CHARACTERIZING NATURAL VARIABILITY OF GROUNDWATER CHEMISTRY: IMPLICATIONS FOR DETECTION OF CO2 LEAKS FROM SUBSURFACE STORAGE SITES

Industrial development and activity over the past 100 years has resulted in a significant increase in atmospheric CO₂ concentrations. Injection of supercritical CO₂ into subsurface reservoirs, termed geologic carbon sequestration and storage, is emerging as a feasible method for reducing CO₂ emissions. Potential storage sites include deep sedimentary basins, depleted oil reservoirs, or coal beds. While geologic storage represents a viable option for reducing carbon dioxide emissions, the possibility of a CO₂ leak from deep storage sites into shallow aquifers is a concern. Leakage of can lead to acidification of ground water and subsequent geochemical reactions that have the potential to release trace metals as well as major elements such as Ca, K, Mg, and Na and degrade water quality. Early detection of CO₂ leaks from subsurface storage sites will be key in mitigating any potential adverse affects associated with increased concentrations of CO₂ in shallow drinking water aquifers.

In order to detect changes in aqueous chemistry driven by CO₂ leaks, it is important to understand the natural variability of groundwater chemical parameters. Changes in ground water chemistry as a result of acidification only will be detectable if the variations resulting from CO₂ leaks exceed natural variability of the groundwater chemistry. Comparing model results of CO₂–driven changes in water chemistry to natural variability will allow us to determine the likelihood of early detection depending on the size and extent of CO₂leaks from subsurface repositories.

To analyze natural variability, we analyzed the concentrations and ratios of major elements as well as trace metals from an existing dataset collected by the Ohio EPA to determine the natural variability of groundwater chemistry in wells from unconsolidated sediments, sandstone, and limestone wells. A key focus of our analysis was within-well variability across wells that have at least 15 years of data. Relationships between concentrations of major elements as well as trace metals were evaluated for significant correlations and should provide insight on the natural variability of major elements as well as trace metals. Reactive transport models will be used to quantify the size of CO₂ fluxes needed to produce changes in groundwater chemistry that exceed natural variability.