Will California’s Central Valley stop sinking?

As pumps in wells hum and pump vast amounts of water to farms in the San Joaquin Valley, the falling water levels leave underground spaces in layers of gravel, sand and clay, causing the ground to collapse and subside.

Satellite measurements have tracked the worsening problem known as land subsidence. In parts of the valley, the land has dropped about 1 foot each year. The ground shifts have damaged canals and wells and threaten to cause even more costly damage in the years to come.

In a new study, Stanford University researchers examined subsidence in an area of ​​the San Joaquin Valley over 65 years and predicted that subsidence will likely continue for decades or centuries even if water tables stopped falling. However, they also found that if aquifers recover when water levels rise significantly, it could slow or halt the subsidence within a few years.

“To get this subsidence issue under wraps, we really need to boost water levels again,” said Matthew Lees, a PhD student in geophysics and lead author of the study.

The research provides new insights into how the ground can continue to sink over a long period of time, even when the water table stops falling. Previously, there was a widespread assumption that stabilizing water levels in an aquifer with reduced pumping would solve the subsidence problem, Less said.

“What we’re showing here is that unfortunately you have this so-called lagged subsidence that continues even as you flatten the water levels,” Lees helps.

And where subsidence continues, the aquifer permanently loses space to hold water. Above ground, the sinking land is bending concrete channels, tearing up roads and other infrastructure, and can tear apart well casings.

Portions of the valley floor have collapsed about 20 feet in the last 65 years, including about 10 feet in the past 20 years, as repeated droughts have added to the stress on groundwater, Lees said.

The study also found that when groundwater levels rise in an area to stem the subsidence, groundwater levels do not need to fully recover.

The research, published this week in the journal Water Resources Research, included data from satellite measurements, well logs and water level measurements dating back to the 1950s in an area near Hanford where farmland relies on well water.

The data allowed Lees and his colleagues to develop a model to study subsidence in the area, including details such as layers of sediment and clay collapsing as water levels drop. They described underground compaction and compared it to a compressed sponge.

Researchers found that water levels in the area had dropped about 100 feet by the end of California’s last major drought, which lasted from 2012 to 2016. Then came 2017, a very wet year, and the water level rose about 10 meters. And the annual rate of subsidence slowed by more than half, from 35 centimeters to about 15 centimeters per year.

“So a kind of rough rule of thumb is that the water level should recover by about a third of what it has dropped,” Lees said, trying to significantly curb the rate at which the bottom surface is sinking.

Subsidence was one of the chronic issues that California lawmakers sought to address when they wrote the state’s groundwater law in 2014. The Sustainable Groundwater Management Act (SGMA) aims to combat overpumping and halt the decline in water levels over the next two decades.

SGMA also requires local authorities tasked with addressing decline to adopt sustainability plans and avoid a list of “undesirable outcomes,” one of which is “significant and inappropriate land subsidence that significantly impacts surface land use.”

What qualifies as “significant” subsidence is open to interpretation, but depends to a large extent on the impacts, including damage that subsidence is already causing or will cause to aqueducts or other infrastructure.

Many of the local groundwater sustainability plans written to date assume that when water levels stop falling, the sinking of water levels will stop, said Rosemary Knight, the lead study author and geophysics professor at the Stanford School of Earth, Energy and Environmental Sciences.

“But that’s not true,” Knight said. “In fact, there is a delayed subsidence that will continue for decades, past the point where you stop the water level.”

In examining the decades-long trends in this part of the San Joaquin Valley, she said, “At no point in those 65 years did the subsidence stop, not even during the recovery period of water levels. It slowed down but didn’t stop. So that’s a glimpse of the targeted effort needed to raise water levels.”

In areas where potential damage suggests subsidence needs to be slowed or halted, the results show that replenishing groundwater through so-called managed aquifer recharge projects could make a big difference, Knight said.

“How can we possibly raise these water levels? With a very aggressive charge program,” Knight said. “California’s future is likely to be more intense floods and more intense droughts. So let’s be ready to catch excess surface water and put it underground during the wet years.”

She said the study also contributes to research that can help prioritize areas where surface water should be diverted so it can seep into the ground to replenish aquifers.

“Let’s be ready to say that we want to bring it underground here. This is an area where we can slow or stop subsidence if we can replenish the groundwater system in that area,” Knight said.

In other research, Knight has been involved in mapping areas where permeable, coarse-grained alluvial soils in channels carved by ancient glaciers provide favorable conditions for surface water distribution to replenish aquifers. She said the research can help prioritize sites for groundwater recharge projects.

The reasons for addressing chronic subsidence of water tables go beyond the damage that subsidence can cause to infrastructure.

According to government data, more than 4,000 household dry wells have been reported since 2015, and the number of dry wells has increased dramatically over the past year. The state received reports of 975 household wells drying up in 2021, many in Central Valley agricultural areas.

So far this year, 258 additional dry wells have been reported.

Where there is land subsidence, it’s a symptom of an area that’s “out of balance,” where far more water is being withdrawn than is being returned through replenishment, Knight said.

Knight and other researchers have also found a strong correlation between land subsidence due to overpumping and elevated concentrations of toxic arsenic in drinking water.

“The whole mechanism of venting clay that causes subsidence can pull arsenic out of the clay as well,” Knight said. “Drawdowns to me are like this alert that there are water volume and water quality issues here.”

The state Department of Water Resources recently reviewed plans submitted by local groundwater authorities and told authorities in San Joaquin Valley agricultural areas that their plans were “incomplete” and would require changes to address the widespread risk of more wells drying up , and other problems to deal with.

State officials told dozens of local agencies in 12 aquifers, 10 of which are in the San Joaquin Valley, that they must submit revised plans in July.

Some smallholder advocates have also expressed concerns about the implementation of the Groundwater Act. Several groups, including Clean Water Action and the Community Alliance with Family Farmers, said in a report this month that their review of 14 local groundwater plans found few agencies considered the impact on farmers, who cultivate smaller areas and typically shallow wells to have. The groups said while large farms are regularly drilling deeper wells, small farms with shallower wells have dried up.

Knight said the modeling work in the study, if conducted in areas across California, would provide important information to guide management and evaluate options to combat land subsidence.

The researchers said in the study that the San Joaquin Valley has an aquifer system with three layers: an upper aquifer, including a layer known as Corcoran Clay, and then a deeper aquifer. They found that pumping from the lower aquifer is worse for triggering subsidence, indicating that pumping water from the upper zone is a better option to limit subsidence.

Sinking ground has already damaged some canals and aqueducts. The researchers said this damage required repairs that would have cost hundreds of millions of dollars.

The state announced in March that it will spend $29.8 million to repair sections of the Friant-Kern Canal damaged by land subsidence and also plans to work on others including the Delta-Mendota Canal, the San Luis Canal and the California Aqueduct.

Plans to build the high-speed train system through the valley also include building through areas where the ground is subsident, which Lee said raises serious concerns.

The California Aqueduct, which supplies water from the Sacramento-San Joaquin River Delta to farms and towns in Southern California, is not currently in a zone of major subsidence, Lees said, but a subsidence “bowl” has been creeping closer to the aqueduct .

“If that starts hitting, then you’re in really big trouble,” Lees said.

“Unless the state gets a handle on this, the state’s vital water-carrying infrastructure will either stop working or require repairs that are so expensive year after year to keep working that you basically can’t do it statewide.” Transporting water the way we rely on it,” said Lees. “That’s why it’s really imperative that the state gets this under control.”

https://www.latimes.com/california/story/2022-06-04/will-californias-central-valley-stop-sinking Will California’s Central Valley stop sinking?

Russell Falcon

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