Thanks melbo. There are added salts in soda water, but as far as I am aware sparkling mineral water has no extra additives except the carbon dioxide.
Jenna has a pretty good gag reflex, so it doesn't have that effect on her. And it doesn't seem to make her burp anymore than normal. Usually its flat after the first half day anyway.
And she certainly thinks she is having a treat. We dont flabour it with anything, she just prefers it plain. Sometimes she wants water, and sometimes "beer", so its very strange. She is just a wierd little girl.