SOQUEL – Focusing on a 1,000-foot-deep two-dimensional slice of ground from Aptos down to Monterey, Stanford University researchers are taking a new tact studying saltwater intrusion along the Central Coast.
Groundwater agencies typically rely on wells to determine when saltwater is creeping into their water supply, with little nuance or advance warning on how quickly the contamination is spreading, officials say. In work being submitted to the Journal of Hydrology for publication, new research is supplementing existing data with geophysical imaging, using electrical currents sent into the ground.
“I often compare geophysical imaging to the challenge doctors faced at the start of the 19th century. I say groundwater managers today face a similar problem,” said Stanford geophysics professor Rosemary Knight during a public presentation on Wednesday afternoon. “Doctors needed to see inside an area, but they couldn’t see directly, or directly sample. What happened, along came medical imaging. You no longer hear about exploratory surgery.”
Knight and Ph.D. student Meredith Goebel offered a comprehensive sneak peek of their research to more than a dozen people at the Soquel Creek Water District office. Their studies use data collected along the coast’s beaches in late 2014, estimating the concentration of saltwater based on resistance the currents meet while flowing through the water. Lower resistance means higher saltwater concentration or possibly other factors, such as the presence of clay, Knight said.
The data shows saltwater intrusion as far inland as 10 miles in the Salinas area, while Soquel Creek Water District “is actually looking pretty good,” Knight said.
“When you start looking at things on a regional scale like this, it highlights to me the importance of cooperation between the various groundwater management districts, which are drawn more on county lines and service water lines,” Knight said. “The groundwater totally ignores those jurisdictional boundaries.”
Water district managers may focus on saltwater intrusion coming inland from the ocean, while a potentially equal threat could come from neighboring jurisdictions, were pressure conditions to change, Knight said.
Soquel Creek Water District general manager Ron Duncan said the district has been following the cutting-edge study since his office provided data from its well sampling for the researchers. He said looking at the magnitude of saltwater intrusion from Monterey upward was “shocking.”
“It substantiates and shows in a little greater detail the amount of seawater intrusion that has occurred along the coast already,” Duncan said of the data’s benefit locally. “We have wells, so we have discreet places where we detect seawater intrusion. What this does is it connects the dots.”
The beach-area northern portion of the Soquel Creek district has remained safe from saltwater intrusion so far, Duncan said, and he said officials are trying to keep it that way.
“That’s the battle, that’s what we’re trying to preserve for future generations,” Duncan said. “We can look around the world and about seven out 10 of the populated regions along the coast that pump groundwater have seawater intrusion. And here we’ve got it directly to the south. I don’t think it’s unfair to say that we’re at a precipice, a tipping point. That’s why we’re working very hard.”
