Which type of well is considered least likely to become contaminated?

Drilled wells are considered the least likely to become contaminated due to their construction techniques and the depth at which they are typically placed. These wells are created by using specialized drilling equipment that allows them to penetrate deep into the ground, often reaching below the water table where the geological layers provide natural filtration and protection against surface contaminants. The deeper location reduces the risk of contamination from surface runoff, human activities, and pollution sources typically found at shallower depths.

In contrast, dug wells, bored wells, and shallow wells are more vulnerable to contamination because they are usually less deep and are often constructed in a way that does not provide the same level of protection from surface pollutants. Dug and bored wells are typically wider and may not have proper sealing or casing, which increases their exposure to potential contaminants. Shallow wells are especially at risk because they draw water from areas that can be more easily impacted by pollution or human activities close to the ground surface. Therefore, the construction and depth of drilled wells contribute significantly to their reduced likelihood of contamination compared to the other types of wells.