Confined aquifers

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. 

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. 

Judging from the hydraulic heads, the vertical flow across the semipermeable interface 1 is in a downward direction, whereas across the semipermeable interface 2 it is in an upward direction. Therefore, unit 2 is being fed by fluid from the phraetic aquifer above it and the confined aquifer below. 

In the case of confined aquifers, the natural recharge and flow is more restrained, and as illustrated in Figure 35, the flow distance will normally be significantly longer and more time consuming. 

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

The relative importance of pressure and gravitational heads depends on whether the water formation is in a free water table condition or in a confined aquifer condition. 

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. 

May cause a lateral spread of dissolved or separate phase contaminant plume Contamination may be transferred from groundwater to die vadose zone Has limited applicability at sites with confined aquifers Low soil permeability or other heterogeneous conditions may reduce effectiveness... 

Has limited applicability at sites with confined aquifers and stratified layers soil heterogeneities may limit effectiveness. 

Surface and groundwater flow within the Canada Creek watershed is SE from the high level of the mill and ARS to the low lying NATA then north through the wetlands (Fig 3). This path is based on local topography, core log data, and hydraulic head values for the confined aquifer. Hydraulic heads show groundwater in the Canada Creek... 

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... 

Certain factors must, however, be considered in choosing the appropriate analytical solution unconsolidated vs. consolidated conditions, fully vs. partially penetrating wells, variable discharge rules, delayed yield, and aquifer boundaries. Most methods are best suited for unconsolidated aquifers with well-defined overlying and underlying boundaries, whereas with consolidated aquifers, the effective aquifer thickness is uncertain. A pumping well that fully penetrates a confined aquifer (i.e.,... 

In addition, monitoring wells screened across LNAPL within confined aquifers will exhibit an exaggerated thickness. This exaggerated thickness reflects relatively high confining pressure, which forces the relatively lower density fluid upward within the borehole. Thus, the measured thickness is a function of the hydrostatic head and not the capillary fringe, which has been destroyed by the confining pressures. 

Note the similarity to the Dupuit equation for confined aquifers ... 

Kimberlin, D. K. andTrimmell, M. L., 1988, Utilization of Optoelectronic Sensing to Determine Hydrocarbon Thicknesses within Confined Aquifers In Proceedings of the National Water Well Association of Ground Water Scientists and Engineers and the American Petroleum Institute Conference on Petroleum Hydrocarbons and Organic Chemicals in Ground Water Prevention, Detection and Restoration, Vol. I, November, pp. 255-274. 

In principle, the recovery of LNAPL is similar in mechanical operation to production of a low-pressure, water-driven reservoir. Almost all documented petroleum remediations have been characterized by subsurface conditions under water table conditions (i.e., the top surface of the fluids are at atmospheric pressure). Few cases of confined aquifer situations have been reported in the literature, and although the mechanical recovery procedures are slightly different, the economic considerations are similar. 

Aquifers may be classified as unconfined or confined. Although unconfined aquifers usually have sediments or soils between them and the surface, the geologic materials are sufficiently permeable so that unconfined aquifers are rapidly influenced by atmospheric conditions, including pressure and precipitation events (Figure 3.5). The upper boundary of an unconfined aquifer is the water table. A confined aquifer is located between two aquitards ((Freeze and Cherry, 1979), 48). At least one aquitard occurs between a confined aquifer and the surface. That is, a confined aquifer is substantially insulated from conditions on the Earth s surface. 

Confined aquifer An aquifer located between two aquitards and with one or more aquitards between it and the surface (compare with unconfined aquifer). 

Unconfined aquifer An aquifer that is not overlain by any aquitards. Precipitation and surface water may readily infiltrate into an unconfined aquifer (compare with confined aquifer). 

The results are compared with solutions determined using fixed-length management periods. The hypothetical homogeneous, isotropic, confined aquifer is comprised of 60 finite elements and 77 nodes, with dimensions 1500 m by 900 m (Culver and Shenk, 1998). The initial contaminant plume, which has a maximum toluene concentration of 40 mg/L, is shown in Figure 1. An easterly steady flow was maintained with a constant hydraulic head of 12.0 m and contaminant concentration of 0.0 mg/L on the left side, a constant hydraulic head of 0.0 m and contaminant concentration of 0.0 mg/L on the right side, and no flow at the top and bottom boundaries. In this example, the sorbed phase is assumed to remain in equilibrium with the... 

Roll-front uranium deposits in confined aquifer systems are amenable to extraction by in situ leach techniques. This method of mining was first tested in Wyoming approximately twenty years... 

The effects of aquifer anisotropy and heterogeneity on NAPL pool dissolution and associated average mass transfer coefficient have been examined by Vogler and Chrysikopoulos [44]. A two-dimensional numerical model was developed to determine the effect of aquifer anisotropy on the average mass transfer coefficient of a 1,1,2-trichloroethane (1,1,2-TCA) DNAPL pool formed on bedrock in a statistically anisotropic confined aquifer. Statistical anisotropy in the aquifer was introduced by representing the spatially variable hydraulic conductivity as a log-normally distributed random field described by an anisotropic exponential covariance function. 

For each hydraulic conductivity field generated, the associated variable hydraulic head field and groundwater velocity field were determined. Each variable hydraulic head field was evaluated numerically by solving the following steady state two-dimensional groundwater flow equation for a heterogeneous confined aquifer [50] ... 

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). 

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.

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