EVALUATION OF GROUNDWATER QUALITY VARIABILITY IN QUATERNARY DEPOSITS DUE TO SURFACE WATER INFLUENCES NEAR THE ILLINOIS BASIN – DECATUR PROJECT, DECATUR, IL, USA

An extensive Monitoring, Verification, and Accounting (MVA) program has been implemented by the Midwest Geological Sequestration Consortium (MGSC) for the Illinois - Basin Decatur Project (IBDP), a large-scale, carbon capture and storage project in Decatur, Illinois. Beginning in October 2008, 17 shallow groundwater monitoring wells ranging from 15-300 feet below land surface were installed to monitor groundwater levels and quality quarterly. This study focuses on one of the shallower wells, 09S, which is 24 feet deep and 200 feet away from Lake Decatur. In well 09S, total inorganic carbon (TIC) and alkalinity concentrations are generally correlated to pH and δ¹³C_DIC. Redox processes (such as microbial respiration and oxidation of reduced iron sulfide minerals) typically control pH, which affects the δ¹³C_DIC, and the types and concentrations of inorganic carbon species present in groundwater. For adequate environmental monitoring of CCS projects, it is important to be able to distinguish natural variation from potential CO₂ impacts from a project.

The chemical and isotopic variability of groundwater from well 09S is interpreted as being the result of a combination of redox processes and lake-water influence. A significant drought from June 2011 through January 2013 likely caused lake water to travel through organic-rich lake sediments (expected to be rich in reduced sulfur and iron species) and infiltrate adjacent shallow aquifers due to changes in local hydraulic gradients. Significant concentration changes for iron (1.3 to 12.4 mg/L) and sulfate (226 to 598 mg/L) have been observed and would support the hypothesis that those constituents were mobilized. Alkalinity, TIC, and δ¹³C_DIC values were correlated to pH values similar to other monitoring wells, however the low δ¹³C_DIC range of -14.6‰ to -16.4‰ (compared to a site-wide average > -10‰) indicates microbial respiration is likely the source of inorganic carbon.