EFFECTS OF ELEVATED CH4 AND CO2 ON THE MOBILITY OF METALS IN NEAR-SURFACE SEDIMENTS

Carbon capture, utilization and storage (CCUS) aims to reduce CO₂ emissions by capturing CO₂ from sources and injecting it into geologic reservoirs for enhanced hydrocarbon recovery and storage. One concern is unintentional CO₂ release to the surface may occur through seepage pathways such as fractures and/or improperly plugged wells.

We hypothesize that CO₂ migration into the vadose zone and subsequent Eh and pH changes could mobilize metals, potentially contaminating ground and surface waters. This potential has not been addressed elsewhere. Goals of this study are to understand if metal mobilization through soil pore water may occur due to CO₂ and CH₄ and to assess potential impact to aquifers and/or the biosphere. The study was conducted at a CCUS site in Cranfield, MS, where localized seepage of CH₄ (45%) from depth reaches the surface and oxidizes to CO₂ (34%) in the vadose zone near a plugged well. Four sediment cores (4.5-9m long) were collected in a transect extending from a background site through the anomalous soil gas. Samples were analyzed for Eh and pH in the field and for occluded gas concentrations, metal concentrations, moisture content, and carbon content (inorganic and organic) in the lab. Data from the background area (occluded gas ~21% O₂, <1% CO₂, 0% CH₄) showed oxidized conditions (Eh from 464-508mV) and neutral pH (7.0-7.8) compared to samples collected near the gas anomaly (13-21% O₂, 0.1-5% CO₂, <0.1% CH₄) that were more reducing (Eh 133-566mV) and of lower pH (5.3-8.0). Strong correlations were found between Eh and O₂ (r=0.95), pH and CO₂ (r=-0.88), and between these parameters and acid-leached metals in samples from within the soil gas anomaly. Correlations quickly weaken away from the anomaly. While total metal and water-mobile metal concentrations are relatively consistent between cores, the percent of acid-mobile metals increases in the gas anomaly. Preliminary conclusions are: 1) Oxidation of CH₄ to CO₂ depletes O₂ causing reducing conditions; 2) high CO₂ and low O₂ alter Eh and pH of sediments affecting metal mobility; 3) geochemical alterations are localized to the extent of gases. Additional work will use modeling to predict potential outcomes of altered sediment geochemistry observed in the field on aquifers – specifically to assess the potential for metals to exceed the MCLs set by the EPA.