Sulphur Odor In Cistern Water

Could be anything. Best to get your cistern water tested ASAP and either drink bottled water or boil before use.
Could be something died in your cistern or there are leaves of certain trees that can acculumlate in the cistern from the gutters and produce foul smelling water. Can't remember the name of the tree.

When was the last time your cistern was cleaned or you looked in to see what was going on or added clorox to the water?
Good Luck resolving the matter.

Sounds like hydrogen sulphide.

Could be leaves composting in the bottom of your cistern.

You should first determine if the odor is coming from only the hot water. Typically I have found the sulfur odor is from the hot water heater, particularly if it has not been used for a week or two, and most likely from a reaction of the sacrificial anode rod with the cistern water. If this is the case, then you can either drain and refill the hot water heater a couple of times to eliminate the issue or run a hose from the hot water heater drain back to your cistern and flush it for a couple of hours (turn your heater off while doing so). If the odor is from your cold water also then the cistern is affected and you visually inspect it. However, while a dead animal in the cistern will definitely smell it will not be a sulfur odor. Also, if you have water filters they will concentrate any odors so you should check them and see if they need to be replaced.

The water testing labs here only check the turbidity (cloudiness) and PH of the water as well for the presence of coliform bacteria and e-coli bacteria, which will not normally be associated with an odor. And unless you chlorinate your cistern it will almost certainly test positive for bacteria. But the presence of bacteria does not mean it is of sufficient concentration to be harmful, or even a harmful type of bacteria. At any rate, you should chlorinate your cistern monthly by adding 4.5 to 6 ounces of regular unscented household bleach per 1000 gallons of water currently in your cistern (not how many gallons your cistern holds when full). This is a batch sterilization of your cistern and after a week or so there will not be any chlorine residue left, so bacteria can start living again in the cistern, which is why you need to do it on a regular basis.

There are many internet articles on the problem (I've used cistern water at numerous places in 30 years, have never experienced it but have always chlorinated my cisterns regularly as JE suggested) but you may find this one helpful:

http://thesurvivalpodcast.com/forum/index.php?topic=30046.0

In one house we took care of we noticed this smell every time under cabinet water filter in the kitchen was overdue for a change. Try the simple fix first. If you have filters in your water system open them up and if they stink change them.

Thank you everyone so much. That was incredibly helpful.

Also, you may want to check for dead critters on your roof.