A rotten egg smell in water is one of the few problems a homeowner can often spot with their nose before any test. The trouble is that the same smell can come from three different places, and each one points to a different fix. Before you price out any treatment, work out which one you have.
Check hot and cold separately
Run the cold tap in the kitchen for a minute, then sniff a clean glass of water from it. Do the same with the hot tap. If both carry the smell, the problem is most likely in the water arriving at the house, which is the usual pattern with a private well. If only the hot water smells, look first at the water heater, because the metal parts inside a heater can react with the water and produce a similar odor. A smell limited to the hot side is a different job from a whole-house water problem, and the company you call should know which one you are describing.
Rule out the drains
Sewer gas also smells like rotten eggs, and it has nothing to do with your water supply. Smell the drain itself with the tap off, then compare that to the glass of water. If the smell is strongest at a floor drain, a sink or shower that is rarely used, or any drain after a long stretch without use, a dry trap or a venting problem is a likely cause. Running water down that drain for a few minutes can refill a dry trap, which tells you the source was the drain and not the water. Drain problems belong to a plumber, not a water treatment company.
Look for other evidence in the house
A problem in the water supply often leaves other marks. Look for dark or black specks in sinks and tub drains, black staining in the toilet tank, and tarnish on silverware or copper fixtures that was not there before. Note whether the smell comes and goes, and whether it shows up after heavy rain, after a power outage, or after the well pump or water heater was serviced. Write down each occurrence and what was happening at the time, since notes will be far more useful than memory when you talk to anyone about it.
What to ask before you agree to any fix
Ask whether the company's testing measures hydrogen sulfide directly and how the sample is collected. A smell judged by nose or taste is not a measurement, so do not accept an opinion in place of a result. Ask whether the test covers the cold and hot water separately. Ask what they would check on the water heater before recommending a treatment unit on the main line, because a problem that starts in the heater will not be fixed by equipment placed ahead of it. Ask which treatment method they propose for the level found, and get the expected result and the steps to take if the smell remains, in writing.
When to call a lab or the health department
If you have a private well, the most useful first step is usually a laboratory test rather than a treatment quote. A certified lab can measure the sulfide level and check for bacteria, and your local health department can tell you which labs are certified in your area. Do not treat the water as safe to drink because it passes a smell test. If the odor is strong, or anyone in the house has a question about whether the water is safe to use, ask a doctor or the local health office rather than relying on your nose. If your water comes from a city or town utility, a sulfur smell is less common, so call the utility, ask whether it has changed anything about the supply, and ask what it would check first.