GEOLOGICAL CARBON SEQUESTRATION AND ENHANCED OIL RECOVERY: POSSIBLE IMPACT ON A SHALLOW FRESHWATER AQUIFER NEAR A PROPOSED SEQUESTRATION SITE, SOUTHERN SAN JOAQUIN VALLEY, CALIFORNIA

The Kern Water Bank (KWB) is a water storage facility on the Kern River alluvial fan located in the southern San Joaquin Basin of California. This freshwater aquifer is at risk of having its groundwater quality impacted from nearby proposed carbon sequestration/enhanced oil recovery targets. Twelve KWB aquifer samples from two wellswere used to quantify changes in aqueous geochemistry and aquifer mineralogy resulting from interactions with a 100% CO₂ atmosphere and groundwater. Samples belong to the Upper Tulare Formation and have been identified to contain elevated concentrations of arsenic. Sediments were combined with deionized water and placed in a glove box where they were exposed to a saturated CO₂ environment at near surface conditions for 31 days. Mineralogical analysis using SEM and XRD focused on changes in aquifer mineralogy and texture. Aqueous geochemistry included pH, alkalinity titrations, ion chromatography for major anions, and ICP-MS for major cations and trace elements.

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.