Contaminants in the accumulated surface water can either leak down along the well casing or penetrate through cracks in the well casing. The top of the well casing cannot, by law, terminate in the basement of any building or in a pit, room, or other space which is below ground level. At least 30 feet from sewer-lines. At least 10 feet from basements or pits. At least 20 feet from overhead powerlines.
Greater distances are always preferable and often necessary, depending on soil conditions.
In looking at potential groundwater contamination, you need to consider casing height. The purpose of these standards is to ensure that surface water cannot run into the well through the top. If your well does not meet these standards, the risk of your drinking water being contaminated is higher. Wellheads located in well pits are more easily flooded during periods of excessive surface water or when the groundwater level rises above normal levels. Flooding of well pits increases the likelihood of well contamination by surface water and accompanying contaminants.