valley. In places, water level declines have resulted in small amounts of aquifer compaction and land subsidence. Earth fissures, some of which may be the result of localized differential subsidence, also have been observed in the valley. Geohydrologic characteristics of Avra Valley that may contribute to potential aquifer compaction, land subsidence, and earth fissures are described. The quantitative evaluation of potential land subsidence in this report is presented as an interim land- and water-use planning tool pending acquisition and study of additional data. Because of data limitations, the evaluation is made on the assumption that future rates of compaction and subsidence in Avra Valley will be similar to those in the Eloy-Picacho area. The evaluation also assumes that water level declines in the aquifer will continue. Several factors that may reduce groundwater withdrawals, and thus the potential for compaction and subsidence in Avra Valley, were not evaluated. These factors include the enactment of the 1980 Arizona Ground-Water Management Act, the planned importation of Colorado River water by the Central Arizona Project, the increased reuse and reclamation of effluent, and the probable long-term decrease in pumping by agricultural and some industrial users. (Lantz-PTT)

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Potential for aquifer compaction, land subsidence, and earth fissures in Avra Valley, Pima and Pinal counties, Arizona
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Open-File Report
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5 maps on 3 sheets ;41 x 23 cm., sheets 91 x 120 cm. and smaller.