Are you running a fountain in the barrel? I would suggest using some
carbon. The water can't be too bad if you have little critters (assuming
they're not mosquitoes, which can survive in some pretty nasty water). Or
you could just get a liner for it and not have to figure this out. They
make drop in liners for 1/2 barrels. ~ jan

The sulphurous smell is anaerobic bacteria breaking down protein on
the bottom of the barrel.

This person should circulate the water in some way. Any kind of small
pump would be fine. Pump from the very bottom of the barrel to
somewhere near the top. The water at the bottom of the barrel is
lowest in oxygen, and this is where the anaerobic bacteria are
thriving.

You can go the decorative route, with some kind of bamboo spout,
like this:

http://www.cherryblossomgardens.com/abamboo.asp

Voila, smell and stagnancy gone, and the barrel looks cool too.

C//