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A30 REGIONAL AQUIFER-SYSTEM ANALYSIS-CENTRAL VALLEY, CALIFORNIA first application of water. Dry areas along the west and south margins of the San Joaquin Valley have subsided in such a manner (fig. 19). Within these areas, subsidence of 5 to 10 ft is common (Poland and Evenson, 1966). Compaction of sediments due to the withdrawal of oil and gas has caused land subsidence locally; however, the magnitude is uncertain. Subsidence of less than 1 ft has been attributed to this process in the oil fields near Bakersfield by Lofgren (1975). Subsidence due to tectonic movement has been negli- gible compared to the other four processes during the last 100 years, according to Williamson and others (1989). Land subsidence in California due to ground-water withdrawal has been extensively studied by the U.S. Geological Survey since the mid-1950's. The pioneer work by Geological Survey hydrologist Joseph F. Poland and his colleagues established many of the principles of the mechanics of land subsidence as well as field measure- ment techniques. Their studies were reported largely in U.S. Geological Survey Professional Papers. Areal in- vestigations of land subsidence are described in Profes- sional Paper 437, chapters A-I. Studies of the geology, physical properties, and compaction mechanisms of sed- iments in subsiding areas are described in Professional Paper 497, chapters A-G. The principal field methods used to determine the magnitude of land subsidence in California have been extensometer wells and precise leveling. A network of bench marks was established and precise leveling was done by the National Geodetic Survey as well as State and municipal government agencies. Extensometer wells were used to measure the change in thickness of the compacting sediments. Such wells consist of a heavy weight anchored into the formation below the bottom of the well casing and a cable attached to the weight on one end and a counterweight at the other end. A recorder provided continuous measurement of the movement of the land surface with respect to the anchor weight. For a summary of field methods to measure land subsidence, the interested reader is referred to a UNESCO guide- book on land subsidence (Poland, 1984). MECHANICS OF LAND SUBSIDENCE Land subsidence due to withdrawal of ground water is caused by compaction of clay within an aquifer system. When pumpage causes the hydraulic head to decline below the preconsolidation stress level, the effective stress (grain-to-grain load) increases and the clay is compacted, releasing water to the aquifer system. A brief summary of the mechanics of land subsidence is given here. This discussion is based largely on detailed analysis of the stresses involved in land subsidence as presented by Lofgren (1968) and Poland (1984, p. 37-54). The classic equation for effective stress (originally devel- oped by Karl Terzaghi and described in Terzaghi and Peck, 1967) is as follows: where P P tO'iJ? P' = P - is effective stress (effective overburden pres- sure or grain-to-grain load), is total stress (geostatic pressure), and is pore pressure (fluid pressure). As the hydraulic head is reduced in a confined aquifer (sand and (or) gravel), the geostatic pressure is not significantly changed. Thus, the decreased pore pressure causes increased grain-to-grain load. The compaction of the aquifer is small, immediate, and largely recoverable. However, for confining beds (clay and silt) with much lower permeability but higher specific storage, the re- sponse is quite different. The adjustment of pore pres- sure in the confining beds to head decline in the aquifer proceeds slowly (after months or years). Compaction is substantial and largely unrecoverable. If pumping ceases, heads recover, and compaction of the confining beds eventually ceases (though it may continue for some time). If pumping resumes, the confining beds will not be compacted until the head declines below the head (critical head, fig. 20) of the previous pumping period (providing the compaction was completed during the previous pump- ing period). The loss of inelastic storage from the compacting clay is not recoverable. The recovery of heads to prepumping levels is not accompanied by a recovery of storage lost to compaction. Inelastic ., (compaction)-"^ storage \ " I ii IBIUSUI; i\ (compaction)' istorage [ TIME FIGURE 20. Relation of ground-water storage to hydraulic head in a compacting aquifer system (modified from Prudic and Williamson, 1986).