How does groundwater pumping cause subsidence, or a drop in land elevation?
Land subsidence is a gradual settling or sudden sinking of the Earth’s surface owing to subsurface movement of earth materials. In alluvial aquifers that include silt and clay layers, such as in the San Joaquin Valley, long-term groundwater-level declines can result in a compaction of those layers, and that manifests itself as land subsidence. Is this the first time subsidence has occurred in the San Joaquin Valley? No. In past decades, extensive withdrawal of groundwater has caused widespread land subsidence in the San Joaquin Valley, in some places exceeding 28 feet. Land subsidence from groundwater pumping began in the 1920’s, and by 1970 significant land subsidence (more than one foot) had occurred in about half of the San Joaquin Valley. During the droughts of 1976–77 and 1986–92, diminished deliveries of imported water prompted pumping of groundwater to meet irrigation demands. This increased pumping resulted in water-level declines and periods of renewed subsidence. Following ea
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