Groundwater pumping causes more land cracks than thought
Source: Tuscon Citizen, by B. POOLE
March 20, 2008
Scientists in Tucson are about to publish the first detailed maps of cracks in the Earth caused by groundwater pumping, and the work shows the fissures are more common in Arizona than previously thought.
"We're getting reports of dozens and dozens of new ones," said M. Lee Allison, director of the Arizona Geological Survey, which has been compiling earth fissure data for about a year.
Only two of the cracks - small ones near the Pinal County line in Marana - are in Pima County. Most are in Pinal County, but the scientists are most concerned about Chandler Heights southeast of Phoenix because it is quickly being developed and the cracks can damage homes.
The state mandated the maps two years ago, and the first of 22 - one covering Chandler Heights - should be on the Internet in coming weeks. The State Land Department will post and maintain the maps after the Geological Survey completes them.
The fissures do not normally occur naturally, Allison said.
"It really only occurs in these dry basins where the groundwater table has dropped hundreds of feet in tens of years," he said.
Though the water table under Tucson has fallen more than 200 feet since 1950, fissures have not appeared because soil here contains less clay than other areas.
There are hundreds of the cracks across the state. The first was found in 1927 near Eloy. Since then, the cracks have been a headache for property owners, businesses and governments.
The CAP canal was routed next to a mountain to avoid fissures, and one part of the canal was reinforced when a fissure was found under it before water started flowing, Allison said.
The earth cracks have only been found in west Texas, southern Utah, New Mexico, Nevada and Arizona, where they occur in Maricopa, Pinal, Pima and Cochise counties.
Scientists have only begun to study the cracks, said geologist Todd Shipman, who is leading the mapping effort.
"One of the issues is that we don't really know much about the development, because they develop from the bottom up," Shipman said.
Most of the fissures are a few inches across but hundreds of feet deep. They start deep underground and often become visible suddenly when rainstorms erode the surface over them.
A crack in Pinal County became a gully several feet wide in just a few minutes during a storm last summer. The next day the same crack opened in a corral a mile away, causing the death of a horse, Allison said.
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