Land subsidence is a settling of the Earth's surface due to the compaction of subsurface materials. Historically, Santa Clara County has experienced as much as 13 feet of subsidence caused by excessive pumping of groundwater.
The photo above shows how land near the Alviso Marina sank more than six feet due to overpumping of local groundwater between the early 20th century, top, and 1970.
County voters approved the creation of the district in the early 1930s partially to protect groundwater resources and minimize land subsidence. Subsidence is costly, as it can lead to flooding that damages properties and infrastructure, and saltwater intrusion that degrades groundwater quality.
The district reduces the demand on groundwater and minimizes subsidence through the conjunctive use of surface water and groundwater. A major component of the district's conjunctive use program is recharging the groundwater basin to replenish the groundwater that is withdrawn.
The district also actively monitors for land subsidence through benchmark surveying, groundwater elevation monitoring, and data from compaction wells. The district surveys hundreds of benchmarks each year to determine if there has been any change in the land surface elevation. The district also monitors groundwater levels to ensure that the amount of groundwater being pumped will not cause further subsidence. Finally, the district collects data from two compaction wells, which are 1,000 foot deep wells designed to measure any changes in the land surface resulting from groundwater extraction.