Well water for the aquarium

If your drinking water comes from a well, whether private or public, then it's coming out of an aquifer, rather than as surface water from a reservoir. A shallow well may be partly recharging with percolation and runoff from precipitation. You might get a better general idea of your own water, if it ultimately derives from groundwater, by checking the incredibly detailed online version of the U.S. Geological Survey's groundwater atlas of the U.S. with regional maps and full descriptions of the geology affecting your drinking water.
 
A good recent test of your well water is a basic tool in understanding what's going on in your aquarium. The Environmental Protection Agency does not regulate private wells that supply water to fewer than 25 people or to fewer than 15 service connections. If you are on private well water, the E.P.A. recommends that you should get your water tested annually — even if you don't keep fish!