Unconfined 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. 

Tricca A, Wasserburg GJ, Porcelli D, Baskaran M (2001) The transport of U- and Th-series nuclides in a sandy unconfined aquifer. Geochim Cosmochim Acta 65 1187-1210... 

Figure 34. Flow of groundwater from an unconfined aquifer, drained by a river and by pumping from a water well...

In an unconfined aquifer the permeability relates to the specific yield. This term express the ability for the aquifer to be drained and is clearly related to the grain size. The specific retention reflects the capillary forces that tend to retain water close to the grains. How these parameters relate to each other is shown in Figure 38. 

Figure 38. The relation of porosity, specific yield, specific retention and grain size in an unconfined aquifer...

Subsurface formations can be divided into the overburden (unconsolidated) and bedrock according to its solidarity. The upper subsurface can be further divided into the unsaturated zone and the saturated zone depending on pore structure and moisture saturation. The saturated zone is the zone in which the voids in the rock or soil are filled with water at a pressure greater than atmospheric. The water table is at the top of a saturated zone in an unconfined aquifer. The unsaturated zone is the zone between the land surface and the water table, and is also called the zone of aeration or the vadose zone. The pore spaces contain water at less than atmospheric pressure, air, and other gases. This zone is unsaturated except during periods of heavy infiltration. 

The NATA site consists of tailings overlying relatively low permeability clay soil, gravel-sand, and bedrock units. There are two aquifers one is confined between the bedrock and clay, and another unconfined aquifer which developed within the NATA tailings. Vertical exchange between aquifers is limited due to the clay separation. This is demonstrated by the higher piezometric levels of the unconfined aquifer compared to the confined one. 

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... 

Fractured rock aquifers may be analyzed in a manner similar to unconfined aquifers, providing the responses exhibit the same general characteristics. During the initial testing, the fracture contributes water to the well, followed by primarily... 

Grubb, S., 1993, Analytical Model for Estimation of Steady-State Capture Zones of Pumping Wells in Confined and Unconfined Aquifers Ground Water, Vol. 31, No. 1, pp. 27-32. 

When the Muskat and Dupuit equations are combined and then integrated to approximate steady-state flow to an unconfined aquifer (as found at most remedial sites), the resulting equation takes the following form ... 

Bouwer, H. and Rice, R. C., 1976, A Slug Test for Determining Hydraulic Conductivity of Unconfined Aquifers with Completely or Partially penetrating Wells Water Resources Research, Vol. 12, No. 13, pp. 423 438. 

Kaluarachchi, J. J., Parker, J. C., and Lenhard, R. J., 1989, A Numerical Model for Areal Migration of Water and Light Hydrocarbon in Unconfined Aquifers Advanced Water Resources, in press. 

FIGURE 8.7 Schematic showing radial flow from a recharge well penetrating an unconfined aquifer. 

FIGURE 13.7 Schematic cross sections of enhanced biorestoration of unconfined aquifers. 

Nwankwor Gl, Gillham RW, van der Kamp G, Akindunni FF (1992) Unsaturated and saturated flow in response to pumping of an unconfined aquifer—field evidence of delayed drainage. Ground Water 30 690-700... 

Goodfellow 2003 Gilliss et al. 2004). Interpretation of hydrology is simplified in regions dominated by porous flow in unconfined aquifers, and significantly more complicated where flow is fracture-dominated, or where significant density contrasts between different groundwater flow systems exist (Carey et al. 2003). 

In situ redox manipulation (ISRM) is an in situ, groundwater remediation technology for manipulating the oxidation-reduction (redox) potential of an unconfined aquifer to immobilize inorganic contaminants (metals, inorganic ions, and radionuclides) and to destroy organic contaminants (primarily chlorinated hydrocarbons). 

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. 

The Indus alluvial plain of Punjab and Sindh provinces in Pakistan is of Quaternary age and is similar to the Ganges alluvial system of India and Bangladesh. The sediments of the plain have their provenance in the western Himalayas and are brought down by the River Indus and its tributaries. The alluvial deposits are widespread and thick (Smedley, 2005) and mostly form unconfined aquifers. However, in contrast to the Bengal basin, the aquifers are relatively oxic (Mahmood et al., 1998 Tasneem, 1999). The area also differs in having a more arid climate and greater proportion of Pleistocene deposits with greater apparent connectivity between the river systems and the aquifers (Smedley, 2005). 

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). 

Optimal Plume Capture Design In Unconfined Aquifers... 

In this paper, the advective control model for groundwater plume capture design is described, algorithmic requirements to accommodate unconfined aquifer simulation are presented, and two- and three- dimensional example problems are used to demonstrate the optimization model capabilities and design implications. The model is applicable for designing long-term plume containment systems and as such assumes steady-state flow and time-invariant pumping. 

During simulation of unconfined aquifers, care must be exercised to avoid dewatering numerical grid cells because of excessive pumping. If extraction well locations are assumed to be most prone to dewatering then either upper bounds on extraction rates or lower bounds on head at the well cell can be imposed on the pumping solution. Imposing explicit upper bounds on extraction may artificially eliminate the best solutions from consideration. With this in mind, the approach taken here is to impose lower... 

Optimal Plume Capture Design in Unconfined Aquifers 33... 

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