Because harmful bacteria do not penetrate very far into the soil, wells are generally free of harmful organisms if:

• A. Less than 30 ft deep
• B. More than 50 ft deep
• C. Drilled into solid rock
• D. Filtered through sand

Answer: (B)

Wells that are more than 50 feet deep are generally considered to be safer from contamination by harmful bacteria because the depth allows for greater natural filtration and attenuation of pathogens. As water moves through the soil and rock layers, various physical and biological processes occur that effectively reduce or eliminate contaminants, including harmful bacteria. The layers of soil and rock provide a natural barrier, and deeper wells often tap into aquifers that are less exposed to surface contaminants.

The geology and hydrology of the local area play a significant role as well; deeper wells may draw from older and more isolated groundwater sources that have had longer residence times in the subsurface, further reducing the likelihood of contamination. In contrast, shallower wells might be more susceptible to surface runoff, agricultural practices, and other sources of contamination.

While other factors such as well construction and the geological composition of the area also contribute to the safety of water from a well, a well depth of over 50 feet is a recognized standard that typically indicates a lower risk of bacterial contamination.