From 1916 to 1966 in the San Jose area of Santa Clara Valley, California, deficient rainfall and runoff was accompanied by a fourfold increase in groundwater withdrawals. In response the artesian head declined 180-240 ft. The land surface subsided 12.7 ft in San Jose, due to compaction of the fine-grained compressible beds. The subsidence resulted in flooding of lands, and the compaction of the sediments caused compressional failure of many well casings. From 1967 to 1975, the artesian head recovered 70 to 100 feet due to a fivefold increase in surface water imports, favorable local water supply, decreased withdrawal, and increased recharge. In 1960, the Geological Survey installed extensometers in core holes 1,000 ft deep in San Jose and Sunnyvale. Measurements of compaction of the confined aquifer system obtained from these extensometers demonstrate the marked decrease in rate of compaction in response to the major head recovery since 1967. In San Jose the rate decreased from about 1 ft/yr in 1961 to 0.1 ft/yr in 1973. The subsidence has been stopped by raising the artesian head in the aquifers until it equaled or exceeded the maximum pore pressures in the fine-grained beds. However, the subsidence will recommence if the artesian head is drawn down appreciably below the levels of 1971-73. (USGS)
