EPA is currently developing rules to regulate geologic CO2 sequestration within the Underground Injection Control (UIC) program in Class VI wells (Code of Federal Regulations: Underground Injection Control Regulations 40 CFR 144-148). The UIC program regulates injection of all fluids in the subsurface, including liquids and gases. The existing UIC program provides a regulatory framework (baseline) for geologic sequestration of CO2. The Geologic Sequestration rule addresses potential endangerment to underground sources of drinking water (USDWs) from CO2 injection activities. The proposed rule uses Safe Drinking Water Act (SDWA) authorities and revises the UIC program for geologic sequestration (Class VI wells). Priority is placed on preventing endangerment of USDWs. Special considerations for geologic sequestration include large volumes, buoyancy, viscosity (mobility), and corrosivity. UIC program elements include site characterization, area of review, well construction, well testing and operation, site monitoring, well plugging and postinjection site care, financial responsibility, and public participation. The rule requires identification of all artificial penetrations into the proposed CO2 reservoir that could allow upward fluid migration and plug or remediate as necessary. Surface casing is required through the lowermost USDW and cemented to the surface. Well materials must be chosen to be compatible with the injectate and formation fluids. The rule requires continuous monitoring of injection rate, pressure, and volume and nature of injected fluid. Plume and pressure fronts must be tracked. Surface-air and soil-gas monitoring are at the director's discretion. Postinjection site care is set at a minimum of 50 yr unless the operator demonstrates that the geologic storage is no longer an endangerment to USDWs.
Special focus is given to sole-source aquifers. Impacts of CO2 leakage on USDWs discussed in the proposed rules rely on the modeling analyses discussed previously and emphasize mineral solubilization as sourcing trace elements, such as As, Pb, and organic compounds. Preventing USDW degradation should not be limited to contaminants with established MCLs but should also include other constituents that could render the groundwater more difficult to treat, such as Si. Si impacts reverse osmosis systems because it interferes with adsorption processes in drinking-water treatment systems. Boron is also a problem because it can readily pass through membranes.
The possibility exists that utilities might want to use some of these CO2 injection site formations as new potable water sources. Waters that were previously considered unusable in many communities throughout the U.S., owing to salinity that exceeded 10,000 mg/L TDS, are now being used as drinking-water sources. With the use of desalination technology, water is treated to EPA's drinking-water standards and provided to water-utility customers. As desalination technology improves, even more saline water may be used in the future.