Aquiclude

More complex arrangements of aquifers, aquiclude, and aquitards, notably in deep sedimentary basins, are systems of interbedded geologic units of variable permeability. These systems are referred to as a multilayered aquifer system. Such systems are considered more of a succession of semiconfined aquifers separated by aquitards. 

Geologic limitations such as the need for an aquiclude and the increasing costs as contaminant depth increases. 

The saturated zone usually contains strata of varying permeabilities, forming aquifers and aquicludes, described in the following sections. 

Clay. Clay minerals have several characteristics that make them good aquicludes ... 

The effectiveness of clay aquicludes depends on the type of clay, montmorillonite being perhaps the best. The thickness of the clay aquiclude is of prime importance—a clay bed 1 m thick may be a significant barrier to underground water movement, but water may slowly leak through. The thicker the aquiclude, the higher is its sealing efficiency. Clay aquicludes with a thickness of several hundred meters are known. 

Shales. Shales are derivatives of clays, formed in slow diagenetic processes. Shales have little or negligible swelling capacity and have medium plasticity. They form effective aquicludes at thicknesses of at least a few meters to tens of meters. 

Igneous rocks. Large igneous bodies, such as stocks or thick sills, often act as aquicludes because they lack interconnected fractures or dissolution conduits. Dikes and sills may act as aquicludes if they are either very fresh and not fractured or are weathered into clay-rich rocks. 

Fig. 2.5 Basic components of a phreatic groundwater system intake outcrops, an aerated zone, the water table, the saturated zone that constitutes a water-bearing aquifer, and impermeable rock beds of the aquiclude that seal the aquifer at its base.

Confined aquifers are water-bearing strata that are sealed at the top and the bottom by aquiclude rocks of low permeability (Fig. 2.6). Confined aquifers are commonly formed in folded terrains (section 3.4) and have a phreatic section, where the aquifer rock beds are exposed to recharge infiltration, and a confined section, where the aquifer rock beds are isolated from the landscape surface by an aquiclude (Fig. 2.6). 

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... 

Fig. 2.16 A lense of impermeable rock creating a local perched water table and a spring an aquiclude sealing off a stagnant zone, turning it into a confined system and karstic features.

Clay and shale are hydroaluminum silicates that by themselves do not add salts to the water that comes in contact with them. However, clay and shale often contain veins and nodules of gypsum, pyrite, and rock salt. Clay and shale are impermeable and form aquicludes rather than aquifers (sections 2.3 and 2.4), but because of the high solubility of gypsum and especially rock salt, groundwater in contact with clay and shale at the base of aquifers often gets saline and is of poor quality. 

Fig. 3.1 Two aquifers of nonsaliferous rocks, separated by an aquiclude of clay with gypsum and salt rock. (I) fully perforated and producing saline water (II) stopped at a safe distance above the clay, abstracting good water from the upper aquifer alone (III) sealed for several meters above and below the clay bed, producing good water from both aquifers.

Clay and shale have a large number of minute pores, totaling up to 55% of the rock volume. Yet these pores are poorly interconnected, resulting in very low permeability or impermeability (section 2.7). Clay and shale significantly slow infiltration and serve as aquicludes. 

Many rock types have a layered structure, individual rock layers varying in thickness from a few centimeters to tens of meters. Layered rocks include marine sediments, most continental sediments, lava flows and volcanic ejecta, and intrusive sills. The hydraulic properties vary from one rock layer to another, often resulting in abrupt changes along the vertical axis. In terms of the permeability coefficient (k) the lateral coefficient (kx) may significantly differ from the vertical coefficient (kz). The alternation of aquifers and aquicludes results from the layered structure of different rocks, and the occurrence of springs is often controlled by the layering of rocks. Fissures may be restricted to individual rock layers or cross several rock beds, in which case water flow is improved, mainly in the vertical direction. 

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.

Geological information of the rock sequence and tectonic settings may be instructive but not definitive, as it is hard to translate field data into hydraulic conductivity values a shale bed may be fractured and let water flow through in one case, and a clay bed may be weathered and act as an aquiclude in another. In addition, a variety of processes lower the local water conductance, occasionally preventing lateral flow. An example of such a process is chemical clogging (Goldenberg et al., 1983). 

Fig. 4.14 Possible effect of a pumping test. The water table of aquifer I is drawn down near the well, a feature called a depression cone. Occasionally water from a lower confined aquifer may breach in (arrows across the aquiclude).

Fig. 6.13 Three wells with water tables similar to those seen in Fig. 6.12 but separated by aquicludes. They have no hydrological connections in spite of the apparent water table gradient.

Fig. 16.11 A chloride concentration cross-section, Gerald City. Leakage from the brine evaporation pit is recognizable. The heavy brine accumulated above the shale aquiclude. (From Fryberger, 1975.)...

Determine the origin of the mixed ground water (i.e. the share of seawater and fresh ground water) taking into account the geological features around the irrigation water well. Keep in mind that there is no distinct aquiclude between the Quaternary and the Cretaceous aquifer. 

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