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GROUND WATER Ili THE CENTRAL VALLEY, CALIFORNIA A31 The relation of storage and hydraulic head in a com- the predominant clay mineral in the major subsiding pacting aquifer system is shown in figure 20. Note that areas of the SanJoaquin Valley. Differences in hydraulic head declines are rapid when pumped water is derived head throughout the pumped interval also affect compac- from elastic storage but are slow when it is derived from tion;the rate of compaction is less when the water table inelastic storage. and artesian head are lowered simultaneously, as in a Water derived from compaction has varied from a few well that is perforated in both water-table and confined percent to more than 60 percent of the pumped water in zones. the subsiding areas.These differences were attributed to The factors influencing inelastic compaction and land variations in geology and well construction, as discussed subsidence are summarized in figure 21.Where wells are in chapter C(Page, 1986),and are summarized here. The perforated in confined zones of an aquifer system that percentage of fine-grained deposits within the stressed contain numerous thin lenses of compressible montmoril- interval and the mineralogy of these deposits are impar- lonite clay, inelastic compaction will be at a maximum- tant factors. In the Los Banos-Kettleman City area, an However,where wells tap both water-table and confined area of maximum land subsidence, the highest percent- zones and much of the perforated section falls within age of fine-grained deposits anywhere in the San Joaquin coarse-grained deposits, compaction will be minimal. Valley lies within the upper 2,606 ft of the aquifer The measured compaction in relation to head decline at system- Bull (1975, p. F49) suggested that within this two wells in subsiding areas from 1960 to 1980 is shown area, interlayering of thin-bedded, compressible fine- in figure 22. At each site, the 1960's were marked by grained sediments with permeable coarse-grained sedi- steady head decline and a high rate of compaction. The ments resulted in rapid and substantial compaction in decrease in ground-water pumpage in the early to middle response to water-level declines. Compaction is less with 1970's was accompanied by a steady recovery of water the same water-level declines in areas of coarse-grained levels and greatly reduced compaction. The resumption beds. of large ground-water withdrawals during the 1976-77 The type of clay mineral present influences subsidence; drought was marked by a sharp decline in water levels montmorilionite is highly susceptible to compaction and is and a short period of renewed compaction. Ireland and .. Water table Sand La' a Water table head difference Sand Small head difference Artesian head .� (lower sand) Gone of depression Artesian head I I (lower sand) t I - f Fine-grained 4ayers; mostly mommonllon+le clay - _- Fine-grained layers; little or no montmorillon-te clay Sand I I _ t ! I l Sand . �r Numerous thin Sand line-grained lenses i of mostly montmoriilonite clay — — =embedded in a sand �•I - f--� Sand __I Fine-grained layers; little or no monimorillonite clay B FicL%E 21—lnfluence of hydrogeology and well construction on potential for land subsidence.Dashed lines denote perforated sections of wet ls-A,High potential for subsidence.B.I..ow potential for subsidence.