EVALUATING THE EFFECTS OF CO2 INTRUSION ONTRACE METAL MOBILITY IN FRESHWATER AQUIFERS

The risk of CO₂ leakage from properly characterized and permitted storage sites is expected to be very low. CO₂ leakage, however, could potentially decrease the pH of potable groundwater, causing trace metals to be mobilized through several pathways that include ion exchange, desorption from surface sites, and mineral dissolution. This study uses a combination of lab experiments, synchrotron analyses and reactive transport modeling to determine the dominant geochemical processes that could occur in shallow aquifers when dissolved CO₂ is introduced at formation pressures.

Preliminary geochemical characterization was done on sediments collected from three different depths of a coastal plain aquifer in Mississippi. Sediment pH buffering capacity was estimated using titrations conducted with 0.01N hydrochloric acid. The observed pH buffering behavior could be fully accounted for by surface protonation; mineral dissolution was ignored since no carbonates were identified in the sediments. Total and inorganic carbon was measured using an NDIR-based TOC analyzer. A significant amount of organic carbon (2.4%) was found in one layer, while no detectable carbon was present at the other two depths. An acid digestion analysis showed that the organic layer also contained elevated metal concentrations compared to the other sediments.

The aquifer material and the organic layer were analyzed using micro X-ray fluorescence spectroscopy. Preliminary analysis showed that the aquifer material contained large quartz grains coated with iron-bearing clays or oxides. In contrast, the organic layer contained large amounts of amorphous material consisting of organic thiol groups associated with alkaline earth elements such as calcium and strontium. Arsenic and lead were found to be associated with discrete pyrite grains in both sediments, though the aquifer material had much smaller amounts of pyrite than the organic layer. This is consistent with observations from SEM images and optical microscopy indicating the presence of pyrite in the organic layer. Based on these results, the amorphous organic material and the pyrite grains could be important sources for CO₂-driven release of metals. This will be tested in sediment leaching experiments using groundwater saturated with CO₂ at aquifer pressures.