Some of the sparking waters I've seen have surprisingly high sodium levels. My wife, who is on a low-sodium diet, has to avoid the ones with a lot of sodium.
Depends on what brand you were buying. If it was from a natural mineral source, the sodium is likely present from the source. I don't know what constitutes as a lot of sodium for your wife, but they are normally not huge, anywhere from 3mg/L (Eska), 12mg/L (Perrier), 32mg/L (Pellegrino) to 118mg/L (Grolsteiner) – none will put you over the recommended intake of sodium, and are considered healthy sources of minerals for people with normal diets.