Click photo to enlarge
Workers have tunneled their way 1,000 feet into the heart of Montara Mountain. The twin tunnels are part of the Devil's Slide highway bypass project. Local water agencies hope the rock will yield some groundwater they can use to offset local water deficits. (Photo Courtesy of John Huseby/Caltrans)

MONTARA —If just one word could be used to describe Devil's Slide in winter, it would be "wet."

Montara Mountain is famous for its ability to retain water above and below Highway 1, creating a waterlogged slide plane and the notorious road failures that gave the stretch of roadway its famous nickname.

When workers attempted to stitch the sagging road together by drilling metal cables into the mountainside in 2006 — the ninth repair in 70 years — they were doused with cannonballs of muddy water trapped between the rocks.

These facts gave the Montara Water and Sanitary District hope that, once the Devil's Slide bypass tunnel project got underway, tunnel workers might discover vast troves of water stored deep in the tunnel rock bed — usable water that could alleviate the water crisis the district has faced since the 1970s.

Unfortunately for the district, and particularly for a group of Montara and Moss Beach residents who have been drawing on unpredictable backyard wells for decades, the twin tunnels being bored straight into the granite heart of Montara Mountain have turned up less water — far less water — than anyone anticipated.

According to a recent assessment by Devil's Slide tunnel project manager Skip Sowko, the largest volume of water discovered so far at about a quarter of the way into the tunnels is 7.92 gallons per minute (or 30 liters per minute). A typical person in the

Advertisement
shower uses six gallons of water per minute.

"It's a pleasant surprise because it would be difficult for us if it were on the higher end," Sowko said.

The state Department of Transportation pumps all the water it finds into storage tanks. The silt and clay must be removed before the water is discharged into the ocean.

It is still possible, and even likely, that work crews will discover a higher volume of water as they bore northward through the mountain toward Pacifica, where less-pervious rock surfaces may spill more water when cracked open, according to Sowko.

The North Coast County Water District, serving Pacifica, has jurisdiction over roughly two-thirds of Montara Mountain, and therefore two-thirds of its water. At 1,000 feet in, the boring machines have already left Montara's territory.

These details came as news to Montara Water and Sanitary District General Manager Clemens Heldmaier.

He said his agency still considered the Devil's Slide tunnel a potential new source of water, along with exploring for new wells, adding a recycled water plant, or building a water desalination plant.

"At this point we are not in contact with Caltrans and we don't know whether there is any water," Heldmaier said. "If there is water available, of course, we would be interested in knowing about it."

The Montara water district had good reason to be hopeful back in 2005, when a report prepared by several Caltrans consultants predicted water flows as high as 118 gallons per minute based on test holes drilled near the future tunnels' south entrance.

The discrepancy is not that unusual, according to Sowko. Rock type and density can vary within a few feet, which means that the amount of water trapped inside is also extremely variable. Workers will bore through several rock "faults," or areas of rock variation, as they go from south to north.

The work area closest to Montara is composed entirely of quartz diorite, or granite, which contains more cracks that allow groundwater to flow westward and escape toward the ocean.

By contrast, the rock at the center of the mountain, composed of fine-grained sandstone and claystone, contains fewer cracks and therefore holds water back more easily. But those stores of water will taper off quickly as well once they are exposed, Sowko said.

When the whole Devil's Slide project is completed in 2010, he expects the average flow from both tunnels to even out at 13 gallons per minute.

"We know that there's higher flows on the north end from our exploration. We don't necessarily know how it's all connected. If you penetrate it and you hit a high zone where there's pressure, you can bleed those out."

The North Coast County Water District will wait until workers push another 500 feet into the mountain before conducting its own study to determine how much water is available, said district General Manager Kevin O'Connell.

Most importantly, officials will have to determine the costs and benefits of building a water treatment plant at Montara Mountain. O'Connell reached out to the Montara water district with a proposal to split the cost of building a plant two years ago but was turned down.

North Coast County Water is working out the final details on a contract with Caltrans to ensure that the tunnels will have a reliable emergency water source, according to O'Connell.

He said the pipelines that would be extended into the mountain also could conceivably be used to transport excess groundwater back to the district someday.

"We need to know exactly what's in the water and we need to know exactly how much water we're going to have," O'Connell said. "If we have to do a lot of treating we don't know how much it would cost — it could be much more than we could afford."

Julia Scott can be reached at 650-348-4340 or julia.scott@bayareanewsgroup.com.