Researchers found that while in very small measurements, Phonenix is slowly sinking each year due to groundwater pumping, and this may lead to not only serious but also expensive impacts over time.

The ground level in some parts of the metro Phoenix is dropping by 0.79 inch a year, with the latest drop pouring at Apache junction at the Valley's east end, Sun City West, Peoria and the north Valley.

Researchers at the Arizona State University (ASU) used elevation data from 1992 to 1996 and 2003 to 2010 to analyze the characteristics of the slowly sinking parts of Phoenix, and published their findings online in The Journal Of Geophysical Research.

The researchers uased synthetic aperture radar which is carried on an Earth-orbiting spacecraft and can measure the elevation of ground at less than an inch, and over wide areas. They were able to map changes in subsidence by measurements repeated over time.

"In parts of Chandler, Mesa and Scottsdale, the ground level has risen in recent years," said Megan Miller, a researcher from ASU. Miller, who is a graduate student at the School of Earth and Space Exploration, explained that this is because of the unused water allotment stored in the ground, which should not be done everywhere. She added that the subsidence pretty much can't be undone.

The basin layers of some sunken areas have now become compacted. According to Miller, as the water was pumped out, the pore spaces in the aquifiers also became empty. The layers had then settled, until the pore spaces disappeared.

In the studied periods of 1992 to 1996 and 2003 to 2010, the water table, or the height of the level of groundwater, has increased. This has increased even at fallen surface elevations, because of groundwater pumping from several years ago.

 "Pumping groundwater alters the elevation of the land surface at different rates around the Valley," said Miller. Because of the varying thickness and properties of sedimentary basins in metro Phoenix, subsidence rates also vary.

At a rate of just a couple of centimeters a year, data on the sinking ground of Phoenix might seem insignificant. However, if this continues for several years and on long distances, structures like the Central Arizona Project (CAP) canals, water and gas mains, utility lines, storage drains and sewers will most likely be affected, further damaging office buildings and homes.

Working side by side with Miller during the study was Professor Manoochehr Shirzaei,also from the School of Earth and Space Exploration.

Photo Chris J | Flickr

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