|MadSci Network: Earth Sciences|
It is difficult to answer your question in a very specific way because both tap water and well water vary a lot from place to place. (I am assuming that by "tap water" you mean water that comes from a public water supply system, because well water can also come out of the tap. Also, I am assuming that by well water you mean water from a private well.)
Public water supply water in the United States and other developed countries has been treated to eliminate bacteria and other materials, mostly organic molecules, that are strictly controlled in drinking water because they are toxic. These dangerous materials include pesticides and byproducts of industrial chemical processes. Naturally occurring dissolved salts and other trace materials that are not harmful may or may not be present. Of course some public water supply systems get their water from wells (in Alabama roughly half of all drinking water comes from wells or springs).
Water from private wells in some jurisdictions (for example, the state of Alabama) is not subject to the kind of strict governmental regulations that public water supply systems have to follow. most private wells are relatively shallow, which means they are vulnerable to contamination from the surface. This contamination can range from animals that fall into the well and drown, to pesticides or herbicides that were mixed or sprayed near the wells, to other kinds of chemicals that get into the wells from nearby rivers or industrial facilities, to lead from lead-based solder in the casing of the well. If a private well is protected from these various sources of contamination the water can be extremely clean. It may contain traces of naturally occurring minerals, some of which are beneficial. it may even be cleaner and healthier than public water supply water.
The well owner may not know how safe or dangerous the water is without testing it. The most commonly used test is for bacteria. This is not very expensive and bacteria are one of the most common risks. Tests for various chemicals can be expensive, but it is possible to find information about well water safety from state and federal agencies. These agencies may have detailed records on wells located near your well. In Alabama, such information is available from the water division of the state geological survey. In other states a separate agency may be responsible for monitoring ground-water quality. The US Geological Survey and US Environmental Protection Agency also are sources of information about ground-water safety.
David C. Kopaska-Merkel
Geological Survey of Alabama
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