In the News

Ground-breaking survey maps coastal saltwater intrusion
Study maps 25 miles of Monterey coastline

By Nicholas Weiler, Santa Cruz Sentinel, 10/24/14

SANTA CRUZ — A team of researchers from Stanford and the University of Calgary say a ground-breaking geophysical survey of saltwater intrusion into groundwater tables along 25 miles of Monterey Bay coastline shows the wells are running a deficit.

Santa Cruz and Monterey counties pump the majority of their fresh water from wells that tap into river-like groundwater aquifers. Normally, water from winter rains soaks into the ground then percolates toward the sea through the aquifers, and only a fraction is removed for domestic and agricultural use.

But as the drought continues, there has not been enough rain to replace the water being withdrawn from the ground.

Mary Bannister, general manager of the Pajaro Valley Water Management Agency, compared the aquifers to a bank account: If withdrawals are bigger than deposits, soon the account will be in the red. Right now, she said, "We're really overdrafting the bank."

A major problem water managers face is that aquifers are effectively invisible. No one knows exactly how far the ocean has penetrated until it contaminates another well. Water districts have monitoring wells in place up and down the coast to help them detect saltwater intrusion before it hits production wells, but they still can't see what's happening between the wells.

This is why Stanford geophysicist Rosemary Knight decided to create a continuous map of where ocean water is crossing into aquifers all along the coast by conducting a geophysical survey of unprecedented scale.

The Canadian-American survey team spent weeks inching up the beach from Fort Ord in Monterey County to Rio del Mar, laying down thousands of feet of cable and pounding 4-foot stainless steel electrodes into the sand every 75 feet. The researchers pumped tiny pulses of electrical current into the ground and measured how they spread through the earth along more than a mile of cable.

"Because we're laying out these cables along such a great distance, we can see to a great depth," said Knight.

The data the team has collected will reveal the boundary between saltwater and freshwater up to 1,000 feet below the ground, Knight said, which will give water managers a warning if the sea is intruding where it shouldn't. Knight estimated her team will have extracted preliminary results from the data as early as February.

Bannister estimated the region needs at least 15 inches of rain each winter to begin to restore groundwater.

"We haven't had that for going on three years now," she said.

Watsonville, near the center of the Pajaro basin, saw less than 10 inches of rain last year, only 40 percent of average, according to data from the National Weather Service.

A serious danger of continuing to pump the depleted aquifers, said Bannister, is that as groundwater levels drop below sea level, ocean water can be drawn into the aquifers, spoiling the water supply farmers and communities rely on. In some places along the coast, saltwater has contaminated wells miles inland, Bannister said.

Saltwater intrusion from overpumping is not a new problem, Bannister said. It has been documented along the Monterey coast since the 1930s. But the current drought has the potential to make it much worse.

"We're kind of in crisis mode," said Bannister.

John E. Eiskamp, former president of the Santa Cruz County Farm Bureau and a berry grower, called saltwater intrusion a basin-wide problem that the bureau is concerned about.

"Once a well is impacted, you're not going to change that," he said.

Meredith Goebel, a graduate student of Knight's who was part of the survey team, said, "I didn't want to write a thesis that would sit on a shelf. This seemed like something that would be helpful to a lot of people."

"It's a critical piece of information," said Taj Dufour, chief engineer at Soquel Creek Water District, where coastal monitoring wells have detected saltwater intrusion.

Dufour said he hopes water districts and scientists can continue to collaborate to create a more complete picture of the problem — for instance by tracking the saltwater boundary over time. Without more continuous data, he said, "we don't necessarily know if it is getting better or worse."

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