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2/11/2016 by mdc
http://www.trivalleycentral.com/casa_grande_dispatch/area_news/eloy-s-elevati...
Tanner Clinch, of the Casa Grande Dispatch, AZ, reports that a State of Arizona geology official says the valley around Eloy is 20 feet lower elevation than it was around 50 years ago. The Arizona Geological Survey has just revised its earth fissure monitoring maps for southern Arizona with six new maps that detail these geological hazards throughout the area.

The maps include study areas east of Picacho Peak that have dozens of reported fissures. The first fissures in southern Arizona appeared near Eloy in 1927 and are thought to be caused by depleting (over pumping?) groundwater aquifers. The fissures can be miles long, according to AZGS.

If you stop drawing the groundwater, we would anticipate at some point the subsidence would stop, said Michael Conway, chief of the geological extension service for AZGS. Subsidence is the gradual lowering of the ground in relation to the sea level. According to Conway, the valley floor around Eloy has lowered as much as 20 feet in the past 50 to 60 years. The geological survey started mapping fissures in 2007 after a horse fell into one in the Chandler Heights area and was killed. While there have been no human fatalities associated with earth fissures in Arizona, there are many hazards involved.

Another concern is that contaminated fluids could enter these fissures and actually migrate quickly into an aquifer. Since earth fissures extend upwards from the groundwater table, the surface evidence of these fissures leaves a direct line to the aquifer. Any pesticides or other fluids that are dissolved by rainwater, could migrate into a fissure and the groundwater below. There have not been any cases reported in Arizona where this has happened yet.

Earth fissures are much different than sinkholes like the one that swallowed up a Queen Creek man on Friday. Sinkholes are caused by what geologists call Karst topography, which is when slightly water-soluble minerals such as limestone and gypsum are dissolved and create sinkholes.

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