Aquifers and Aquicludes

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

Rock beds at the saturated zone that host flowing groundwater are called aquifers (from Latin aqua, water ferre, to bear or carry). Aquifer rocks contain the water in voids—pores and fissures. The size and number of voids and the degree of interconnection between those pores and fissures define the qualities of the aquifers. The same properties define infiltration efficiency and capacity of intake of recharge water. These properties are discussed below for different rocks. 

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Metamorphic rocks. Quartzite, schist, gneiss, and other metamorphic rocks and metasediments vary in their hydrological properties, but in most cases are fractured and transmit water. In rainy zones weathering may produce clays that occasionally clog the fractures. 

The rocks discussed in this section have intrinsically medium to good infiltration efficiency, aquifer conductance, and storage capacities. These features may be developed to different degrees as a result of secondary 

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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). 

Cracks do not aquiclude. Cracks do not aquifer and aquiclude is Cracks do not aquifer and aquiclude is Cracks do not aquifer. 

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.

Indeed, often many shallow and deeper aquifers are separated by less permeable geological horizons, known as aquitards, which retard, reduce and deflect subsurface water flow in these cases, hydraulic connections do exist between adjacent aquifers, and significant water exchange can take place between them. Sets of hydraulically connected aquifers form aquifer systems, and this is today often the unit taken or the scale used to study, protect and manage groundwater resources. Aquifer systems are in turn separated by aquicludes, made of highly impermeable material, which do not allow water to get across. 

Hydrogeology is a complex science studying the interrelations between the lithological conditions of rock formations (consolidated and unconsolidated), the structural and tectonic setting of aquifers, aquitards and aquicludes, tire input to and output from an aquifer in terms of fluxes, the water-rock interactions, as well as the chemical composition and evolution of the groundwater. 

During underground sequestration operations, supercritical CO2 will be injected into a deep aquifer and rise, as a result of buoyancy, until it reaches the overlying aquiclude where it will form a bubble . The presence of supercritical CO2 will result in chemical disequilibria and hence the initiation of reactions. It is important to understand the direction, rate and magnitude of such reactions, both in terms of their impact upon the ability of the aquifer to safely contain the injected CO2, and in terms of the longevity of CO2 containment. Three broad areas of reaction can be considered ... 

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. 

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... 

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.

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. 

FIGURE 3-1 A representative subsurface environment, showing an upper unsaturated zone and a lower saturated zone with an aquiclude beneath. An aquiclude is nearly impermeable to water. The saturated zone above the aquiclude is a water table aquifer. A well is used to withdraw water from the aquifer. 

DNAPL located at the bottom of an aquifer serves as a source of pollution to groundwater flowing past. Considering the low solubility of many chlorinated solvents and the unacceptability of their presence at concentrations higher than a few parts per billion, it is apparent that even a relatively small volume of DNAPL can contaminate an immense volume of groundwater unfortunately, centuries or even millennia may be required for the ultimate removal of DNAPL from an aquifer under natural conditions. DNAPL also commonly will enter channels, fractures, holes, and cracks in an underlying aquitard or aquiclude, thereby sinking even farther (see Fig. 3-26). No satisfactory technique has yet been developed to locate DNAPL in bedrock fractures, much less to predict its movement. 

As typical water-contained karst water mine, Gujiatai Eon Mine is characterized with complicated hydrogeological conditions, a river cuts through middle of the ore body on the earth s surface and there are mainly two aquifers, which are quaternary glutenite and Ordovician system limestone developed from karst, and tertiary aquiclude between the two aquifers. The ore body lies in contact zone of marble and diorite and presents stratoid. Direct roof of ore body is mostly marble and its footwall is skam, sometimes skarnized diorite and alteration diorite. 

The engineering geological condition in the study area is relatively simple. The excavation causes cracks. These cracks will damage bedrock s and the imified aquiclude s integrity which can keep sandy aquifer from leaking. And the severity of the... 

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