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Surficial Aquifers

General Definition and Distinction from Confined Aquifers

“Surficial Aquifer” is one term that can be used to describe a shallow, unconsolidated aquifer. Aquifers are the result of ground water that is not absorbed by soil occupying the lowest accessible voids. This is known as gravity water. The ability of water to percolate through a particular soil composition is dependent on its porosity. The velocity also depends on the gradient at which the water is travelling, and is known as hydraulic conductivity. In the case of an unconsolidated aquifer, the water occupies the interstices of an aggregated material, as opposed to being bounded by solid materials in a consolidated aquifer. The upper limit of this ground water is known as the water table; although due to complex relations between the mixture of material and water, this does not necessarily form a planar surface. The pressure conditions for water in a surficial aquifer are atmospheric, and gravity is the defining factor for the movement of this groundwater. This is in contrast to a confined aquifer; where the greater depth beneath the earth’s surface, combined with the impermeable confining layer, can put the water under much greater pressure than the external atmosphere. These conditions are reflected when a well is created in these so-called artesian aquifers: the water rises until it finds equilibrium with the atmospheric pressure.

Types of Surface Aquifers

In addition to sharing atmospheric conditions with the surrounding landscape, surface aquifers are much more closely integrated with it in many other ways. As a result of local geology, topography, and climate, they can take on a number of forms. In general, all types have a granular composition, and can be divided into four basic categories:

1.      Basin or valley fill aquifers are composed of layers of fine silt and clay; this mixture gradually gains compaction with depth. These are deposited in basins or valleys formed by less permeable rocks, which have generally been shaped by either erosion, faults, or a combination.

2.      Blanket sand and gravel aquifers consist of alluvial deposits and may be bounded below by less permeable layers such as lava flows.

3.      Glacial deposit aquifers may contain outwash, terrace, or ice-contact deposits.

4.      Semi-consolidated aquifers comprise a layering of semi-consolidated sand with silt, clay, and small carbonate rocks. They may be fluvial, deltaic, or shallow marine in type.

More detailed information on these subcategories and their distribution in the United States is available from the United States Geological Survey. https://water.usgs.gov/ogw/aquiferbasics/uncon.html

Interaction with the Landscape

Due to their close interaction with the surrounding environment, a rise in the water table due to local rain or contributing runoff from slopes can exceed the capacity of the surface aquifer, generating seepage into the landscape (also known as discharge). The amount of water and nature of localized geography produces various instances of seepage. Firstly, discharge may rise up through the soil due to capillary action. It may form a small spring. In other cases, it can flow into existing lakes, streams, and wetlands, contributing to their seasonal fluctuations in water level. Similarly, it can constitute the base flow of a stream. Surface aquifers may also be exposed at certain points of topography, due to changes in landscape slope occurring more suddenly than the water table can accommodate. Furthermore, rises in the elevation of bedrock below the aquifer can also push it to the surface at points of its course.

Bibliography

Marsh, William M. Landscape Planning: Environmental Applications, 4th ed. Hoboken: John Wiley & Sons, Inc., 2005.

U.S. Department of the Interior, Bureau of Reclamation. Ground Water Manual: A Water Resources Technical Publication, 2nd ed. Washington: United States Government Printing Office, 1995.

also:

https://www.usbr.gov/tsc/techreferences/mands/mands-pdfs/GndWater.pdf (retrieved 04.08.2019).

USGS Groundwater Information. “Unconsolicated and Semiconsolidated Sand and Gravel Aquifers.” United States Geological Survey, 2018. (retrieved 03.01.2018).

https://water.usgs.gov/ogw/aquiferbasics/uncon.html