Carbon sequestration and groundwater - thoughts on a recent Duke study
If geologic formations are to be used to keep carbon dioxide (CO2) out of the atmosphere, it is essential that we know what the potential leakage pathways are, and establish effective rules to ensure that injected CO2 stays permanently trapped. It is also essential to know if any harm could result if CO2 were to migrate from the formations where it is injected into drinking water aquifers.
Carbon capture and storage (CCS) researchers have pointed out since the beginning of the study of the topic that if CO2 were to leak from injection formations into groundwater supplies, chemical reactions could result in the release of harmful contaminants, potentially rendering the groundwater unsafe for human use. A number of studies have demonstrated the potential for such contamination – in simple terms – by soaking specimens of typical groundwater rock in CO2 solutions.
For this reason, scientists and policymakers studying CCS have called for rigorous requirements for licensing any future CO2 injection operations. These requirements include criteria for site selection aimed at avoiding sites where significant pathways might exist that could not be eliminated, and monitoring, verification and reporting requirements for injection operations to ensure that injection projects will not result in risks to any groundwater resources. With those requirements in place, extensive study, as well as practical experience with projects, suggests very strongly that no CO2 leakage should be expected.
The most important aspect of this defense in depth approach is to limit injection projects to deep formations, much deeper than any groundwater resources and to assure that secure barriers of impermeable rock layers are located between the CO2 injection zone and any nearby groundwater resources. This approach is often not well understood, with the result sometimes being that routine studies providing more information on the chemical contamination that CO2 could produce if it leaked and came into contact with groundwater are treated as news by the popular media and generate alarm in the blogosphere.
The recent coverage in Climatewire and the New York TImes regarding one such study by Duke University are good examples of how coverage of these studies can lead to mistaken impressions about what scientists know about the risks of CO2 injection and what policymakers are doing to assure that such risks are avoided when and if future CCS projects go forward.
The Duke study essentially confirmed what has appeared a number of times in published scientific literature for almost a decade: that CO2 in groundwater could mobilize potentially dangerous trace elements and constituents (see, for example, here, and here). This is not a new finding. The study is a simple leaching experiment that identifies which elements may be of concern. It confirms previous results and is an example of research that needs to be done to address the potential impacts of groundwater quality if CO2 leaks from a storage reservoir into a drinking water aquifer.
However, the study is not the complete picture. For example, it does not identify key geochemical reactions that control trace element groundwater chemistry or the rates of these reactions. Without this information, it is not possible to deduce how fast these elements might be released, groundwater quality as a function of time, the spatial extent of those releases, and whether groundwater flow or other factors would render the increased concentrations at hazardous levels temporary or localized. Further work is needed in these areas, and some experiments currently under way are expected to yield valuable results.
It is worth noting that in excess of 35 million tons per year of CO2 are being injected today in the U.S. for enhancing production in oil fields. The regulatory framework for larger scale injection projects to isolate CO2 captured from power plants and other large sources will be much more rigorous than current EOR regulatory practice. The operating permits for sequestration projects under the Class VI Underground Injection Control rule that EPA is preparing to promulgate any day now are specifically designed to prevent any such migration out of the containment zone. This does not mean that leaks are impossible. But the risks are manageable and proper enforcement – which we should strive for – will prevent leaks.
The flip side of the coin from the study is that the documented changes in geochemistry can be used to monitor for leaks. Monitoring for these geochemical signatures and increased concentrations of trace elements or constituents can provide a proactive tool for the early detection of leaks and timely intervention. Current results suggest that these monitoring methods are sensitive and effective (see here).
Finally, the study seems to suggest that areas with aquifers that have a high potential of contamination in the event of a leak should not be selected as sequestration sites. This might be prudent in some cases, but a decision like this should be made on the basis of a full risk assessment that considers all parameters for the site. For example, a site with multiple seals that would make an excellent trap for CO2 but is overlaid by such an aquifer might yet be preferable to a site with poor sealing qualities but more benign geochemistry.
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