Central Valley (California)

The Central Valley of California is one of the most productive agricultural regions in the world. It is also one of the most over-pumped aquifer systems in the United States, with cumulative subsidence in the San Joaquin Valley exceeding 30 feet in some locations since the 1920s — meaning the ground surface is more than three stories lower than it was a century ago. The water that pulled it down went into irrigation, into row crops, into dairies, into orchards. It is not coming back.

For private well owners, this is a story with two layers: the quantity problem (the aquifer is shrinking, your well will eventually go dry, the cost of going deeper compounds), and the quality problem (the water that remains is concentrated, agriculturally polluted, and contaminated with naturally occurring metals that mobilize under the chemistry of an over-pumped basin).

What it is, geologically

The Central Valley is a structural trough between the Sierra Nevada to the east and the Coast Ranges to the west, filled with hundreds to thousands of feet of Quaternary alluvial sediments — sands, gravels, silts, clays — laid down by the rivers that drain the Sierra. The aquifer system is divided geographically into the Sacramento Valley (north) and the San Joaquin Valley (south), with the Tulare Basin as the southernmost closed sub-basin.

The aquifer is multi-layered. Shallow wells tap the unconfined zone above the Corcoran Clay (a regional aquitard); deeper wells tap the semi-confined and confined zones below it. Many private wells in agricultural communities are shallow — 100-300 feet — and draw from the most heavily impacted upper aquifer.

The subsidence receipt

Land subsidence in the San Joaquin Valley has been measured continuously since the 1920s. The pattern is unmistakable:

• Cumulative subsidence in parts of western Fresno and Kings counties: 28-30+ feet since 1925.
• Current rates in active subsidence hotspots (Tulare County, around Corcoran): 1-2 feet per year during drought periods.
• The California Aqueduct, which crosses the Valley, has had to be raised multiple times because the ground beneath it is dropping.
• Highway 152, the Friant-Kern Canal, and well casings throughout the Valley are damaged annually by differential subsidence.

Subsidence is essentially permanent. When fine-grained sediments compact under the loss of pore-water pressure, the clay structure collapses irreversibly. Even if pumping stopped tomorrow and the aquifer refilled, the ground would not rise. This is the irreversibility cost of mining a confined aquifer — separate from the recoverable depletion.

The 2014 Sustainable Groundwater Management Act (SGMA) requires local groundwater agencies to bring critically over-drafted basins into balance by 2040 — twenty-six years from passage. Whether enforcement actually constrains pumping in the most over-drafted sub-basins remains an open question.

Water quality: the four problems

Arsenic — naturally occurring in the alluvial sediments, with the highest concentrations in the southern San Joaquin and Tulare Basins. About 10% of San Joaquin Valley wells exceed the EPA MCL of 10 μg/L. Arsenic mobilizes more aggressively under reducing conditions, which intensify as the aquifer is drawn down. See arsenic.

Nitrate — the dominant agricultural contaminant. The 2012 California State Water Resources Control Board's Nitrate Report to the Legislature documented that hundreds of thousands of Valley residents are exposed to drinking water above the 10 mg/L MCL. Affected communities are concentrated in dairy belts (Tulare, Kings, Stanislaus) and intensive vegetable production areas. See nitrates.

Chromium-6 (hexavalent chromium) — naturally occurring in the alluvium derived from ultramafic rocks of the Coast Ranges, plus industrial legacy contamination (the famous Hinkley case). California adopted a state MCL of 10 μg/L for chromium-6 in 2024, the first such standard in the US. Many Central Valley wells exceed it.

Legacy pesticides — DBCP (dibromochloropropane, banned 1977 after the Lathrop case), 1,2,3-trichloropropane (TCP, a Shell/Dow soil-fumigant residue), atrazine, and other agricultural chemicals persist in Valley groundwater decades after their last use. Many small water systems have been forced to abandon wells specifically for TCP.

Bottled-water communities

The Central Valley is the location of multiple rural communities — East Porterville, parts of Allensworth, Tooleville, Seville, Yettem — where private wells have either gone dry from subsidence-related declines or become contaminated beyond domestic use. Bottled water has been the primary drinking-water source for some Valley communities for over a decade. State and county programs (the SAFER fund) provide emergency water deliveries; long-term solutions (consolidation with adjacent municipal systems, treatment plant construction, deeper well replacement) move slowly.

If you're on a private well in the southern San Joaquin Valley, do not assume your situation is stable. Test annually. Watch the regional water-table data published by your local groundwater sustainability agency. Plan for replacement.

PFAS risk modeling

The USGS 2024 national PFAS-in-groundwater predictive model estimates a 19.3% mean probability of detecting any PFAS compound at typical private-well drinking-water depth across the Central Valley (California). Roughly 8.0% of the aquifer's area falls in the model's >50% probability band — land where a freshly drilled domestic well is predicted more likely than not to show some detectable PFAS. That is close to the national CONUS aquifer median of 19%, and places the Central Valley (California) in the upper half of CONUS Principal Aquifers.

This is a 1 km-resolution statistical prediction, not a measurement of any specific well. What PFAS is and how it gets into groundwater → · How to test a private well for PFAS → · Full state-and-aquifer breakdown with methodology →