Compress the helium into tanks and use atmosphere as ballast. Ballast can be dropped by directly purging with helium. Alternatively, contain the ballast in bla

Compress the helium into tanks and use atmosphere as ballast. Ballast can be dropped by directly purging with helium. Alternatively, contain the ballast in bladders that vent as helium pressure rises. Designing flexible bladders that don't leak too badly would be an engineering problem. A terribly inefficient option would be to allow the gasses to mix and just use fractionation to separate the heavy elements when dropping ballast.