Areas in the Antelope Valley have sunk nearly six feet since 1930 and no one knows how long or at what rate the region will continue to drop, according to the U. S. Geological Survey.
Known as subsidence, the irreversible sinking is the result of ground-water pumping, USGS scientists said Wednesday. They were giving a presentation to members of the technical committee of the Antelope Valley Water Group, a collection of water purveyors and local governments that commissioned a valleywide study of the area's water resources and demand.
Most of the sinking has occurred in Lancaster, in the center of the Antelope Valley, primarily because of the fine-grained soil there, said Steve Phillips, a USGS hydrologist. Palmdale, next to the valley's southern mountains, has larger-grained soil less prone to subsidence.
An area near Avenue I and Sierra Highway, north of Lancaster's center, dropped 5.9 feet between 1930 and 1992, said USGS hydrologist Marti Ikehara. And a benchmark site to the northeast, at Avenue G-8 and 70th Street East, sank the same amount during that period. The two locations show the greatest subsidence discovered in the Antelope Valley.
Other areas where substantial sinking occurred include Avenue I and 45th Street East, which dropped 5.1 feet, and Avenue B at 120th Street East, which sank as much as 4.3 feet, Ikehara said. By contrast, areas within Palmdale have dropped less than a foot.
USGS used about 200 benchmarks and a satellite survey system to determine the subsidence amounts, which Ikehara said are accurate to 2 to 4 centimeters.
While portions of the Antelope Valley's ground was sinking, so too was the level of its ground water, Ikehara said. An area on the west side of Lancaster, for example, sank 2.7 feet between 1930 and 1960 while the ground-water level declined 180 feet between 1920 and 1962.
At Edwards Air Force Base, in the north part of the Antelope Valley, USGS scientists conducting a separate study there discovered that pumping caused portions of the military base to drop as much as 3.3 feet since 1961.
While the USGS scientists are able to determine how much subsidence has occurred, they have no way of knowing when it will stop. And even if all the ground-water pumping came to a halt in the Antelope Valley the sinking would continue.
"Subsidence is not an on-off process," Phillips said. "It stops very slowly."
The best guess is that some sinking will occur for a few decades, the consequences of which are unknown, he said.
Already the subsidence has caused cracks in the earth--the largest at Edwards AFB measuring three-quarters of a mile long, as much as 14 feet deep and 6 feet wide--and breaks in well casings.
Subsidence in Arizona, Nevada, Texas and other areas of California has led to such things as flood control problems and damage to buildings and agricultural land.
The findings presented in Palmdale Wednesday are expected to be part of a formal written report that will be released in about one year.