Perched aquifer

Above the water table, groundwater can also occur in perched aquifer conditions. In these instances, groundwater occurs in relatively permeable soil that is suspended over a relatively low permeability layer of limited lateral extent and thickness at some elevation above the water table. Perched groundwater occurrences are common within the vadose zone high-permeability zones overlie low-permeability zones of limited lateral extent in unconsolidated deposits. However, perched conditions can also occur within low-permeability units overlying zones of higher permeability in both unconsolidated and consolidated deposits. In the latter case, for example, a siltstone or clay stone overlies jointed and fractured bedrock such that groundwater presence reflects the inability of the water to drain at a rate that exceeds replenishment from above. 

Site No. 3, situated in the southern portion of the West Coast Basin, is underlain by, in descending stratigraphic position, the Semi-perched, Gage, Lynwood, and Silverado aquifers (see Figure 12.25). Most pertinent to LNAPL hydrocarbon occurrence is the Semi-perched aquifer, which is encountered under water table conditions. 

Certainly, if protection of the Silverado aquifer is the primary objective, and the water table aquifers (i.e., Gage, Old Dune Sand, or Semi-perched aquifers) are essentially a write-off, then the intervening aquifer(s), although not used for water supply, may serve the purpose of guardian aquifers. 

Perched aquifer A locally saturated zone or lense in the unsaturated zone that may overlay a low-permeability unit. 

Marley et al. (1994) reported on the typical field application of a trench. The site consisted of clayey silts with an unconfined, perched aquifer with the water table residing 2 ft below ground surface. The trench used was built in 40-ft segments for a total of 360 ft. The trench measured 1 meter in width, and air was injected at a rate of 0.005 m /s per meter of trench. Volatile VOCs were emitted directly to the atmosphere. Bittner and Hoffman (1996) reported on the use of an aeration curtain at a site in Utah. The subsurface was contaminated with trichloroethylene with concentrations as high as 1,000 Mg/L. Air was injected into the trench 21.7 L/s. Approximately 90% of the initial concentrations were removed within 48 hours of operations, and 99% was removed within 8 days. 

In such a case the aquifer and water table are said to be perched. Perched aquifers are usually localized. An unconfined aquifer, often called a water table aquifer or a phreatic aquifer, is bounded above by unsaturated porous media that is in contact with the overlying atmosphere. By contrast, in a confined aquifer, the water-bearing layer is bounded on the top and bottom by low-permeability layers a common example occurs when clay layers separate the confined aquifer from overlying and underl)dng materials. Confined aquifers transmit water much as imconfined aquifers do when water is removed by a well, however, there can be no corresponding drainage of pores or movement of air to fill the space vacated by water, as occurs in an unconfined aquifer. Instead, compression of the confined aquifer, as well as expansion of the water as its pressure is released at the well, accommodate the space previously occupied by water. Flawing artesian aquifers are confined aquifers in which water flows out of wells without any need for pumps. 

Maguire, T. F., 1988 Transport of a Non-Aqueous Phase Liquid within a Combined Perched and Water Table Aquifer System In Proceedings of the National Water Well Association of Ground Water Scientists and Engineers FOCUS Conference on Eastern Regional Ground Water Issues, September, 1988. 

Groundwater beneath the site occurs within the Gage aquifer under perched and water table conditions. Beneath the northern half of the main refinery shallow groundwater appears to occur only under water table conditions. Beneath the southwestern portion and southern half of the main refinery, groundwater occurs under perched conditions upon and within the middle unit. Groundwater is present under water table conditions within the lower unit. 

A limited number of aquifer pumping tests have been conducted within the perched zone. Two separate tests were conducted on a recovery well located in the southwestern portion of the refinery. Low transmissivity values of 100 and 150 gpd/ft were calculated. 

Ground Water Geochemistry. Ground water from the lower aquifer in the lower Perch Lake Basin at the Chalk River Nuclear Laboratories, Figure 2, was used for field elution studies to provide water of different redox characteristics. Water from piezometer "0" in the transition area, and from KNEW in the discharge area was used as being representative of "neutral" and... 

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