Land subsidence is a settling of the Earth's surface due to the compaction of subsurface materials. Between about 1915 and 1969, the northern portion of Santa Clara County experienced as much as 13 feet of subsidence caused by excessive pumping of groundwater. Over 100 square miles from San Jose to southern San Francisco Bay were impacted. Subsidence has never been observed in the Coyote Valley or the Llagas Subbasin.
The two photos above show how land near the Alviso Marina sank more than six feet due to subsidence from overpumping of local groundwater between the early 20th century (top photo) and 1970 (bottom photo).
County voters approved the creation of Valley Water in 1929 to protect groundwater resources and minimize land subsidence. Subsidence has been costly to Santa Clara County, as it caused infrastructure damage, increased flooding, and saltwater intrusion that degraded groundwater quality.
By about 1970, subsidence was essentially halted because of Valley Water’s investments in reservoirs, diverse water supplies, and groundwater recharge, along with management programs that allowed groundwater conditions to recover.
Through careful water management and ongoing monitoring, Valley Water is vigilant in minimizing the risk of additional subsidence. Valley Water uses local and imported surface water to recharge the groundwater basin to replenish the groundwater that is withdrawn. Valley Water also reduces the demand on groundwater by providing alternate water supplies and through water conservation and recycling.
Valley Water actively monitors for land subsidence by monitoring hundreds of benchmarks, wells, and extensometers, which are deep, specialized monitoring instruments designed to detect changes in land surface. Valley Water has developed goals and metrics to ensure that any changes in the land surface are within acceptable levels. More information about subsidence and Valley Water’s related monitoring, goals, and metrics can be found in Valley Water’s Groundwater Management Plan, which is available at https://www.valleywater.org/your-water/where-your-water-comes/groundwater/sustainable.
Annual monitoring by Valley Water indicates that there has been no change in the land surface in northern Santa Clara County that is associated with subsidence related to groundwater pumping. Recent monitoring information can be found in Valley Water’s Annual Groundwater Report, available at https://www.valleywater.org/your-water/groundwater.