Horizontal strain critical to characterizing aquifer properties and to understanding land subsidence

January 23, 2001

BLACKSBURG, VA, January 24, 2001 -- To understand how subsidence and fissures result from pumping aquifers, scientists and engineers need to measure horizontal as well as vertical strain, geologists have now demonstrated.

Since the analytical work of Theis and Jacob over half a century ago, scientists have used only vertical strain to measure storage in aquifers, ignoring what is going on in the horizontal direction, says Thomas J. Burbey of Virginia Tech's Department of Geological Sciences. But water is released from aquifers through water expansion and aquifer compression or compaction.

Burbey says that the aquifer compression or compaction of water is occurring in a horizontal as well as vertical direction. In many parts of the country -- particularly the west and southwest -- aquifers, or naturally forming underground reservoirs, are the source of water for agriculture and communities.

When an aquifer is pumped to remove the water, the water pressure decreases in the space between the grains of sand, particles of clay, rock, and other soil components. Thus pore water pressure is lowered. No longer buoyed by the water, the particles compress.



Clay compresses vertically, which can result in subsidence. The classic example is California's San Joaquin Valley, where there was 27 feet of subsidence from 1927 to 1977 (see 1977 photo* of Joe Poland, the "father of subsidence research").

"The same thing that happens vertically, also happens horizontally. Sand particles, in particular, release water when they are compressed horizontally. Earth fissures, such as are occurring in Las Vegas Valley and other desert communities (see photos), are a result of horizontal and vertical compression of soils as water is removed from aquifers.

"Compression of coarse-grained deposits, such as sands, tend to contribute much of the horizontal strain and compression, whereas fine-grained deposits, such as clays, tend to contribute most of the strain and compaction (subsidence) in the vertical direction," says Burbey.



"The only way to understand and characterize what is happening, is to look at horizontal as well as vertical strains," he says. "The entire matrix is dynamic, moving. The aquifer does not remain fixed. The whole granular matrix is moving -- in smaller amounts than the water, but a couple of centimeters in an arid region can initiate a fissure," Burbey says. "Pumping a well can affect an aquifer for miles horizontally, whereas, vertically, the aquifer is not as thick. Where the fissures occur depends on faults and weaknesses in the deposits."

For his study, Burbey used numerical models that take into consideration only vertical strains, then wrote a model for three-dimensional strain, including horizontal, and compared the results with those using the vertical strain only.

While his studies show that calculated hydraulic head values, or water levels, and the production of water in terms of volume strain are nearly identical for both models, he also discovered that, over time, the location of the maximum production, or place from which water is pumped, moves outward, horizontally, from the well.

Burbey also found that, when the model incorporates the horizontal strain, about half of the water originates from horizontal strain and that the percentage increases over time to as much as 70 percent.





Full size image available through contact




Finally, Burbey found that producing the same quantity of water using just one dimension requires much more compaction, or land subsidence, to accommodate the volume of water pumped out. He explains further: "Due to mass balance, pumping out water reduces water pressure, increases effective stress, and increases stress equally in all directions.

If we assume the strain resulting from that stress occurs only in the vertical direction, then there will be more subsidence than would occur if we assumed that horizontal strain could also accommodate some of the stress.

"Results indicate that small changes in porosity resulting from horizontal strain can yield extremely large quantities of water to the pumping well," Burbey reports. "This study suggests that the assumption of purely vertical strain used in the definition of the storage coefficient is not valid."





Full size image available through contact




Burbey presented his findings at the Geological Society of American meeting in Reno in November. He is doing research to answer questions about where subsidence will occur, and also to determine what compression means in terms of the future water storage capacity of an aquifer. "Water is stored within the pore spaces. If we are only taking water out, what does compression mean for future capacity?" he asks.

He is also evaluating water flows and how horizontal deformation may affect how we analyze aquifer tests.
-end-
The first three jpegs are earth fissures in various stages of growth or progression.
The "lvhouse.jpg" is a house experiencing differential subsidence.
The picture of Joe Poland in the San Joaquin Valley of California was taken in 1977 where the markers on the telephone pole show the land surface elevation at various periods in time. A total of 27 feet of subsidence occurred here. Burbey is doing research in collaboration with Don Helm of Morgan State University. Funding is being provided by the National Science Foundation.

This release is an update to the previously posted " Study shows that purely vertical strain assumption not valid during aquifer testing, " posted on Nov. 28, 2000. This updated release contains more information about the relevance of horizontal strain in aquifers to land subsidence.

*Photos are posted at http://www.rgs.vt.edu/resmag/burbey_photos /

PR CONTACT: Sally Harris, 540-231-6759; slharris@vt.edu

FACULTY MEMBER: Thomas J. Burbey, 540-231-6696; tjburbey@vt.edu

Virginia Tech

Related Water Articles from Brightsurf:

Transport of water to mars' upper atmosphere dominates planet's water loss to space
Instead of its scarce atmospheric water being confined in Mars' lower atmosphere, a new study finds evidence that water on Mars is directly transported to the upper atmosphere, where it is converted to atomic hydrogen that escapes to space.

Water striders learn from experience how to jump up safely from water surface
Water striders jump upwards from the water surface without breaking it.

'Pregnancy test for water' delivers fast, easy results on water quality
A new platform technology can assess water safety and quality with just a single drop and a few minutes.

Something in the water
Between 2015 and 2016, Brazil suffered from an epidemic outbreak of the Zika virus, whose infections occurred throughout the country states.

Researchers create new tools to monitor water quality, measure water insecurity
A wife-husband team will present both high-tech and low-tech solutions for improving water security at this year's American Association for the Advancement of Science (AAAS) annual meeting in Seattle on Sunday, Feb.

The shape of water: What water molecules look like on the surface of materials
Water is a familiar substance that is present virtually everywhere.

Water, water everywhere -- and it's weirder than you think
Researchers at The University of Tokyo show that liquid water has 2 distinct molecular arrangements: tetrahedral and non-tetrahedral.

What's in your water?
Mixing drinking water with chlorine, the United States' most common method of disinfecting drinking water, creates previously unidentified toxic byproducts, says Carsten Prasse from Johns Hopkins University and his collaborators from the University of California, Berkeley and Switzerland.

How we transport water in our bodies inspires new water filtration method
A multidisciplinary group of engineers and scientists has discovered a new method for water filtration that could have implications for a variety of technologies, such as desalination plants, breathable and protective fabrics, and carbon capture in gas separations.

Source water key to bacterial water safety in remote Northern Australia
In the wet-dry topics of Australia, drinking water in remote communities is often sourced from groundwater bores.

Read More: Water News and Water Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.