re: Well water smells terrible

Hydrogen sulfide is a very chemically reactive gas and can be smelled in a concentration in the low parts per billion (ppb) and is present in anaerobic (no oxygen) conditions which is the norm in wells.

In fish hatcheries or other similar applications it’s usually removed, fairly easily, by vigorous aeration of the source water, usually in a retention pond, which blows the gas off to the atmosphere before it is used. But you’d obviously need a different approach for a home water system.