Hi, can anyone offer any advise on our smelly cisterna which is huge and under the house, I have read a lot of threads on this, but pumping it out and cleaning it would be hard, it is practically impossible to go down into it, if  I were to put chlorine tabs in, how to stir it? if that is what you have to do, It has a huge amount of water but afraid to use it.

Chlorine tablets - the quick dispersing variety - might do the trick.  No need to stir as Brownian motion would distribute the chemical which as a gas would then evaporate naturally leaving you with just water again and hopefully killing off the cause of the smell. 

Has anything crawled in and died....sorry to put the frighteners on it but we have a covered rainwater butt, only for plant watering, and it was stinking.Found something floating in it with dead maggots floating on it.No amount of chlorine will clear that up.

Thanks Nobrot, but it is quite impossible to go down & check, I hope it is not as you say, I will try the chlorine tablets suggested by Michael and see where we go from there.

Such 'things' as described do tend to float so can you dangle a video camera and light in to have a look?

Is it not feaseable to drop a submersible pump in?

We would need to know more info really.

Can you get a feel for what the smell might be? Is it musty, bad eggs, rotting vegetation or worse? Is the water cloudy or clear? Does the water itself smell or just the cisterna?
Household bleach which is labelled as safe for drinking water is fast dispersing and very cheap. the stuff we get from Suma is safe but from Lidl is marked as not suitable. Check the bottle.
Pool shops sell an enzyme based product which, like biological washing power, eats up any organic matter. If it's safe for pools then it's safe for people if you follow the instructions.

Water is clear, but smelling of rotten eggs, Where is Suma.

http://www.cleanwaterstore.com/technical/well-water-odor.php?Screen=USL&Code=well-water-odor

Might have the answer you're looking for

Many thanks,