Artesian aquifers

Confined aquifer (artesian aquifer) An aquifer that is confined between two aquitards. 

The term aquifer is used to denote an extensive region of saturated material. There are many types of aquifers. The primary distinction between types involves the boundaries that define the aquifer. An unconfined aquifer, also known as a phraetic or water table aquifer, is assumed to have an upper boundary of saturated soil at a pressure of zero gauge, or atmospheric pressure. A confined aquifer has a low permeabiUty upper boundary that maintains the interstitial water within the aquifer at pressures greater than atmospheric. For both types of aquifers, the lower boundary is frequendy a low permeabihty soil or rock formation. Further distinctions exist. An artesian aquifer is a confined aquifer for which the interstitial water pressure is sufficient to allow the aquifer water entering the monitoring well to rise above the local ground surface. Figure 1 identifies the primary types of aquifers. 

The confined type of aquifer has a hydraulic pressure (static head) that is on a higher level than the top of the aquifer. This artesian pressure can sometimes reach above the surface level resulting in self flowing wells (artesian wells). 

Figure 35. Flow of groundwater in an confined aquifer with potential artesian wells...

For the confined aquifer, the pressure head becomes more important than the elevation head. As can usually be seen in an artesian aquifer condition, the groundwater may flow from a lower elevation to a higher elevation if the water pressure at the lower elevation is higher. 

Confined or artesian aquifer (fresh) jlXa SisConfining zone... 

Confined or artesian aquifer (saline) O Intentional input... 

Kanivetsky, R. (2000) Arsenic in Minnesota Ground Water Hydrochemical Modeling of the Quaternary Buried Artesian Aquifer and Cretaceous Aquifer Systems, Report of Investigations — Minnesota Geological Survey, St. Paul, MN. 

The water in the saturated zone of the phreatic section of a confined system exerts a hydrostatic pressure that causes water to ascend in wells. In fact, a confined aquifer can often be identified by the observation that water ascends in a borehole to a level higher than the level at which the water was first struck. In extreme cases the water ascends to the surface, constituting an artesian well. This phenomenon of water ascending in a well and flowing by itself was first described in 1750 in the area of Artois, a province in... 

Fig. 2.6 Components of a confined aquifer with through-flow tilted, or folded, water-bearing rock strata, sealed at the top and the base by aquicludes. Each active confined system also has a phreatic section at outcrops of the aquifer rocks. The level of the water table in the phreatic section defines the piezometric head in the confined section. Water ascends in boreholes drilled into confined aquifers. Water reaches the surface in artesian flow in boreholes that are drilled at altitudes lower than the piezometric head.

The level water reaches in an artesian well reflects its pressure, called the piezometric, or confined, water head (Fig 2.6). In boreholes drilled at altitudes that are lower than the piezometric head, water will reach the surface in a jet (or wellhead pressure) with a pressure that is proportional to the difference between the altitude of the wellhead and the piezometric head. The piezometric head is slightly lower than the water level in the relevant phreatic section of the system due to the flow resistance of the aquifer. Confined aquifers often underlay a phreatic aquifer, as shown in Fig. 2.7. The nature of such groundwater systems may be revealed by data measured in boreholes and wells. The water levels in wells 1 and 2 of Fig. 2.7 did not rise after the water was encountered, and both wells reached a phreatic aquifer. Well 3 is artesian, and the drillers account should include the depth in which the water was struck and the depth and nature of the aquiclude. The hydraulic interconnection between well 1 and well 3 may be established by... 

Compaction caused by the weight of overlying rocks is called upon to explain the water pressure in deep-seated artesian aquifers (Mazor, 1995). Compaction counteracts through-flow in deep aquifers, as it causes collapse features, and pressurizes the entrapped water. If through-flow would take place, the open ends of the hypothetical aquifer ducts would serve as... 

Artesian Aquifer, South Coast of South Africa... 

An often quoted case study (Vogel, 1970) is that of an artesian aquifer in an area near the south coast of South Africa (Fig. 11.8). The decrease in 14C downslope from the aquifer has been taken by the researcher to indicate continuity. One can even calculate the velocity of groundwater flow in the aquifer by selecting two points on the lines of Fig. 11.8, for example, 2 km-4000 years and 18 km-28,000 years. The average flow velocity in the aquifer is... 

Fig. 12.11 Sample locations in the Great Artesian Basin and concentrations of 36C1 (107 atoms/1) (data from Bentley et al., 1986a), and Cl, Ca, and S04 (mg/1) (data from Water Resources Commission of Queensland). Line DD marks a discontinuity between the 36C1, Ca, and S04 results from unconfined wells in the Great Dividing Range recharge area (located at the northeast corner) and the results from the buried confined J aquifer of the GAB.

The Great Artesian Basin, Australia, in which a Jurassic aquifer is up to 2000 m deep (Habermehl, 1980)... 

Torgersen and Clarke (1985) found for the confined Jurassic aquifer in the Great Artesian Basin, Australia, that the observed helium concentrations were 70 times higher than expected for their calculation of the hydraulic ages. 

Fig. 14.1 Helium concentrations as a function of aquifer depth or temperature (following Mazor and Bosch, 1992a) Bunter sandstone aquifer, eastern England (data from Andrews et al., 1984) Stripa granite, Sweden (data from Andrews et al., 1982) Blumau, Austria (data from Andrews et al., 1984) Molasse Basin, Austria (data from Andrews et al., 1981) Great Artesian Basin, Australia (data from Torgersen and Clarke, 1985).

Good correlation with depth of confinement, as seen for the Great Artesian Basin (Fig. 14.4), and as indicated by Andrews et al. (1984) for the Bunter sandstone aquifer. 

Good correlation with local depth, as seen in Blumau, the Bunter sandstone aquifer of eastern England, the Stripa granite in Sweden (Fig. 14.1), and in other examples included in Table 14.1, or good correlation with temperature, as seen at the Molasse Basin in Austria and the Great Artesian Basin in Australia (Fig. 14.1). 

The practice of exploiting artesian systems has produced a large number of depressurized stagnant aquifers that are at present of no use. Such aquifers, which supplied high-quality water, can be reused by reinjecting fresh water to create pollution-immune underground water reservoirs. 

Mazor, E. (1995) Stagnant aquifer concept I. Large scale artesian systems—Great Artesian Basin, Australia. J. of Hydrology 174, 219-240. 

Talma, A.S., Vogel, J.C. and Heaton, T.H.E. (1984) The geochemistry of the Uitenhage artesian aquifer (carbonate solution in a closed system). Isotope Hydrology in Water Resources Development 1983, IAEA Vienna, 481 497. 

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