- Associated Press - Thursday, August 13, 2015

PHOENIX (AP) - A new Arizona State University study shows parts of metropolitan Phoenix are sinking by about three-quarters of an inch a year because of groundwater pumping decades ago.

The study shows the ground-level elevation in Apache Junction is dropping the fastest and suburbs north and west of Phoenix also are descending.

The long-term impact of what scientists call subsidence are cracks that can develop in home foundations and ground fissures that damage canals, water mains and sewers. Researchers says there’s no need to panic.

“If anything this is slow,” said ASU researcher Megan Miller told the Arizona Republic (http://bit.ly/1HJcYUS). “It’s rarely going to cause anything you would associate with a disaster. It can be a nuisance but has the potential to cause costly structural damages, and is something to keep an eye on.”

The study attributes the dropping water levels to water pumped from subsurface aquifers before 1980. Legislation passed that year limited to what is recharged to preserve the resources.

When water was pumped out, the sediment layers essentially resettled, leaving less available space for water than before and causing the ground level above to drop.

State officials have been aware of what’s called “land subsidence” - where the earth collapses and drops -for years.

The Arizona Department of Water Resources is working with NASA to collect radar data to compliment the department’s data and maps on where land has subsided. The department has been collecting and processing data since 2002 to monitor land subsidence, which is occurring over 2,800 square miles in Arizona.

Not all areas of the Valley are sinking, the ASU study found. Parts of Scottsdale, Chandler and Mesa have risen by as much as half a centimeter. ASU scientists say they did not observe a change in most of the city of Phoenix.

The ground sinking is not unique to metro Phoenix. It’s also occurring in southwestern Arizona and agricultural valleys in California. The U.S. Geological Survey has identified more than 17,000 square miles of land subsidence in 45 states.

The Bureau of Reclamation has projected about a 1-in-3 chance that as a result of the prolonged Southwestern drought Lake Mead will drop low enough to force Arizona to forgo some of its usual Colorado River water deliveries. The bureau has also forecast a better than a 2-in-3 chance that it will happen in 2017. The agency plans to release a new 24-month projection on Monday.

Any water shortage will initially affect central Arizona farmers, but a prolonged or deepening cut in supplies could force the state to start drawing water from its underground storage - potentially leading to more subsidence.

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Information from: The Arizona Republic, http://www.azcentral.com

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