Insects, rodents or animals entering the well are other sources of contamination. These wells usually have large openings and casings that often are not well-sealed. This makes it easy for insects, rodents, or animals to enter the well. Another way bacteria can enter a water supply is through inundation or infiltration by flood waters or by surface runoff. Flood waters commonly contain high levels of bacteria.
Small depressions filled with flood water provide an excellent breeding ground for bacteria.
Whenever a well is inundated by flood waters or surface runoff, bacterial contamination is likely. Any system with casings or caps that are not water-tight are vulnerable. This is particularly true if the well is located so surface runoff might be able to enter the well. During the last five to 10 years, well and water distribution system construction has improved to the point where bacterial contamination is rare in newer wells. Indications of Bacteria Bacterial contamination cannot be detected by sight, smell or taste.