and thus lessens the potential for contamination. The depth to the water table can be important because short flow paths decrease the opportunity for sorption and biodegradation and thus increase the potential for contamination. Conversely, longer flow paths from land surface to the water table can lessen the potential for contamination by chemicals that sorb or degrade along the flowpath. The recharge rate is important because it affects the extent and rate of transport of contaminants through the unsaturated zone. Finally, environmental factors, such as temperature and water content, can significantly influence the loss of contaminants by microbial transformations.

Some general geologic and hydrologic factors that influence an aquifer's vulnerability to contamination are shown in Table 3.1, along with examples of features that lead to low or high vulnerability. Although these factors may seem quite simple at first inspection, many of them interact in the

TABLE 3.1 Principal Geologic and Hydrologic Features that Influence an Aquifer's Vulnerability to Contamination (After Johnston 1988)

Feature Determining Aquifer Vulnerability to Contamination

Low Vulnerability

High Vulnerability

A. Hydrogeologic Framework

Unsaturated Zone

Thick unsaturated zone, with high levels of clay and organic materials.

Thin unsaturated zone, with high levels of sand, gravel, limestone, or basalt of high permeability.

Confining Unit

Thick confining unit of clay or shale above aquifer.

No confining unit.

Aquifer Properties

Silty sandstone or shaley limestone of low permeability.

Cavernous limestone, sand and gravel, gravel, or basalt of high permeability.

B. Ground Water Flow System

Recharge Rate

Negligible recharge rate, as in arid regions.

Large recharge rate, as in humid regions.

Location within flow system (proximity to recharge or discharge area)

Located in the deep, sluggish part of a regional flow system.

Located within a recharge area or within the cone of depression of a pumped well.



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