Environmental risk assessment for aquifer disposal of carbon dioxide

Considerable discussion has lasted for over a decade in both scientific and industrial circles as to how anthropogenic emissions of carbon dioxide can be controlled to reduce greenhouse effect that contributes to global warming. Methods of underground-aquifer disposal of CO2 were suggested, with some practical application being reported. Examples include injection of high pressure CO2 solution into underground aquifer, and pumping of slurry, super critical CO2 fluid into depths of greater than 3000 m. However, environmental impacts and risks exist in a number of disposal sites over the world. Specific concerns include safety of transport and injection processes for CO2, leakage of liquid CO2, escaping of CO2 (from saturated aquifer), and chemical reaction between CO2 and other chemicals in aquifer. Consequently, the surrounding environment would be under threat. Examples of the impacts and risks include (a) lowering pH value of surrounding water zone due to dissolved CO2 disposal could introduce a risk of disturbing short- or long-term physical and biological processes in the aquifer; and (b) a variety of pathways exist for CO2 (and the associated contaminants) to return to the atmosphere and thus generate impacts on agriculture, forest, and public health. Therefore, decisions of detailed disposal activities should be based on effective analyses of these risks. By systematically analyzing factors related to CO2 disposal and the related environmental and economic concerns, criteria and objectives, this study introduces a risk assessment approach based on stochastic risk analysis (SRA). It is useful for comprehensively evaluating risks within a system containing many subsystems with complicated interrelationships. It can incorporate effects of different impact factors and prevention/protection techniques within a general framework. The modelling results have provided bases for determining desirable management actions. The SRA is applied to a hypothetical case study of aquifer disposal for CO2 generated by four large coal-fired power plants. The result indicates that reasonable solutions have been obtained. They provide useful decision support for identifying appropriate plans of not only CO2 disposal actions but also the related environmental management activities.