Types of Aquifers

Unconfined or water table aquifers maintain a saturated surface that is exposed directly to the atmosphere. These are often similar to a bathtub full of sand or gravel to which water has been added. A well drilled through the water table would fill with water to the common water elevation in the tub. Thus, the potentiometric head in the aquifer is at the elevation of the water table. Unconfined aquifers are also characterized by a fluctuating water table, which responds seasonally. With unconfined aquifers, the water table is at atmospheric pressure, and only the lower portion of the aquifer is saturated. Recharge to a water table aquifer comes from rainfall that seeps downward to the water table. The water table level in this type of aquifer rises in direct proportion to the effective porosity. If the equivalent of 2 in. of rainfall seeps into the water table (actually reaches the water table) in an aquifer with an effective porosity of 0.3, the water table would rise 6.7 in. Alternatively, if the same water is pumped and removed from a well, the water table aquifer is then derived from the storage in the formation in the immediate vicinity of the well. Natural 

FIGURE 3.17 Example of water table contour map and hydrogeologic cross section showing two groundwater flow regimes. 

Semiconfined or leaky aquifers are similar to confined aquifers with the exception that the overlying and/or underlying units are not impermeable and some leakage 

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. 

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. 

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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). 

Types of Aquifer Test Analyses for Unconsolidated Environments... 

Depending on the desired treatment methodology and goals, addition of surfactants to a subsurface system should either increase HOC distribution coefficients (i.e., immobilization approach) or decrease them (i.e., mobilization objective as in many SEAR applications). For example, distribution coefficients for phenanthrene and naphthalene to kaoiinite are 0.002 and 0.0003 L/g, respectively (Table 4). Therefore, if enhanced mobilization of these HOCs in a similar type of aquifer system was desired, addition of a surfactant would have to bring the distribution coefficients below these values. However, as can be seen in Fig. 5, all distribution coefficients for the surfactant doses investigated here are larger than these values, even when the doses and subsequent aqueous surfactant concentrations are well above the CMC. This observation results from a combination of surfactant sorption followed by HOC partitioning to the sorbed surfactant. 

A specific numerical example is enlightening. The KD value of naphthalene for an initial Tween 80 dose of 7.63 mM is 0.0075 L/g (i.e., the last point in Fig. 5d corresponding to Ssurf = 5.32 mM this dose also corresponds to a sorbed Tween 80 concentration of 29 pmol/g as can be seen in Fig. 1). This value greatly exceeds the A/Dwhen no Tween 80 is present in the system. In other words, addition of Tween 80 to this model system or a similar type of aquifer system would lead to an increase in naphthalene retardation, not a decrease as would be desired for SEAR applications. Because the plateau for Tween 80 sorption occurs near the above dose (i.e., 770 x CMC, Fig. 1), it is expected that much higher doses would be needed to overcome the enhanced retardation effects caused by Tween 80 sorption to the aquifer matrix. These very high surfactant doses, however, would likely not be practical for real world applications. A more useful approach, therefore, would be to utilize the Tween 80 to increase retardation in this particular system the optimum dose to use would correspond to a maximum KD value. 

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... 

Groundwater is abstracted from many different types of aquifers, some of which may be highly susceptible to pollution as a consequence of human activity. The vulnerability of groundwater sources is important when assessing the risks to groundwater posed by various activities, Some aquifers are protected by one or more layers of impermeable material, such as clay, that lie above the saturated zone and that will prevent or retard the transport of chemicals from their sources to the saturated zone. Also, aquifers at certain depths may be protected from chemicals (even from some naturally occurring chemicals) that may be present at other depths in the geological profile. 

Figure 6.4. Types of aquifers, wells, and groundwater flow. (Adapted from Environment Canada).

Microbial populations in landfill leachate-contaminated aquifers are dominated by bacteria (eubacteria and archaea), as shown by analysis of the phospho lipid fatty acids (PLFAs) (Ludvigsen et al, 1999). The total number of bacteria reported in landfill leachate plumes are in the range of 4 X 10" -1.5 X 10 cells g dry weight (dw) and the number of colony-forming units, hving cells, are in the range of 60-10 CFU g dw (Christensen et al, 2001). However, the large variation caused by different analytical methods and the fact that different types of aquifers were studied mask the difference in the number of... 

Table 1 shows the median concentrations of organic carbon from seven different types of aquifers. The median concentration of dissolved organic carbon is 0.7 mg C/L for sand and gravel, limestone, and sandstone aquifers. Only igneous aquifers with 0.5 mg C/L had a lower median concentration of organic carbon. In a study based on 50 samples from various types of aqui-... 

TABLE 1. Median Concentration of Organic Carbon in Various Types of Aquifers"... 

For various aquifer minerals, porosity varies over a fairly narrow range (ca. 0.3 to 0.5) but hydraulic conductivity varies over many orders of magnitude.2 Even for a specific type of aquifer material, ranges of 1-4 orders of magnitude are common (e.g., 10 8 5 to 10 4 m/s for fractured rock, 10-5 to 10 3 m/s for well-sorted sand). The lowest hydraulic conductivities are found for crystalline rock (10 14 to 10 10 m/s) and the highest for well-sorted gravel (10-2 to 1 m/s) and clean sand or cavernous limestone (10 6 to 10 2 m/s). 

Possible water sources for injection are sea water, fresh surface water, produced water or aquifer water (not from the producing reservoir). Once it has been established that there is enough water to meet demand (not an issue in the case of sea water), it is important to determine what type of treatment is required to make the water suitable for injection. This is investigated by performing laboratory tests on representative water samples. 

Nested wells can also be used to analyze multilayer aquifer flow. There are many situations involving interaquifer transport owing to leaky boundaries between the aquifers. The primary case of interest involves the vertical transport of fluid across a horizontal semipermeable boundary between two or more aquifers. Figure 4 sets out the details of this type of problem. Unit 1 is a phraetic aquifer, bound from below by two confined aquifers, having semipermeable formations at each interface. 

The demand for gas is highly seasonal. Thus pipeline companies economi2e by si2ing production faciUties to accommodate less than the system s maximum wintertime demand. Underground storage faciUties are used to meet seasonal and daily demand peaks. In North America, gas is stored in three main types of underground formations depleted oil or gas fields, aquifers that originally contained water, and caverns formed by salt domes or mines. 

Contaminants. The type and concentration of contaminants in an aquifer dictate what type of in situ bioremediation system, aerobic, anaerobic, or combination, ate the most effective. 

Porous Media Packed beds of granular solids are one type of the general class referred to as porous media, which include geological formations such as petroleum reservoirs and aquifers, manufactured materials such as sintered metals and porous catalysts, burning coal or char particles, and textile fabrics, to name a few. Pressure drop for incompressible flow across a porous medium has the same quahtative behavior as that given by Leva s correlation in the preceding. At low Reynolds numbers, viscous forces dominate and pressure drop is proportional to fluid viscosity and superficial velocity, and at high Reynolds numbers, pressure drop is proportional to fluid density and to the square of superficial velocity. 

Even when horticultural crops leave substantial amounts of nitrate in the soil at harvest, they are not usually a very important factor in the nitrate problem because horticulture occupies only about 6% of the cultivable land. However, there could be a problem if, because of the type of soil, a number of market gardens were concentrated above an aquifer that was an important source of potable water. 

The modeling of a groundwater chemical pollution problem may be one-, two-, or tlu-cc-dimcnsional. The proper approach is dependent on the problem context. For c.xamplc, tlie vertical migration of a chemical from a surface source to the water table is generally treated as a one-dimensional problem. Within an aquifer, this type of analysis may be valid if the chemical nipidly penetrates the aquifer so that concentrations are uniform vertically and laterally. This is likely to be the case when the vertical and latcrtil dimensions of the aquifer arc small relative to the longitudinal scale of the problem or when the source fully penetrates the aquifer and forms a strip source. 

C22-0083. Many places have underground water, called aquifers, that can be tapped using wells. Depending on the types of geological formations above these aquifers, water from the surface may or may not reach these underground reservoirs. How could radioactive tracers be used to determine if rainwater falling on a particular location makes Its way down into an aquifer beneath that location ... 

Bachu et al. proposed a theoretical approach to C02 storage estimation considering each type of trapping mechanism in deep saline aquifers [26]. They introduced a simple time-independ-... 

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