It is of utmost importance that we can assess the potential impacts of CO2 leakage from a Carbon Capture & Storage (CCS) system on the marine ecosystem, even though leakage is understood to be unlikely. Existing work has used laboratory experiments and observations of natural CO2 seepage sites in the marine environment. Whilst these are both informative laboratory mock ups can never fully replicate the complexity of the real world, whilst at natural seep sites we cannot easily quantify effects because no pre-seepage (baseline) measurements exist.
Carbon dioxide (CO2) controlled release
To increase our knowledge of the impact of increased CO2 in the marine environment we have conducted a unique controlled CO2 release beneath a Scottish sea loch. A borehole was drilled from land underneath the bed of the loch and CO2 released directly into the sediments 12 metres under the seabed.
This approach ensures that the overlying sediments remain undisturbed, which is important when mimicking a real sub-sea bed leak of CO2. The chosen depth is optimal from a biological and biogeochemical point of view as it is below the zone where sea floor communities live and the important biochemical processes occur. Before drilling could go ahead, we had to conduct mapping surveys to find out more about the sea bed topography, the underlying bedrock and geotechnical properties of the sea bed sediments at a number of potential sites. These properties include porosity (how much free space there is within the sediment), permeability (the flow of liquid through the sediment) and sediment strength.
How and when did we do this?
The controlled CO2 release took place in sandy mud sediments that are typical of UK shelf seas. The release started in May 2012 and took place over 36 days followed by a 90 day monitoring of recovery. Carefully timed measurements of the physical characteristics, biology and biogeochemistry of the sediment and overlying water column took place before, during and after the 36-day release and at specified distances from the point of release. Measurements were also made at a control site nearby which was not affected by the introduced CO2. This allows us to rule out any effects caused by natural changes due to the weather or seasons.
What have we measured?
We measured a comprehensive suite of physical, chemical and biological properties, including:
Geochemical and physical properties of the sediments and waters contained within the sediment will be analysed for changes in pH, CO2 content, oxygen and porosity.
Video and photographic surveys (both pre and post surveys).
Continuous logging of sediments and water below the sea bed.
Digital hydrophones will be deployed to monitor any bubble release from the sediment.
Water sampling to measure flow and dispersion of CO2.
Activity of microbial, microscopic and larger animals within the sediment and on the sea bed.
Before conducting the controlled release, it is crucial that we made sure that there would be no long term damage to the local environment. We worked with the appropriate regulatory bodies to make sure that the release is acceptable and supported by them, including: the Scottish Government, which has regulatory control of Scottish inshore waters; Marine Scotland; The Crown Estate, who own the sea floor; and the Scottish Environment Protection Agency (SEPA). We also consulted with users of the marine system in the vicinity of our release site to ensure that we did not interfere with local activities.