Carbon capture must also include an approach for sequestering the captured CO2. Current approaches to carbon sequestration rely on storage of CO2 in underground geological formations, usually either deep saline formations or depleted petroleum reservoirs. Direct injection of liquid CO2 into oceans has also been proposed, but this approach has not been proven at even large pilot scale . Geological sequestration has been demonstrated to work in enhanced oil recovery applications and in a handful of large-scale pilot tests. Other approaches, such as ocean capture of CO2 by natural processes, are often considered to be geoengineering approaches and are more fully discussed in Chap. 9.
Wilson et al.  identified several environmental risks associated with such geological CO2 sequestration approaches and placed these risks in taxonomy according to the processes involved and whether the risks were local or global. A further study identified research needs and policy approaches to mitigate these risks . In summary, those risks were identified as:
• Suffocation of humans or animals above ground
• Effects on plants above ground
• Biological impacts below ground on roots, insects, and burrowing animals
• Contamination of potable water directly, through increased mobilization of metals or other contaminants, or by displaced brines
• Interference with deep-subsurface ecosystems
• Induced seismicity
The Department of Energy's carbon sequestration roadmap also pointed out the need to ensure protection of groundwater resources and minimize CO2 leakage . The roadmap identified other components of a sequestration program that will have environmental impacts, including construction of CO2 pipelines and the need for drilling, perhaps in areas that are not now subject to deep well drilling for oil or gas. The DOE roadmap and the regional partnerships that are developing pilot sequestration projects are also evaluating monitoring, mitigation, and verification (MM&V) activities and approaches that will be required to minimize the environmental, legal, and financial risks associated with geological sequestration.
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