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A30 REGIONAL AQUIFER-SYSTEM ANALYSIS—CENTRAL VALLEY, CALIFORNIA first application of water. Dry areas along the west and The classic equation for effective stress(originally devel- south margins of the San Joaquin Valley have subsided in oped by Kari Tetzaghi and described in Terzaghi and such a manner(fig. 19).Within these areas,subsidence of Peck, 1967) is as follows: 5 to 10 ft is common (Poland and Evenson, 1966). I. Compaction of sediments due to the withdrawal of oil P, = P-2l.w and gas has caused land subsidence locally;however,the magnitude is uncertain. Subsidence of less than 1 ft has where been attributed to this process in the oil fields near P' is effective stress (effective overburden pres- Bakersfield by Lofgren (1975), sure or grain-to-grain load), Subsidence due to tectonic movement has been negli- P is total stress (geostatic pressure), and gible compared to the other four processes during the last 14µ, is pore pressure (fluid pressure). 100 years, according to Williamson and others (1989)- Land subsidence in California due to ground-water As the hydraulic head is reduced in a confined aquifer withdrawal has been extensively studied by the U.S. (sand and (or) gravel), the geostatic pressure is not Geological Survey since the mid-195[1'8.The pioneer work significantly changed.Thus,the decreased pore pressure by Geological Survey hydrologist Joseph F. Poland and causes increased grain-to-grain load. The compaction of his colleagues established many of the principles of the the aquifer is small, immediate,and largely recoverable. mechanics of land subsidence as well as field measure- However, for confining beds (clay and silt) with much ment techniques. Their studies were reported largely in lower permeability but higher specific storage, the re- U.S. Geological Survey Professional Papers. Areal in- sponse is quite different. The adjustment of pore pres- vestigations of land subsidence are described in Profes- sure in the confining beds to head decline in the aquifer sional Paper 437, chapters A-1. Studies of the geology, proceeds slowly (after months or years). Compaction is physical properties, and compaction mechanisms of sed- substantial and largely unrecoverable. If pumping iments in subsiding areas are described in Professional ceases, heads recover, and compaction of the confining Paper 497, chapters A-G. beds eventually ceases(though it may continue for some The principal field methods used to determine the time). If pumping resumes,the confining beds will not be magnitude of land subsidence in California have been compacted until the head declines below the head(critical extensometer wells and precise leveling. A network of head, fig. 2(1)of the previous pumping period (providing bench marks was established and precise leveling was the compaction was completed during the previous pump- done by the National Geodetic Survey as well as State ing period). The loss of inelastic storage from the and municipal government agencies. Extensometer wells compacting clay is not recoverable. The recovery of were used to measure the change in thickness of the heads to prepumping levels is not accompanied by a compacting sediments. Such wells consist of a heavy I recovery of storage lost to compaction. 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 he I hC2I he3.hC4= I roads the land surface with respect to the anchor weight. For a summary of field methods to measure land subsidence, w the interested reader is referred to a UNESCO guide- P > hO' book on land subsidence (Poland, 1984). o i 'O2 - _ I i r + �1'tC4HAt�l(3 I1F LAII'l1 SL'H51 r'EltiLE � F hC4 1 ..J C3Elastic - ----r---}---- I I Land subsidence due to withdrawal of and water is a storage stol g i r restac I I I `storages caused by compaction of clay within an aquifer system. a ❑ When pumpage causes the hydraulic head to decline x ❑ Inelastic t: �-;Inelesuc (compactlony-- { I �(cpmpactlo'n) below the preconsolidation stress level, the effective storage I i I storage stress (grain-to-grain load) increases and the clay is I - compacted, releasing water to the aquifer system. A T1 Tz T3 T4 T6 T6 brief summary of the mechanics of land subsidence is TIME given here. This discussion is based largely on detailed FIGURE 20.—Relation of ground-water storage to hydraulic head in a analysis of the stresses involved in land subsidence as compacting aquifer system (modified from Prudic and Williamson, presented by Lofgren(1968)and Poland(1984,p.37-64). 1986).