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Great looking, smelling, and tasting water is no guarantee that you have safe water!
Many contaminants, lead, mercury, E. coli, disinfection byproducts - in fact the majority of the harmful contaminants listed below - have no taste or smell, nor would they be visible in harmful quantities.
water that looks, tastes, or smells bad may be a sign of unsafe water, but often that is because they serve as indicators that the normal purification process has failed and dumped the undetectable contaminants into your water along with the detectable indicators.
Unless you have read a water quality report, had water quality tests done, or talked with neighbors who have the same water source and know the safety of the water, you can not really know how safe your water is.
Water companies are required to test the treated water constantly - that is because conditions, both in the water source and in the treatment plant, are continually changing.
It is important to test private wells, but there are two limitations to be aware of:
1) You can only test for a limited set of contaminants - others could be present that you do not test for. You need to figure out the contaminants that are most liable to be in your well so that the tests will be of most value. The Wisconsin Department of Natural Resources Water testing for private well owners site provides some guidelines for well testing. In the US, you could also contact the local EPA, the USGS, local health department, or other local well owners for information about water quality in your area.
2) You are looking at a "snapshot" of your water at a moment in time. Confined aquifers tend to be more stable since water usually moves through them more slowly than through unconfined aquifers, but all ground water contaminant levels can change over time.
Other resources for well owners:
EPA document Private drinking water wells. University of Nevada Cooperative Extension Drinking Water Testing for Private Well Owners