When you walk through Jeannie Williams's sunny orchard, you don't notice anything wrong. But the problem's there, underfoot. The land around her — about 250 square kilometres — is sinking.

"It's frightening," Williams says. "Is the land going to come back up? I don't know."

She points out the well from which she obtains all of the water she needs to grow organic fruits and vegetables. The well is small and shallow; she only has two acres of crops to water. But her neighbours are far more thirsty, and have been for a very long time.

Her farm in Hanford is located in California's Central Valley, a 47,000 square kilometre area that produces about a quarter of America's food supply — and a large part of the $3.6 billion US worth of produce that California exports to Canada.

For more than a century farmers in the Central Valley have been pumping water out of the ground — so much so that the land is slowly sinking, a process known as subsidence. In less than 100 years, it's dropped 8½ metres.

"Some areas actually are sinking one to two feet [30-60 centimetres] a year," says recently retired NASA geologist Tom Farr. "That's a huge amount."

For years, researchers like Farr have used satellites to measure exactly how far and how fast the ground is sinking. A recent graphic of the southern Central Valley produced by the Scripps Institution of Oceanography shows a dark purple area indicating the most serious area of subsidence.

"It's relatively confined to that area around Corcoran, and it's about 30 or 40 miles [50 to 65 km] across, so it's not small," Farr says.

This image produced by the Scripps Institution of Oceanography shows subsidence in 2017. The darker the area, the more the ground has sunk. (Wesley Neely/Scripps Institution of Oceanography)

Corcoran, only 30 kilometres south of Williams's farm in Hanford, bills itself as the "Farming Capital of California." No surprise that it's also the state's subsidence capital. But it's far from alone. The latest survey by the California Department of Water Resources found almost 40 sites with significant levels of subsidence.

"We're just pumping too much groundwater, especially in the agricultural areas in the Central Valley," Farr says.

A Stanford University study published in the journal Water Resources Research found that unless action is taken, the ground in that region will sink more than four metres over the next 20 years.

And the region is already feeling the effects.

Tom Farr, a now-retired geologist with NASA, has been tracking the Central Valley's sinking ground for years. (Kim Brunhuber/CBC)

In a tiny white shed next to a vineyard in Terra Bella, Calif., groundwater hydrologist Michelle Sneed unspools a cable that will snake more than 300 metres down into the earth.

"As the land subsides, more of this pipe will appear to protrude from the casing," Sneed says. She checks some gauges and taps a few notes into her laptop. This was a wet year, Sneed says, and still the ground sank more than 2.5 centimetres.

"It is quite a bit when you're talking about the impact to canals," Sneed says.

She's talking about canals like the one just metres up an embankment from the shed: the Friant-Kern Canal. It's a 250-kilometre-long aqueduct that delivers water to 15,000 farms.

Michelle Sneed, a groundwater hydrologist with the U.S. Geological Survey, says even in this relatively wet year, the ground she's been measuring has sunk more than 2.5 centimetres so far in 2019. (Kim Brunhuber/CBC News)

"This canal really is critical for water delivery," Sneed says, walking along its bank.

But the system depends on gravity, and now the ground has sunk so much in some spots the water has to travel uphill, turning the canal in some places into essentially a deep pool.

"And so that impacts the ability of the canal to actually move water," Sneed says.

"It can only transmit about 40 per cent of the water that it was designed to transmit. And there are other things too: roads, railways, pipelines, any sort of long infrastructure that crosses these areas of what we call differential subsidence are impacted."

Subsidence also reduces the ability of the earth itself — composed largely of clay — to store water.

"Those randomly oriented clay grains will start to realign themselves into a stack," Sneed says, which takes up less space, leading to the land sinking. "But also now there's less space to store water."

Because of subsidence, the Friant-Kern Canal can only transmit about 40 per cent of the water that it was designed to carry. (Kim Brunhuber/CBC News)

A 2019 study by researchers at Arizona State University found that the aquifer system in the San Joaquin Valley at the heart of the Central Valley lost up to three per cent of its storage capacity from 2012 to 2015.

And once that capacity is lost, Sneed says, it's lost for good.

"You can't undo it. Even if everybody stopped pumping today and we just let the groundwater aquifer systems just recharge, you don't get that subsidence back."

According to the Arizona State researchers, if California experiences only one drought per decade, the state's aquifers could permanently lose more than a quarter of their storage capacity this century.

And the problems caused by subsidence don't end there.

In the tiny community of Allensworth, about 60 kilometres south of Williams's Hanford farm, no one dares drink from the tap.

Allensworth resident Israel Sanchez walks out of his home tapping a large refillable water bottle, the kind one might see topping an office water cooler. Sanchez says he spends about $200 a month on bottled water.

"This is what we have to go buying all the time in the town," Sanchez says. "That's how we survive here."

That's because the water is dangerously polluted with arsenic. Subsidence is kind of like squeezing a sponge: the sinking ground presses contaminants like arsenic out of the soil and into the groundwater.

A recent Stanford study in the journal Nature Communications found that in areas where land sank more than 7.6 cm per year, the risk of finding dangerous levels of arsenic in groundwater tripled.

That study also found that around 10 per cent of wells tested in this agricultural hotspot showed dangerous levels of arsenic.

Israel Sanchez says he spends about $200 a month on bottled water because the water from his taps is undrinkable due to high arsenic levels. (Kim Brunhuber/CBC)

And just across from Sanchez's house, a cautionary tale: historic Allensworth.

Driving around the small community composed of several homes, a schoolhouse, a church, and a few other simple structures, one might easily get the idea that you were visiting a ghost town.

"It's never been a ghost town," insists Jerelyn Oliveira, an interpreter at the Colonel Allensworth State Historic Park. "There has actually been a continually operating elementary school here since 1910."

Oliveira uses her key to open the door of a white house. Inside are pictures of several African-American women: Colonel Allen Allensworth's daughters. This was his house, at the centre of the town he founded. In the early 1900s it was a thriving African-American community. By the 1960s it became an empty tourist attraction.

One of the reasons this town was eventually abandoned, according to Oliveira: arsenic.

The historic part of Allensworth, which was once a thriving African-American community, is now abandoned, largely because of arsenic polluting the water. (Kim Brunhuber/CBC News)

She points to a row of trees several hundred metres away.

"It's hard to believe that's an old lake bed," Oliveira says.

Now, even less remains of the lake than of historic Allensworth. So much water was diverted for agricultural use, the lake vanished. And a new problem emerged.

"Arsenic is something that was already in the land as it's a natural occurring mineral," Oliveira says. But with more farmers using the water and the water table not being replenished, she says, "the concentrations [of arsenic] were going to get higher. And that's what eventually happened…. The community faded. Its wells were deemed unsafe."

With the added threat of climate change, experts say more needs to be done to recharge the aquifers and curb water use or the ground will keep sinking and the aquifers will keep shrinking. (Kim Brunhuber/CBC)

Historic Allensworth provides a dramatic lesson, but it's one California has been slow to learn. It was the last state in the U.S. to adopt a groundwater law. The Sustainable Groundwater Management Act of 2014 requires aquifers to become sustainable, but only by 2040.

With the added threat of more frequent droughts due to climate change, experts like Farr say more needs to be done to recharge the aquifers and curb water use right now, or the ground will keep sinking and the aquifers will keep shrinking.

"The farmers now are coming to terms with that, and that they may have to pump less," Farr says.

A new well being dug in Hanford, Calif. (Kim Brunhuber/CBC News)

But many farmers like Williams say that's not likely unless the state of California builds more reservoirs.

Until then, she says, "farmers have no choice but to pump it out of the ground. Either that or we don't grow crops, and people need to eat."

On the land next to hers, you can hear the roar of a huge machine: a giant hydraulic drilling rig. It turns out her neighbour is digging a new well for his farm. It's big. And it's deep.