(685d) Shallow Ground Water Monitoring at the Secarb CO2 Injection Site Near Citronelle, Alabama

The SECARB Anthropogenic Test is a fully integrated U.S. Department of Energy sponsored-research project located near Citronelle Alabama designed to demonstrate the safe, reliable capture, transportation and storage of CO₂. Starting in August 2012, the project team injected over 114,000 metric tonnes of CO₂ into the Paluxy Formation at a depth exceeding 2,865 m over a two-year period. Injection concluded in September 2014 and the project is currently in the post-injection site care period. Several monitoring techniques are being used to demonstrate post-injection containment of the CO₂ in the injection zone and non-endangerment of groundwater resources during the three-year post-injection monitoring effort. These techniques include time-lapse cross-well and vertical seismic profile surveys, geophysical logging, reservoir pressure and temperature measurements, soil CO₂ flux measurements, tracer surveys, and shallow groundwater sampling and analyses.

The ground water monitoring program consists of collecting water samples from three dedicated monitoring wells and one water supply well installed in the shallow surficial aquifers in close proximity to the projectâ??s deep CO₂ injection and observation wells. A risk assessment performed during the early stages of the project identified the deep wellbores, which penetrate the confining unit overlying the injection zone, as the only potential migration pathway. No known faults are present based on available geologic and seismic data. Although the risk of CO₂ leakage is expected to be extremely small, SECARB installed the shallow ground water monitoring wells on the injection well pads to check for a CO₂ release as part of the regulatory permitting program.

Water samples are collected from each monitoring well on a quarterly schedule. Samples are analyzed on-site for time-sensitive parameters including pH, electrical conductivity, dissolved oxygen, temperature and oxidation-reduction potential and preserved for off-site analysis for 17 metals, total inorganic carbon (TIC) and other inorganic parameters by a contract laboratory. The ground water analytical results are compared to U.S. Environmental Protection Agency (EPA) drinking water standards and to background samples collected prior to CO₂ injection using EPA-approved statistical methods.

The study results indicate that shallow ground water systems are dynamic and complex, resulting in water quality results for individual naturally-occurring constituents that can fluctuate considerably with time due to natural processes or sampling-induced mobilization of inorganics. This may lead to false detection and reporting of a CO₂ release over a 40 to 100-year life span of a commercial CO₂ storage project. Therefore, multiple constituents (e.g., pH, Fe, TIC and EC), or lines of evidence, should be collectively used in the analysis to detect a release and not just one or two (e.g., pH). The duration and location of background sampling should also be carefully considered in the initial design of the storage facility due to the ubiquitous occurrence of inorganics found naturally in ground water.