Tell the band to keep playing...the aquifer is shrinking, the country is sinking: central California by 30 feet in 60 years, Houston, Las Vegas and Pheonix as well...

And now the sewage won't flow uphill 


By Robert Roy Britt
Senior Science Writer
posted: 02:00 pm ET
22 August 2001

There's a new "ride" at Disneyland. Not too dramatic for visitors, but remarkable from a scientific standpoint. The entire theme park moves up and down every year. Just a couple of centimeters. No extra charge.

Nearby, the annual swelling of the ground is far worse.

A new satellite-based study shows parts of the Santa Ana basin, near Los Angeles, rise and fall more than 4 inches (11 centimeters) every year. That would be fine if the land masses returned to some equilibrium. Instead, over time, the elevation of much of the land in the greater Los Angeles area and beyond is gradually dropping as a result of the land's seasonal movements.

Vast areas of the Golden State are sinking, threatening to disrupt the flow of water and sewage. It's a growing problem not confined to California, and difficult to spot in other parts of the country.

Nature has little to do with it.

For Los Angeles, as ever, it's about water. Local water districts that purchase water from outside the area pump it into huge, natural underground reservoirs to store it in winter months. These aquifers, as they are called, are regions of rock, sand and soil that can extend for many square miles and through which water flows freely.

Every summer, the water is withdrawn from these subterranean banks. But each year, the withdrawals exceed the deposits.

"Every year when they pump water out, they're pumping it to a new, lower level," said Gerald Bawden, a U.S. Geological Survey scientist who led the new study, which appears in the Aug. 23 issue of the journal Nature.

The seasonal draining causes rock and soil to compact to an extent that prevents an aquifer from ever filling to its previous capacity -- something scientists have understood since the late 1920s. Like a hardened sponge, the compacted material simply can't hold water as well as before.

The result is that in addition to the seasonal fluctuation, the ground in some places also sinks steadily over time. It's a process geologists call subsidence.

 

Faulty fault data

Potentially the greatest impact of the process is that it renders meaningless some of the important data used to study earthquake potential in the region. The Global Positioning System (GPS), which geologists rely on to monitor long-term ground movement along earthquake faults that might presage sudden and deadly quakes, is being fooled by the thirst of millions of people.

After a pair of large Southern California earthquakes, including the Northridge temblor that killed 56 people in 1994, geologists installed 250 GPS stations along faults throughout the region to monitor ground movement.

Plates of the Earth's crust are known to creep over the years, and the movement can be detected along fault lines, where two plates meet and move in different directions. This creep is known to produce stress in the rocks near the fault. When the stress builds to a certain point, it breaks, and an earthquake results.

But the seasonal swelling caused by the pumping of water, as well as withdrawals of oil and natural gas in the area, obscure and in some cases mimic the tectonic signals detected by the stations, the new study found.

As the ground rises and falls over an aquifer, it can actually pull at the edges, creating horizontal movement near the perimeter of the aquifer. This relationship is important because earthquake faults often form the boundaries of an aquifer. Along a fault, rock is ground into a fine powder the consistency of potter's clay, and water cannot penetrate this soil. Fault lines can extend a mile or more below the surface.

So the horizontal ground movement caused by seasonal water pumping sometimes occurs right where GPS data collection is most critical -- along the fault.

 

Working around the problem

Bawden said the new findings will make it more challenging for geologists to study fault movement in Southern California.

"It's not unrecoverable," he said. "We can still learn what's going on. It just has been made a lot more difficult."

One way to work around the problem is to ignore areas where groundwater pumping alters the surface, and instead take measurements in places that are not affected by the problem. And since fault lines crisscross much of California, pockets of ground can be identified where there is no effect from aquifers. Another solution would be to figure out the pattern of seasonal variation over the next three or four years and account for it.

It's the long-term sinking that will be the most challenging to separate from fault-movement data, and Bawden working on ways to mathematically account for it.

But it will take several years, he said, to accumulate enough information and develop confidence in whether ground movement is caused by human-induced long-term or seasonal fluctuation or a creeping earthquake fault.

 
Sewers don't flow uphill

Devin Galloway, a USGS scientist who specializes in subsidence but did not participate in the new study, said he was surprised by the extent of both seasonal and long-term movement caused by groundwater siphoning. Galloway said the study, which he has read, was based on solid methods and good data.

"Water managers should be aware of it," Galloway said. "But subsidence is one of those issues that's really hard to get on peoples' radar screens. It's so subtle, that people don't really notice it unless something happens."

They may notice before too long.

"The city of Santa Ana [just south of L.A. proper] is sinking at the rate of about a half-inch a year," Bawden said. California's huge system of aqueducts, used to move water into the region from Northern California and from the Colorado River, currently slopes just 4 inches per mile to provide enough grade to keep water flowing.

"It's not going to take too many years before water is not flowing the proper direction," he said.

Stephen Kashiwada, chief of the Division of Operations and Maintenance for the California Department of Water Resources, said mild subsidence is not a problem. Pumping stations help move water along, he said, and the canals are surveyed each year to check for any change.

But sewer pipes typically rely on gravity alone, and they may soon be fighting an uphill battle to get waste out of sunken areas. In the more distant future, a depression could form that would cause portions of the Santa Ana river to become a lake, Bawden said.

If it comes to that, no one can say there were no warnings. As early as the mid-1950s, geologists noted that groundwater pumping near the Santa Ana river had caused a primary aquifer to drop below sea level, and saltwater had flowed into the aquifer up to 5 miles inland.

To fight this problem, a series of 23 wells were drilled so that officials could pump up to 1.3 million gallons of water into the ground each day. The effort creates a hydraulic wall of sorts that keeps seawater at bay.

 

Elsewhere: That sinking feeling

Subsidence caused by groundwater pumping is nothing new. One region of California's Central Valley, well north of the Los Angles area, is known to have subsided roughly 30 feet (9 meters) since the late 1940s. Other parts of the country are also affected.

In 1997, the USGS measured long-term sinking in select locations in or near these cities:

  • West of Phoenix, Arizona: 18 feet (5.5 meters)
  • Houston, Texas: 9 feet (2.7 meters)
  • Las Vegas, Nevada: 6 feet (1.8 meters)

Measurable side effects have already occurred. At Edwards Air Force Base in California, a giant crack in the ground caused by groundwater siphoning once rendered a backup runway designed for the space shuttle unusable.

Bob Pierotti saw that huge fissure back in the early 1990s. He called it "the mother of all fissures, a big gaping crack" that was some 20 feet wide and hundreds of yards long. Pierotti is a geologist with the southern division of the California Department of Water Resources. He says that in general, subsidence is viewed seriously by those who manage water districts.

He said studies like the new one provide valuable information for water managers charged with planning current and future water storage practices.

But addressing subsidence in the Santa Ana basin would require the cooperation of many agencies. There are several water districts in the greater Los Angeles area, some operating locally and some regionally, with multiple layers of bureaucracy.

 

Under your feet, but undetected

Subsidence is not confined to the West or to the areas studied so far. It could be happening under your feet, too, geologists say, especially if you live above an aquifer and near a highly populated area or in an agricultural region. Much of southern and coastal New Jersey, for example, sits above a large underground water system.

Wells have pumped the New Jersey groundwater to the point that seawater backs into the aquifer, rendering the water undrinkable in many low-lying places far inland.

Such thirst is a growing problem around the country.

Since the mid-1950's, the amount of water in the United States supplied by groundwater pumping has grown from less than 20 percent of the total to more than 30 percent. Much of this rise occurred in the Southwest, where the population burgeoned after World War II. Officials expect the problem to grow as more rivers and lakes are tapped out.

But in many parts of the country, the effect is hard to spot. Bawden and his colleagues used satellites to study ground movement over time by bouncing radar off the surface on subsequent passes in the satellite's orbit. The technique does not work well in heavily forested areas because the radar bounces off tree leaves instead of the ground.

For Californians, as well as lovers of Mickey Mouse, there is some comforting news.

For locals who worry that drinking tap water might actually help bring on the next Big One, Bawden and his colleagues did a little investigating: They took a close at one active fault and determined that the human activity "has no bearing" on the current seismic activity of the fault.

Nor is Disneyland in any imminent danger. The annual rise and fall is over such a broad area, Bawden said, that it doesn't threaten to cause any structural damage at the park.