There are numerous ways well water can get contaminated. If you live near an industrial or agricultural site, effluents and runoff from them can get into your well contaminating your water supply.
Many people store or mix pesticides, insecticides, fuels, degreasers, near their private wells. This could prove to be a costly mistake. If flooding occurs, harmful chemicals can seep into the ground and eventually end up in your water.
Pollution can also occur from wastes and effluents from on-site sanitation systems, wastewater treatment plants and leaking sewers.