As groundwater scientists we’ve known that groundwater can be surcharged with CO2, compared to atmospheric levels. We sometimes see bubbles escaping our samples. An interesting article on CO2 and groundwater quantifies just how much CO2 is released from groundwater depletion in the US and the world. It turns out to be a pretty impressive amount, greater than about 1/3 of the 23 major sources of CO2 reported by EPA, and has been entirely overlooked until this study. Note that as long as groundwater recharge balances discharge, the net effect on CO2 emissions is neutral. But when depletion of groundwater storage occurs, more CO2 is released than is kept in subsurface storage.
Full article is posted here: http://onlinelibrary.wiley.com/doi/10.1002/2017EF000586/full