The Oil Drum

This is the original BBC report from 1986:

The eventual number of people who died in the Lake Nyos gas leak was put at more than 1,700.

Scientists debated the cause of the disaster for some time afterwards.

It was finally concluded that the lake's lower levels had become saturated by carbon dioxide gas (CO2) due to gaseous springs which bubbled up from the extinct volcano beneath.

It is thought that recent high rainfall had displaced the CO2-rich water at the bottom, releasing a massive bubble of carbon dioxide gas from the lake in a natural phenomenon now referred to as "lake overturn".

The heavy gas then sank to the ground and rolled in a cloud several tens of metres deep across the surrounding countryside.

Pipes have now been put in place in Lake Nyos and nearby Lake Monoun to siphon water from the lower layers up to the surface and allow the CO2 at the bottom of the lake to slowly bubble out, preventing a repeat of 1986 tragedy.

Following the Nyos tragedy, a survey was carried out into the CO2 content of other African lakes.

It revealed that Lake Kivu, in Rwanda, is becoming saturated with carbon dioxide just as Lake Nyos was, and is seriously at risk of lake overturn.

Scientists have warned that if nothing is done, millions of people living around Lake Kivu are in danger.

http://news.bbc.co.uk/onthisday/hi/dates/stories/august/21/newsid_338000...

This shows how dangerous CO2 storage can become. If this storage were ever done onshore, permanent monitoring ad infinitum would have to be done similar to nuclear dumps. I cannot imagine any EIS would be accepted by a local population once the dangers are known.
NASA climatologist Hansen therefore proposes storage in deep ocean sediments. This means that only coal fired power plants near the coast could have CCS if those suitable sediments were found.