The union of several profiles' unmeasured ions, in WATER_IONS order.
It's the union and not the intersection because if one water in a blend never had its sulfate measured, the blend's sulfate isn't known either.
The union of several profiles' unmeasured ions, in WATER_IONS order.
It's the union and not the intersection because if one water in a blend never had its sulfate measured, the blend's sulfate isn't known either.