USGS using magnetic fields to map underground water reservoir
U.S. Water News Online
SAN ANTONIO -- Geologists are making a map of the 11-county-wide groundwater aquifer that supplies Texas' largest springs.
A plane chartered by the U.S. Geological Survey is conducting an aeromagnetic survey of the limestone and other rock layers that make up the Edwards Aquifer. San Antonio is the largest city in the country to rely solely upon such an underground reservoir for municipal water supplies.
The subterranean map to be completed after several weeks' work may help locate wells for monitoring water quality and quantity and small dams to boost recharge to the aquifer. Its levels have dropped in past summer droughts, cutting flow to Comal and San Marcos Springs where threatened and endangered species live.
Crews in a Piper Navajo owned by Spectra Exploration Geoscience Corp. of Calgary, Canada are making the survey with technology used by the U.S. Navy to hunt submarines and by American companies to explore for oil, gas, and mineral deposits.
The planes fly out of Hondo Municipal Airport, recording patterns in the Earth's magnetic fields to map geological features as deep as 20,000 feet below the surface.
Of particular interest to geologists and hydrologists are locations in Uvalde County and elsewhere where magma penetrated Edwards limestone to form barriers to the present-day flow of water.
``Being able to better identify the ones we can't see on the surface will help us understand the flow paths,'' said Geary Schindel, the Edwards Aquifer Authority's chief technical officer.
The USGS and San Antonio Water System are paying Spectra a total of $120,000 to conduct the low-level flights, which could be completed within three weeks. Analysis likely will take at least six months.
Parts of Bexar, Uvalde, and Medina counties are the first to be examined.
``It's not necessarily pinpointed at the Edwards [Aquifer]. It's the structure the Edwards limestone is sitting on,'' said George B. Ozuna, a hydrologist and chief of the USGS office in San Antonio.
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