Where does the calcium come from?

Calcium in water is in the form of Ca(OH)2 and CaSO4, in addition to Ca2 and CaOH , so at least the first one can release water if reacted with carbonic acid.

At the moment, pilot studies for Carbon Capture and Storage (CCS) systems propose the removal of CO2 by pumping it into holes deep underground. However, this is a costly and difficult process and carries with it a long term risk of the gas leaking back out - possibly many miles away from the original downward source.

If I was in the 'carbon sequestering business' that's what I'd do: Pump it down, get paid by the cubic meter I 'sequester' and engineer a leak somewhere where it can escape undetected. Voila: infinite storage space (you can always 'blame' your increased storage space on carbon dissolving into rocks or groundwater or make up some PR babble about larger than expected compression ability...or just lie about your intial storage capacity. No one is going to check)

PhysOrg seems to remove plus-signs from comments for some reason. Ca2 means Ca2plus and CaOH means CaOHplus