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Geological carbon sequestration: possible impact on a shallow freshwater aquifer near a proposed sequestration site, southern San Joaquin Valley, California
Carbon Capture and Sequestration (CCS) of carbon dioxide (CO2) from high emission point-source sites such as power plants is one strategy to reduce greenhouse gas emissions and mobilize residual hydrocarbon in reservoirs for enhanced oil recovery. This has led to much work on the effects of injected CO2 on brine geochemistry and mineralogy of proposed geologic storage formations under reservoir conditions specific to injection sites. However, another aspect that needs consideration is the unintentional migration of CO2 into shallow fresh groundwater aquifers through the conduits such as fractures and faulty deep well casings. This could have a profound impact on the quality of the groundwater by mobilization of heavy trace metals such as arsenic, lead, or uranium. The Kern Water Bank (KWB) is a water storage facility on the Kern River alluvial fan located in the southern San Joaquin Valley Basin of California. This freshwater aquifer is at possible risk of having its groundwater quality impacted from nearby proposed CCS targets. Twelve KWB aquifer samples from wells 23H1 and 24K1 were used to quantify changes in aqueous geochemistry and aquifer mineralogy resulting from the sediment interactions with a near 100% CO2 atmosphere and groundwater. Samples belong to the Upper Tulare Formation and have been identified to contain elevated concentrations of arsenic. Mobilizations of heavy trace metals due to CO2 exposure is of special interest in this study. Sediment samples were combined with a deionized water solution and placed in a precision controlled atmosphere glove box where they were exposed to and sampled in a saturated CO2 environment at near surface pressures and temperatures for 31 days. "Mineralogical analysis conducted before and after the experiment using a scanning electron" "microscope (SEM) and X-ray diffractometer (XRD) focused on changes in aquifer" "mineralogy and texture. Aqueous geochemistry included pH, alkalinity titrations, ion" "chromatography (IC) for major anions, and inductively coupled plasma mass spectrometry" "(ICP-MS) for major cations and trace elements. After CO2 exposure, pH of reactors dropped two units from ~8 to 6 and gradually increased to a new equilibrium just below the pH secondary drinking water standard of 6.5. Alkalinity increased throughout the experiments indicating enhanced aqueous carbonation. Two cation behaviors in response to CO2 were identified: cation I (increase) and cation II (decrease). Ca, K, Sr, Fe, Ba, Se, Mn, and U were the main elements that displayed a significant degree of increased mobilization. As and V had the greatest decline in aqueous concentration. Undesirable concentrations of iron (926 ppb), manganese (660 ppb), selenium (1027 ppb), and uranium (145 ppb) are reached during the CO2 phase and well above drinking water standards. CO2 exposure had a positive impact on water quality in relation to arsenic as most concentrations fell below maximum contamination levels (MCLs) for potable water. Anions had three distinct behaviors: I (increase), II (decrease), and III (stable) with no potential hazards on groundwater quality. Dissolved minerals and the presence of clay and oxide coatings in SEM analysis suggested evidence of experiment related reactions. XRD analysis showed that CO2 reactions in aquifer conditions have insignificant effects on bulk and clay mineralogy. A suggested geochemical indicator for CO2 intrusion is pH because it drops immediately upon exposure to high levels of CO2. Additional work is necessary to confidently characterized shallow CO2 intrusion into the KWB aquifer and associated hazards on groundwater quality.
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