Sooner or later, bacterial growth will occur if organic material is present in the well. The presence of coliform bacteria may be due to a structural defect that allows contamination to enter the well. A good drilled and cased well normally has a 6 inch casing which extends down at least 100 feet. The casing extends above the ground surface 1 foot. The top of the casing has a cap which is called a sanitary seal.
Some wells are just hand dug down to the water table.
Hand dug wells usually have larger openings and are difficult to protect. Any openings in the well structure that could allow entry of surface water, rodents, or insects into the well should be corrected before trying to eliminate the bacteria. Openings should be covered to provide a water tight seal. Sometimes a cover which overlaps a rim can be made to provide a shoe box-type fit. Any vent should be directed downward and screened off.