GEOLOGICAL CARBON SEQUESTRATION AND ENHANCED OIL RECOVERY: POSSIBLE IMPACT ON A SHALLOW FRESHWATER AQUIFER NEAR A PROPOSED SEQUESTRATION SITE, SOUTHERN SAN JOAQUIN VALLEY, CALIFORNIA
After CO2 exposure, pH dropped from ~8 to 6 and increased to a new equilibrium just below pH secondary drinking water standard of 6.5. Alkalinity increased indicating dissolution of primary minerals. Two cation behaviors in response to CO2 were identified: I (increase) and II (decrease). Ca, K, Sr, Fe, Ba, Se, Mn, and U displayed significant mobilization. As and V had the greatest decline in aqueous concentration. Undesirable concentrations of U (145 ppb), Se (1027 ppb), Fe (926 ppb), and Mn (660 ppb) were observed and are well above maximum contamination levels (MCL) for drinking water. CO2 exposure had a positive impact on arsenic as concentrations for most samples fell below the MCL. Anions had three distinct behaviors: I (increase), II (decrease), and III (stable), with no potential hazards for groundwater quality. SEM suggested evidence of experiment related alteration of pyrite into iron oxides, dissolution of calcite, feldspars, and other accessory minerals, and the formation of clays and oxides from primary mineral assemblage. XRD showed that changes in weight percent of bulk and clay mineralogy fell within a margin of error of 2%. This suggested that CO2 had little to no effect on mineral abundance. A suggested geochemical indicator for CO2 intrusion is pH because it drops immediately upon exposure to high levels of CO2.