Horizontal Strain Critical To Characterizing Aquifer Properties And To Understanding Land Subsidence

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.

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."

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.