Consequently, contamination in shallow wells is a site-specific rather than a regional problem. Local land use, chemical use, well characteristics, and surface geology, such as the type of soil in the area, determine contamination levels. Dug wells are more likely than bored wells to be contaminated. Dug wells often are not properly covered at the land surface. They also tend to be older and in poor repair.
Nitrate contamination is of greatest concern.
Virtually every well sampled had detectable levels of nitrate. Levels frequently were several times higher than the acceptable limit of 10 milligrams of nitrogen per liter of water. Fourteen pesticides were detected in the well-water samples. Atrazine was the most commonly found chemical, followed by dieldrin and chlordane. Other chemicals detected were acifluorfen, alachlor, butylate, cyanazine, diazinon, dinoseb, ethalfluralin, metolachlor, metribuzin, simazine, and trifluralin.