Water table aquifer

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. 

Fig. 3. Three-dimeiisioiial flow for stream recharge via a water table aquifer where (a) is the elevation view and (b) is the plan view.

Wilson JT, McNabb JF, Balkwill DL, et al. 1983a. Enumeration and characterization of bacteria indigenous to a shallow water-table aquifer. Ground Water 21 134-142. 

Large regional-scale dissolved contaminant plumes have been reported in many parts of the country. In California, many of the alluvial basin aquifers (i.e., major groundwater basins) were found to be impacted by chlorinated solvents during the period between 1979 to 1981. Beneath one valley a dissolved TCE plume incorporated the upper 50 ft of the saturated zone of a water table aquifer, was over 3 miles wide, and extended hydraulically downgradient over 14 miles. 

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... 

Maguire, T. F., 1988 Transport of a Non-Aqueous Phase Liquid within a Combined Perched and Water Table Aquifer System In Proceedings of the National Water Well Association of Ground Water Scientists and Engineers FOCUS Conference on Eastern Regional Ground Water Issues, September, 1988. 

Certainly, if protection of the Silverado aquifer is the primary objective, and the water table aquifers (i.e., Gage, Old Dune Sand, or Semi-perched aquifers) are essentially a write-off, then the intervening aquifer(s), although not used for water supply, may serve the purpose of guardian aquifers. 

Bromacll leached to a shallow water table aquifer In a Lakeland sand soil In a Florida test plot (61). The concentrations seemed to be related to rainfall, and at one point exceeded 1 ppm, though values of several hundred parts per billion were more typical. 

Carbofuran has been found In ground water In Wisconsin and New York, In areas with sandy soils and water table aquifers, at levels typically between 1 and 50 ppb (19). 

Goni, M.A., Gardner, R.L. (2003) Seasonal dynamics in dissolved organic carbon concentrations in a coastal water-table aquifer at the forest-marsh interface. Aquat. Geochem. 9, 209-232. 

Jorgensen (1968) investigated a case of water quality deterioration in a municipal well at Avon, South Dakota. The city wells pumped the free water table aquifer of the Codell sandstone (Fig. 16.9). Well A had a significantly... 

Andersen, L.J., Kelstrup, V., and Kristiansen, H. (1980) Chemical profiles in the Karup water-table aquifer, Denmark. In Nuclear Techniques in Groundwater Pollution Research, IAEA, Vienna, 47-60. 

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. 

FIGURE 3-4 A representative water table aquifer of sand and gravel with fractured bedrock beneath it. Several paths by which chemicals can enter the subsurface environment are shown. 

If the density of the bulk product is greater than that of water, the product will continue to migrate downward through the water table aquifer under the continued influence of gravity. Downward migration ceases when the product is converted to residual saturation or when an impermeable surface is encountered. Polychlorinated biphenyls and other chlorinated organic solvents are usually denser than water and are characterized as DNAPLs. 

Unconfined aquifer (water-table aquifer) An aquifer that is not separated from the surface by an aquitard or confining bed. 

Quantitative assessment of water-table aquifers vulnerability is being carried out when there is risk of bacteriological infechon. This method is based on groundwater downward time-of-travel during which percolated bacteria or polluted water may rich to water table. 

At present qnalitative and qnantitative evaluation of water table aquifers and compiling of respective maps are of highly practical importance (Goldberg 1988 Grozdova 1979). 

Vulnerability of water-table aquifers we have evaluated by depths to water table from land surface and by thickness of poorly permeable soils in aeration zone. According to this method in the cross-section of aeration zone following types of soils with low infiltration rates have been distinguished ... 

Table 2 Classification of water-table aquifer by vulnerability degree...

Table 3 Evaluation of water-table aquifer vulnerability by bacteriological contamination...

Vulnerability assessment from bacteriological contamination is given in Table 5. The water-table aquifer is evaluated as slowly protected (TOT = 50-100 days) and protected (100-200 days). 

Table 4 Water-table aquifer intrinsic vulnerability assessment by numerical scores... 

Table 5 Water-table aquifer natural vulnerabihty assessment from bacteriological contamination ...

The primary hydrologic units of interest in this study are, in descending vertical order (1) the residuum which forms the water-table aquifer and acts as an upper confining unit, (2) the Upper Floridan aquifer (formerly the principal artesian aquifer), which consists of the Ocala Limestone, the Clinchfield Sand, and the upper part of the Lisbon Formation, and (3) the basal part of the Lisbon... 

The decreased infiltration of water into the residuum caused a decrease in the vertical leakage through the base of the residuum, and so recharge to the Upper Floridan aquifer decreased. Water levels continued to decline in both the water-table aquifer and the... 

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