Phreatic aquifer

Rouen D, Scher H, Blunt M (1997) On the structure and flow processes in the capillary fringe of phreatic aquifers. Transp Porous Media 28 159-180 Rose CW (1993) The transport of adsorbed chemicals in eroded sediments. In Russo D, Dagan G (eds) Water flow and solute transport in soils. Springer, Heidelberg, pp 180-199 Rosenberry DO, Winter TC (1997) Dynamics of water-table fluctuations in an upland between two prairie-pothole wetlands in North Dakota. J Hydrol 191 266-289 Russo D (1997) On the estimation of parameters of log-unsaturated conductivity covariance from solute transport data. Adv Water Resour 20 191-205 Russo D, Toiber-Yasur 1, Laufer A, Yaron B (1998) Numerical analysis of field scale transport of bromacU. Adv Water Resour 21 637-647... 

Zhang H, Barry DA, Hocking GC (1999) Analysis of continuous and pulsed pumping of a phreatic aquifer. Adv Water Resour 22 623-632... 

Phreatic aquifers have free communication with the aerated zone. The synonym free surface aquifer relates to the free communication between the aquifer and the vadose zone. An example is shown in Fig. 2.5. The term phreatic originates from the Greek word for a well. 

Phreatic aquifers are the most exploited type of aquifer and most of the hydrochemist s work is performed on them. Phreatic aquifers are the collectors of infiltrating recharge water, and this process is well reflected in... 

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... 

Fig. 2.7 A confined aquifer underlying a phreatic aquifer. The nature of such a system may be established by parameters measured at boreholes and wells (see text).

Confined aquifers (section 2.8) are rare in tectonically undisturbed regions with horizontal rock beds (Fig. 3.5). Tilting of the aquifer and aquiclude sandwich makes room for the formation of confined aquifers. It provides each case with a recharge outcrop section, forming a phreatic aquifer (section 2.8) and a confined section fed by the former (Fig. 3.6). 

Fig. 3.5 Horizontal rock beds often allow for the formation of only one recharged (phreatic) aquifer, the first aquiclude preventing recharge water from reaching lower potential aquifers.

The common interpretation of pumping test data is based on the assumption that only one aquifer is pumped and tested. However, the intensive pumping during the test causes a significant local pressure drop in the pumped aquifer that may cause water from an adjacent aquifer to breach in (Fig. 4.14). If the pumping test is done in a phreatic aquifer, water of a lower confined aquifer may flow in. Similarly, in pumping tests in confined aquifers, an overlying phreatic aquifer may be drawn in. 

Fig. 4.17 Tritium and 14C measurements during a pumping test conducted in a confined aquifer at the Aravaipa Valley, Arizona (Adar, 1984). Recent water of high tritium and 14C concentrations intruded from the overlying phreatic aquifer.

Phreatic aquifers are often regarded as recharge zones feeding adjacent confined systems. A continuous through-flow is commonly envisaged, controlled by (and deduced from) water level gradients and transmissivities. However, in certain cases a discontinuity is observed between the phreatic and confined parts of a system, reflected in abrupt changes in the chemical... 

Fig. 11.16 Study area of the Judean Mountains, central Israel. Limestone and dolomite (Cenomanian-Turonian) outcrops serve as recharge areas into a phreatic aquifer, confined on the flanks by younger chalk (Senonian). (From Mazor and Kroitoru, 1987.)...

Fig. 11.28 14C values (pmc) in wells of the Sokoto basin, northern Nigeria. The arrow denotes the orginally suggested groundwater flow direction. The phreatic aquifer (A) coincides with the rock outcrops (stippled) and has high 14C values. An abrupt drop in 14C values is observed in the adjacent confined section of the system (B). (Data from Geyh and Wirth, 1980.)... 

Answer 2.5 The first water occurrence indicates the existence of a phreatic aquifer, and the second occurrence of water may indicate the existence of an underlying confined aquifer. However, the drillers could have difficulties in telling water occurrences apart if the drilling itself is conducted with water. Additional data are always needed—water table, water temperature, and water chemistry. Differences between the properties of the deeper and shallower waters will establish that two separated aquifers exist and that the deeper one is confined. 

Answer 11.5 The Parly-Chennons well is tapping a phreatic aquifer with post-1952 water. 

Ronen D., Magaritz M., Almon E., and Amiel A. (1987) Anthropogenic anoxification ( Eutrophication ) of the water table region of a deep phreatic aquifer. Water Resour. Res. 23, 1554-1560. 

Equations [3-6], [3-7a], and [3-7b] assume that the saturated aquifer thickness b is constant (as in a confined aquifer). In unconfined (phreatic) aquifers, which are somewhat more susceptible to subsurface contamination, the saturated thickness varies as the hydraulic head changes thus b is not, strictly speaking, constant. Unless otherwise stated, however, it is assumed in the following discussions that changes in the water table height of a phreatic aquifer are relatively small compared with the saturated thickness. When this is not the case, more complex expressions are needed to describe the hydrodynamics, and the reader is referred to Bear (1979). 

Equations [3-8a] to [3-8c] are different applications of the Thiem equation, which estimates drawdown in an aquifer or well under steady-state conditions. As previously mentioned, it is assumed that the changes in saturated aquifer thickness are small compared with the total saturated depth. This is necessarily true in a confined aquifer, but not always in an unconfined (phreatic) aquifer. If drawdown becomes a significant fraction of the saturated aquifer thickness, more complicated expressions for drawdown are obtained see Bear (1979). For an unconfined aquifer in which drawdown is a significant fraction of the saturated thickness, Eq. [3-8a] must be expressed in terms of head instead of drawdown ... 

Superposition in phreatic aquifers is more complicated than superposition in confined aquifers, because drawdown and hydraulic head are not additive. However, for unconfined aquifers in which drawdown is a sufficiently small fraction of aquifer thickness, the technique of superposition can be used as an approximation. The reader is referred to Strack (1989) for further details on superposition in unconfined aquifers. 

FIGURE 3-27 Typical schemes to recover a floating NAPL, such as gasoline, from a phreatic aquifer. In each case a cone of depression is created by pumping to encourage the NAPL to flow toward the collection point. Either a NAPL/water separator or two pumps are required. 

A well in a phreatic aquifer 100 meter thick supplies a small manufacturing plant, and is pumped continuously at a rate of 1200 liter/min. The difference in water levels between observation wells 01 and 02, located 10 and 20 m away, respectively, is seen to stabilize at 3 cm, whereas before pumping the level in Ox was 1 cm higher than in 02. The specific yield of the aquifer material is estimated as 0.2. 

The Quaternary Gravel Phreatic Aquifer. The thickness is less than 2 m and the water type is HCOj Ca. The permeability coefficient is 0.22-206.1 iti/d and the specific capacity is about 0.029-6.71 L/(s m). This aquifer receives precipitation and river with big seasonal change. 

As the eight monitoring sites all locate in the hanging wall and the foot wall of FI, their seepage situations are crucial for studying the hydraulic connection between the orefield and the Quaternary water-rich phreatic aquifer, and seawater. While the results make clear that the upper drifts have not severe connection with the Quaternary water-rich phreatic aquifer and seawater. It reflects, at present, the water-resistant layers haven t been disturbed seriously by the excavation. 

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. 

Air-equilibrated precipitation having a pH of 3.9 (due to sulfuric acid, H2SO4) falls on a watershed and ultimately recharges the phreatic aquifer. During the process of percolating to the water table, the water... 

---