I have well and septic on 1/2 acre. One month ago I had a new gas hot water heater and well tank installed. My water has always been hard with a slight iron odor but, 2 weeks ago the water started to smell very foul. The odor is sort of like methane or natural gas. The odor seems to be only in the HOT water not the COLD. Could there be any relationship between the new hot water heater and the odor?
It is likely that your new water heater has a magnesium anode. If so that is the likely cause. You can either remove it or replace it with an aluminum anode to get rid of that odor. Replacing or removing must be done with care. You don’t want to leave a piece of the magnesium rod in the tank. You should also chlorinate the tank with 2 cups of household unscented bleach. Leave it in the unit for 2-4 hours and then flush it. This job is best done, either way, by a plumber.
If the odor continues after that then you will likely need some water treatment. If it does it is likely caused by Sulfate Reducing Bacteria. The solution for that is to either occasionally shock treat the well with bleach or buy a continuous chlorination system.