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Photograph by Jacqueline Ramseyer
Deep Dirt: Mark Newhouse, geologist with the USGS and site supervisor for the new monitoring well being dug near the Los Gatos Creek off Lincoln Avenue, shows off a core sample that was taken at about 400 feet below the ground.
Water district and USGS drilling wells to study the local geology
Willow Glen site one of six planned to learn more about valley's underground water
By Kate Carter
Employees of the U.S. Geological Survey made an interesting find in Willow Glen last week that will change the way scientists understand the earth and water beneath us.
USGS hydrologist Mark Newhouse, who was overseeing the work on the north bank of the Los Gatos Creek east of Lincoln Avenue, says what he and his team found deep under the ground contradicts expectations held by experts at the USGS and San Jose State University.
"This caught us by surprise," he said. "It was so much of a surprise that no one believed me when I told them. This is more interesting geologically now, as well as hydrologically."
The crew was drilling down to create a well, as part of a collaboration between the USGS and the Santa Clara Valley Water District, to learn more about the valley's groundwater basin and geologic formation. They began drilling on March 23, expecting it would take two weeks to complete the well.
But on April 3, they unexpectedly hit bedrock. Newhouse says they had only drilled about 810 feet down when the nature of the drilling changed and indicated something was different down there. That was surprising, he says, because USGS and SJSU scientists had expected the bedrock in that area to be about 1,500 feet down.
USGS scientists at their Palo Alto offices confirmed a sample of the rock at the bottom of the well as bedrock, Newhouse said.
The discovery, though not anticipated, is just the kind of thing USGS and water district officials hope to find out by drilling a half-dozen wells throughout the valley over the next few years.
The USGS wants to gather as much information as it can about the valley's geology to help it better understand the area's earthquake risk and to improve the accuracy of its geologic models, Newhouse said.
The water district hopes to learn better ways of protecting the underground water basin that provides half of the county's drinking water, district spokesman Mike DiMarco says.
"It's a way to double-check the way the recharging zones are working," he said.
The water district imports about half its annual water supply from state and federal water projects further north in California, DiMarco said. The district "recharges" most of the imported water--puts it into streambeds, ponds and other bodies of water--where it seeps into the groundwater basin. It can then be pumped directly into the county's water supply, "because Mother Nature cleans it beautifully," he said. The other half of the county's water is kept in reservoirs and must be sent to treatment plants before it can be used by customers.
By analyzing the water in the basin, scientists can tell how long it has been there and figure out how much of it is the imported, recharged water. DiMarco said the water district wants to make sure that as much of the imported water is making it into the groundwater basin and not being lost through processes such as evaporation.
The analysis will also help the water district better understand the valley's ground subsidence, or the sinking of the ground when too much water is removed from the groundwater basin. The water district came into existence to combat subsidence caused by early efforts to draw water from the ground, DiMarco said, and was only able to stop the sinking of the ground in the late 1960s.
Photograph by Jacqueline Ramseyer
Analyzing the Ground: Newhouse sifts through a tray of depth-dependent soil samples. This information is being collected as part of a regional project with the Santa Clara Valley Water District and the USGS.
Newhouse said that removing water from different types of rock in the ground can make subsidence either recoverable or nonrecoverable. The wells will help the district understand how their pumping affects the ground.
"The water district wants to be able to know how much subsidence they can get away with," Newhouse said.
In times of drought, the water district uses the groundwater basin as its "emergency supplies," DiMarco said. Figuring out how much it can demand of the basin will help the district learn better ways of dealing with a prolonged drought.
DiMarco said district officials aren't worried about a water shortage this summer. However, he said they do anticipate getting significantly less water from outside sources this year, because there has been less snowfall and is less runoff water. Although there is enough water to meet this year's needs, a water shortage next year would be more serious, he said. This well project with the USGS is one way the water district is preparing for droughts.
Newhouse said the well is actually one hole with multiple wells. The Willow Glen hole will likely have between four and five wells, each descending to a different depth. They will be enclosed in plastic casing and have screens at the bottom to allow water to seep into the tube. From them, the water district can take readings of the water at various levels in the ground, he said.
"Each layer in the groundwater has the ability to tell us something different," Newhouse said. "We can get an idea of the lateral, as well as the vertical distribution of water."
The USGS is trying to create a three-dimensional model of the valley's geology with money from a National Science Foundation grant, Newhouse says. The information it gathers from these wells, combined with other analyses, will make the model more accurate and specific, he says.
The hole for Willow Glen's well off Lonus Street will be just north of what will become part of the Los Gatos Creek Trail, DiMarco says. It will be in a subsurface vault and with a manhole-like cover about 4 feet in diameter, even with the ground.
Once the construction of the well is completed, USGS scientists will spend a couple of weeks taking readings and samples. After that, water district scientists will be able to visit it and check the status of the groundwater there.
The Willow Glen well is only the second of about six the two organizations are planning to dig in the next few years. The first, built last fall, is on district-owned land on William Street along the Coyote Creek and across from the William Street Park. DiMarco says the site will include an outdoor youth educational facility, which should be completed by next fall.
The agencies almost weren't able to dig the Willow Glen well because of its proximity to some PG&E power lines, DiMarco says. The team was concerned that the electric current might arc from the lines and hit the drill boom, interrupting power or hurting someone operating the machinery. DiMarco says the crew was able to change the work's logistics so they were comfortable about going ahead with the drilling.
Both the water agency and USGS were pleased that the work could finally occur, especially when it disproved what scientists had previously thought about the structure of the earth beneath the valley's floor.
"It just goes to show how much more information we can get," Newhouse says. "There's so much information that we can gather from doing projects like these."
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Willow Glen, California Since 1992
