We just drilled a new well and the water smells like rotten eggs. The smell is most likely hydrogen sulfide gas - a fairly common, mostly natural contaminant that occurs in groundwater. It is often associated with certain rock formations like black shales that can have a high sulfur content. It does not usually pose a health threat, but it can be a terrible nuisance. It also can be corrosive to metals.
Higher concentrations can cause a laxative affect and produce a bitter taste.
Hydrogen sulfide in a well can appear and disappear, but will probably need to be dealt with when its odor is present. Proper treatments can typically control the problem. We would recommend getting your well tested for hydrogen sulfide. The treatment method typically depends on the concentration of hydrogen sulfide. A new method for treating hydrogen sulfide uses catalytic carbon.