SR0084717_SSNL - Laserfiche WebLink

Home Browse Search

GROUND WATER IN THE CENTRAL VALLEY, CALIFORNIA A31 The relation of storage and hydraulic head in a com- pacting aquifer system is shown in figure 20. Note that head declines are rapid when pumped water is derived from elastic storage but are slow when it is derived from inelastic storage. Water derived from compaction has varied from a few percent to more than 60 percent of the pumped water in the subsiding areas. These differences were attributed to variations in geology and well construction, as discussed in chapter C (Page, 1986), and are summarized here. The percentage of fine-grained deposits within the stressed interval and the mineralogy of these deposits are impor- tant factors. In the Los Banos-Kettleman City area, an area of maximum land subsidence, the highest percent- age of fine-grained deposits anywhere in the San Joaquin Valley lies within the upper 2,000 ft of the aquifer system. Bull (1975, p. F49) suggested that within this area, interlayering of thin-bedded, compressible fine- grained sediments with permeable coarse-grained sedi- ments resulted in rapid and substantial compaction in response to water-level declines. Compaction is less with the same water-level declines in areas of coarse-grained beds. The type of clay mineral present influences subsidence; montmorillonite is highly susceptible to compaction and is the predominant clay mineral in the major subsiding areas of the San Joaquin Valley. Differences in hydraulic head throughout the pumped interval also affect compac- tion; the rate of compaction is less when the water table and artesian head are lowered simultaneously, as in a well that is perforated in both water-table and confined zones. The factors influencing inelastic compaction and land subsidence are summarized in figure 21. Where wells are perforated in confined zones of an aquifer system that contain numerous thin lenses of compressible montmoril- lonite clay, inelastic compaction will be at a maximum. However, where wells tap both water-table and confined zones and much of the perforated section falls within coarse-grained deposits, compaction will be minimal. The measured compaction in relation to head decline at two wells in subsiding areas from 1960 to 1980 is shown in figure 22. At each site, the 1960's were marked by steady head decline and a high rate of compaction. The decrease in ground-water pumpage in the early to middle 1970's was accompanied by a steady recovery of water levels and greatly reduced compaction. The resumption of large ground-water withdrawals during the 1976-77 drought was marked by a sharp decline in water levels and a short period of renewed compaction. Ireland and Large head difference-i^ Fine-grained layers; ^=^=^^1^-=E^-- ' - . .'.'.- Sand " ." .-.' ". '. "i Cone of depression . . . mostly montmorillonite clay ~^=^- _^ .'.-.-'. -. Artesian head (lower sand) Artesian head (lower sand) Sand "~~ ~- .' .. Numerous thin 0 " './ : fine-grained lensesSand . . .-.-.-. Of mostly j ;. ' ' ' ' montmorillonite clay I I Sand : . -. -. ' j ]...- .' .'. .-.'.-.'. . Fine-grained layers; little or no montmorillonite clay I I.1 i:i i i i Sand B FIGURE 21. Influence of hydrogeology and well construction on potential for land subsidence. Dashed lines denote perforated sections of wells. A, High potential for subsidence. B, Low potential for subsidence.