The vulnerability potential of an aquifer to ground water contamination is in large part a function of the susceptibility of its recharge area to infiltration. Areas that are replenished at a high rate are generally more vulnerable to pollution than those replenished at a slower rate. Unconfined aquifers that do not have a cover of dense material are susceptible to contamination. Bedrock areas with large fractures are also susceptible by providing pathways for the contaminants. Confined, deep aquifers tend to be better protected with a dense layer of clay material. Wells that connect two aquifers increase the chance of cross contamination between the aquifers.

In addition to serving as a source of drinking ware, a well can act as a direct pathway from the land surface into the water supply. Thus a major consideration in ground water contamination is the position and condition of the well. Most of the contaminants that commonly cause concern originate above ground, often as the the result of human activities.

Soil overlying the water table provides the primary protection against ground water pollution. Bacteria, sediment and other insoluble forms of contamination become trapped within the soil pores. Some chemicals are absorbed or react chemically with various soil constituents, thereby preventing or slowing the migration of these pollutants into the ground water. In addition, plants and soil microorganisms use some potential pollutants, such as nitrogen, as nutrients for growth, thereby depleting the amount that reaches the ground water.

Just as any man-made filtering device can be overloaded, so can the natural filtering capacity of soil. Large amounts of potential pollutants concentrated in a small area can cause localized ground water contamination, depending on the depth and type of soil above the water table.

To help protect water wells against contamination, it is important to use the natural protection that soil provides by maintaining adequate separation distances between wells and potential sources of contamination.

After an aquifer has been contaminanted it is difficult to entirely define or isolate a contaminant plume. It is also difficult and extremely costly to remove it. Even after the source of contamination has been removed, an aquifer may remain contaminated for anywhere from a few years to a few centuries. Thus, it is often unrealistic to talk about a "cure" for ground water contamination. Prevention is the key, and prevention includes finding the major sources of contamination, and learning to control them.

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