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The Sinking Oxnard Plain

A cache by TerryDad2 Send Message to Owner Message this owner
Hidden : 11/19/2007
1.5 out of 5
1 out of 5

Size: Size: not chosen (not chosen)

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Geocache Description:

The Oxnard Plain has been sinking. This sinking is in large part due to extracting more ground water than is replaced. Requires elevation enabled GPSr.

Parking is in the parking lot of an abandoned building on the corner. Do not expect beautiful scenery or such. This is the location of a benchmark E 584 that was used in a study of land subsidence on the Oxnard Plain. This study showed that the land settled approximately 2.6 feet between 1939 and 1978. One of the primary causes of this settling is thought to be the over-pumping of ground water.

The demand for water has grown with the population of the Oxnard Plain. Much of this demand has been met by pumping ground water out of the various aquifers beneath the area faster than it is replenished. This has resulted in the lowering of the water table. In some cases, the level of the water has gone down hundreds of feet.

Ground water actually helps keep individual grains in the ground apart. Once the water is removed, the grains are compacted closer together by the weight of the grains above. This decreases the elevation of the area. This is also a permanent, so even if the ground water level comes up again, the ground surface does not move. The compaction also reduces the volume of water that can be stored in the aquifer.

You will be taking your own elevation measurement at this location and comparing it to the benchmark elevation of 17.65 ft above mean sea level. Currently the benchmark is lost, but it was in the general area. If you want to find an actual benchmark use one of the nearby benchmarks.

For those of you that want to be sticklers for details, the elevation provided by most gpsr are based on barometric pressure which changes based on temperature, weather, and various other factors, so this will not be a detailed scientific measurement, but it should give you a good idea. To get a better elevation measurement, you should go to a known benchmark on bedrock, calibrate your elevation, then come back.

Benchmark Z 583 at N 34° 05.733 W 119° 04.317 and an Altitude: 13.23 is located on bedrock and was included in the subsidence study mentioned above. In that study, they found very little subsidence due to regional tectoncis.

Oil and natural gas extraction also has similar effects. While not direct factor at this location, the oil fields on Oxnard Plain have had enough oil and gas extracted to be equivalent the lowering of the ground water table by 1000s of feet.

Logging requirements:
Send me a note with :

  1. The text "GC17GBN The Sinking Oxnard Plain " on the first line
  2. The number of people in your group.
  3. With your elevation measurement (be sure to put the gps on the ground)
  4. Compare your measurement to the benchmark’s elevation
  5. Assuming your measurement is accurate, what is the rate of subsidence at this location since the end of prior study in 1978.
  6. Based on your answer to 2, do you think over extraction of ground water is continuing?

The above information was compiled from the following sources:

  • U.S. GEOLOGICAL SURVEY SUBSIDENCE INTEREST GROUP CONFERENCE, EDWARDS AIR FORCE BASE, ANTELOPE VALLEY, CALIFORNIA, NOVEMBER 18-19, 1992: ABSTRACTS AND SUMMARY, Keith R. Prince, Devin L. Galloway, and Stanley A. Leake, editors; U.S. Geological Survey Open-File Report 94-532, Sacramento, California 1995
  • EARTH FISSURES: ARIZONA’S CRACK PROBLEM An interdisciplinary science module incorporating geological, chemical, biological, and engineering concepts in the evaluation of earth fissures. Departments of Physical and Life Sciences Mesa Community College, December 21, 2001

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