Yes, it is, most homebrewed ciders and seltzers have a eggy/sulphur smell - this is a common occurrence with commercial producers as well.
This is down to the yeast producing volatile sulphur compounds, and unfortunately not much can be done about that.
However, the seltzer itself should turn out fine and hopefully the smell will go within a few days. If the smell persists at the end of fermentation it may help to stir to agitate and aid release of the sulphur-containing gases trapped in the liquid before bottling.
Leaving the seltzer to stand an extra week or so will help by allowing the volatile sulphur compounds to dissipate to some degree, however there are a few ways we suggest to speed up the process;
Stir the seltzer with some clean, shiny, sanitised copper (such as copper tubing, copper wine etc – important that it is bright and shiny, not dull and oxidised otherwise it may not be effective).
‘Splash rack’ the seltzer by passing it from one vessel to another 2-3 times to force the volatile sulphur compounds out of solution.
Stir the seltzer vigorously for several minutes to degas (as with ‘splash racking’ this will force the residual carbon dioxide gas out of solution, taking the volatile sulphur compounds with it).