The quality of your well water depends on the where the water recharging the well comes from, the geological mineral characteristics surrounding the water, and many other characteristics. A shallow aquifer just north of Denver, CO was fed from cattle ranches on the surface. Nitrates were exceedingly high. In the Front Range Mountains west of Denver, natural uranium deposits resulted in very high levels of radon, making the water unusable. Each long term use groundwater source of water is unique and should be tested. However, a quick drink now and then from mountain spring likely won't do you harm. I have had giardia and para-typhoid after being exposed to contaminated water. You really don't want to experience these diseases. They are no fun at all!