Thusly "NetMax" Spake Unto All:

Does anyone know the characteristics of the source of this smell? Does it
take long to leave the water, and is the water left harmless after?

Hydrogen sulphide is weakly toxic, and smells like, well, like
someone's farted. It's said to smell like rotten eggs, but to me it
smells like fart. Anyway, it's a completely different smell than the
sharp, putrid, smell of ammonia & mercaptans you get in aquaria which
are cycling, and there's no risk of confusing those two smells.

However are these sulphide gases the same
as what cause the sulphur smell in well water?

Probably, yes. Fortunately you can smell hydrogen sulphide at levels
much lower than they're lethal to fish, and water changes helps.

This link might be of interest to you:
http://www1.agric.gov.ab.ca/$department/deptdocs.nsf/all/agdex1160?opendocument

Incidentally, it seems crustaceans are far more sensitive to hydrogen
sulphide than fish.