Solubility could replace the aquifer tag as well. Any soluble layer could in theory hold an aquifer, the higher it's solubility the faster it pours water out. Additionally, it could also mark certain inorganic materials as being unfit for liquid storage without adding any additional tags.
Might you mean "porosity"?
Solubility would be a different[1] quality and give a different purpose, especially in the formation of caverns[2] and the like, with flows of water (current
or historic) removing an amount of soluble rock (at a rate determined by the degrees that the rock has this quality) until saturated with the mineral. (Also consider deposition of this solute, with stalactites and stalagmites and joined-up pairs in columns.)
Porousness (don't we already have a 'drainage' value of some kind?[3]) would allow water to escape at a given rate through a rock layer (perhaps down
to a soluble layer, there to carve away disconnected voids) from the surface or any other place where water is present (away from a limestone cave?[4]), and of course allow it to bubble upwards from deep-water heated reservoirs. Both water and the rock being unchanged during transit (barring any soluble components, intrusions or other neighbouring strata encountered along the way). To bring us back to the OP, somewhat.
[1] Potentially independent, so that you could have high or low solubility rocks having (prior to being dissolved, in the latter case) high or low porousness.
[2] The current caverns don't seem to have any geological 'guidance' applied to their formation, but a brute-force overlay across whatever strata there just happens to be in their defined areas of incidence. Which is not to say that traditionally eroded/silt-scoured caverns could not also exist, as well as man(/dwarf)-assisted caverns, perhaps following veins of now long-extracted minerals. Worldgen (and embark-tile loading, on demand) could be rather complicated if trying to emulate/store/procedurally-recreate all the myriad possible processes, though...
[3] Though I've not done enough comparable worldgens/site-findings to work out exactly how this affects a site. Perhaps it encourages dry sandy deserts over clay-floored plains, all other qualities else being equal or left unconsidered.
[4] ...and I must check to see if limestone-rich waters intersticially deposit minerals into subsequent porous rocks, because that'd be an interesting 'metamorphosis' of a rock-type. Been ages since I did any practical geology, though.