Ongoing land subsidence even if aquifer levels stop declining

deferred subsidence tracked using satellite measurements

NASA satellite measurements are being used to track land subsidence in parts of California’s Central Valley, where land has been sinking about one foot each year.

A recently published study in the AGU’s journal Water Resources Research by Stanford University researchers examined over 65 years of data in an agricultural area near Hanford, California and projected that subsidence will likely continue for decades or centuries, even if aquifer levels were to stop declining.

However, researchers also report that if aquifers recover with a significant rise in water levels, subsidence could slow or stop within a few years.

"A kind of rough rule of thumb (to significantly curb the rate at which the ground surface is dropping) is that the water levels should recover about a third of the amount that they fall," said Matthew Lees, a doctoral student in geophysics and the study's lead author.

Rosemary Knight, senior study author and geophysics professor at Stanford's School of Earth, Energy and Environmental Sciences, calls attention to deferred subsidence, which “will continue for decades, beyond the point at which the water levels going down."

"At no point in those 65 years did subsidence stop, even during the water level recovery period. It (subsidence) slowed, but it didn't stop,” Knight shed light on the “targeted effort needed to raise water levels."

She is optimistic about the potential of very aggressive programs such as managed aquifer recharge (MAR) projects.

Knight and other researchers have also found a strong correlation between land subsidence due to over-pumping and water quality contamination, such as increased concentrations of toxic arsenic in drinking water.

"Draining clays that cause subsidence also can draw arsenic out of the clays," Knight said.

The study identifies a three-layer aquifer system in the San Joaquin Valley: an upper aquifer, below that a layer known as the Corcoran Clay, and then a deeper aquifer. Since pumping from the lower aquifer is likely to trigger subsidence, pumping water from the upper zone is a better option to limit the sinking.