U.S. Environmental Protection Agency (1993)

Aquifer Sensitivity - "The relative ease with which a contaminant (in this case a pesticide) applied on or near the land surface can migrate to the aquifer of interest. Aquifer sensitivity is a function of the intrinsic characteristics of the geologic materials of interest, any overlying saturated materials, and the overlying unsaturated zone. Sensitivity is not dependent on agronomic practices or pesticide characteristics."

Ground Water Vulnerability - "The relative ease with which a contaminant (in this case a pesticide) applied on or near the land surface can migrate to the aquifer of interest under a given set of agronomic management practices, pesticide characteristics and hydrogeologic sensitivity conditions.

the quality of the ground water over a region. Contamination resulting from brine injection wells, enhanced oil recovery wells, artificial recharge wells, and subsurface nuclear detonations are not considered because they represent purposeful placement of contaminants in the ground water system; it is obvious that any ground water system is vulnerable to such activity. The mobilization of naturally occurring trace elements and salt water intrusion into coastal aquifers as a result of pumping are also excluded. While in many places these sources and pathways may be the dominant cause of contamination, the concept of ground water vulnerability addresses only contaminants introduced by humans above the water table at or near the land surface. Other potential contamination must be addressed on a case-by-case basis using other means.

In certain circumstances, a large number of certain types of point sources—such as septic tank systems—distributed over a region could be considered a regional nonpoint source problem and are included in this definition. Also, cracks and fractures on a regional scale would be considered. In all cases considered under this definition, the contaminant must move at least partially through surficial material. Any mechanism that causes a complete bypassing of this material, such as back siphoning during chemigation, is not directly addressed by the methods examined in this study.

This conception of ground water vulnerability is bounded, as are any others, by a fundamental principle which is stated here as the First Law of Ground Water Vulnerability:

All ground water is vulnerable.

Vulnerability is not an absolute or measurable property, but an indication of the relative likelihood with which contamination will occur; no ground water

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