3302011_Notes - Michael Gregg San Joaquin Valley Subsidence...

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Michael Gregg San Joaquin Valley Subsidence The San Joaquin Valley: -Part of California’s Central Valley -Bounded by the Sierra Nevada in the East and the Diablo and Temblor mountain ranges in the West - Very important valley for food production - Currently the location of great land subsidence Causes of Subsidence in the San Joaquin Valley: - Four Identified Causes: 1) Sustained groundwater overdraft has caused aquifer system compaction 2) Hydrocompaction (near surface compaction) 3) Subsidence caused by withdrawal of oil and natural gas from beneath the earth 4) Subsidence caused by tectonic forces Geology of the San Joaquin Valley: - Filled with marine sediment overlain by continental sediment -Water table aquifer separated from an underlying confined aquifer by the Corcoran Clay layer Pre-Development San Joaquin Valley Water Budget: - Aquifer recharge from rainfall, and from rivers flowing out of the Sierra Nevada -Discharge occurred through evapotranspiration and flow of groundwater into rivers flowing out of the valley Post-Development San Joaquin Valley Water Budget: - Water discharge and recharge are no longer balanced. Discharge is now favored. - Croplands undergo more evapotranspiration than natural lands - Confined aquifer water used to water crops is exported away with the crops -Agriculture in the valley caused a deficit in the water budget as high as 800,000 acre-feet in 1960 Agricultural History of the San Joaquin Valley: -Valley first used for agriculture after the 1849 Gold Rush - Initially surface water was diverted to water crops - Unconfined aquifer was then pumped for water -Finally in the 1930s the spread of electricity to the region and new pumping technology allowed the confined aquifer to be pumped for water Effects of Groundwater Extraction: - Areas of the deep aquifer experienced a four hundred foot drop in water level -The unconfined aquifer experienced an average water table drop of forty feet Land Subsidence: - Pumping of the deep aquifer caused 75% of land subsidence witnessed in the San
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Joaquin Valley. -Maximum subsidence of 28 feet -Average subsidence of 1 foot Hydrocompaction: - Subsidence that occurs when water is added to the land surface - Dried-out, high porosity sediment structures collapse under their own weight when water is added Subsidence Mitigation -Increased reliance of surface water -More efficient water usage Problems with Subsidence
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3302011_Notes - Michael Gregg San Joaquin Valley Subsidence...

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