In an ideal world, we'd be able to stash most of our planet-warming carbon dioxide emissions in underground formations, where they would turn to stone. As High Country News has reported in the past, the carbon in C02 can be incorporated into calcium carbonate, or limestone, through chemical reactions. That's a good thing for climate change because calcium carbonate stores carbon for the long haul. Few climatologists lie awake at night worrying that limestone will bubble out of the ground and into the atmosphere.
But the cover article of last week's Nature (subscription required) suggests that Mother Nature sees long term carbon storage in a different light. A team of British and Canadian researchers have discovered that over the course of many thousands of years, subterranean C02 generally ends up mixing with water instead of stone. In oil and gas fields in New Mexico, Arizona and Colorado (as well as China and Europe), no more than 20 percent of naturally-occurring carbon dioxide has ended up in mineral formations. Spent hydrocarbon reserves are generally viewed as convenient places to store greenhouse gas, but if we inject our carbon waste back into them, not much of it will spend the ensuing centuries as limestone. Most will dissolve into any available groundwater deposit, carbonating it like a bottle of Crystal Geyser.
On the one hand, the study offers a measure of vindication to researchers who have already predicted that deep groundwater deposits can effectively store C02. On the other hand, the new findings mean that anyone planning to sequester carbon had better thoroughly understand the hydrology of a potential site. Barbara Sherwood Lollar, a University of Toronto professor who participated in the study, thinks it's unlikely that the fizzy water will contaminate water supplies. After all, the "formation water" in question would generally lie more than a thousand feet below the surface. But since our water supplies already get along so poorly with gas drilling, it's tough not to worry about the ramifications of infusing groundwater -- no matter how deeply buried -- with C02. Lollar does note that when C02 is sequestered in oil and gas fields -- where the topography has been punctured by drill sites -- leakage becomes more of a concern.
(On a side note, the HCN article referenced above discusses the sequestration potential of basalt, one of the minerals most conducive to calcium carbonate precipitation. None of the recent research described in Nature took place in basalt flows.)