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Tracers are used to tag the injected carbon dioxide (CO2) for verification of the storage performance and containment in geosequestration. Yet the characterization of the transport behavior of a chemical tracer in two‐phase flow system has not been fully investigated. We present experimental observations together with numerical results for both homogeneous and heterogeneous media from core scale to field scale. The key features of the breakthrough curves (BTCs) that include the tracer arrival time, peak concentration, and tailing pattern were examined. We identify distinct differences in BTCs depending on whether the formation was previously swept by CO2. When the tracer is released before CO2 sweeps the formation, the tracer transports at the front of gas‐liquid interfaces with a narrow tracer plume distribution and the BTCs reflect the effect of local heterogeneity. Furthermore, a series of tracer pulse can be used to identify whether new pathways have been developed during CO2 injection. Plain Language Summary: Geologic storage of carbon dioxide (CO2) has become an attractive climate change mitigation option, and ensuring the containment of injected CO2 is of prime importance for successful geological storage. Tracers are an effective means of tracking injected CO2 and verifying containment within the storage formation. Here we show that distinctive features in observed tracer data are highly related to whether or not the storage formation was previously swept by CO2. When the tracer is released at the beginning of CO2 injection, the tracer moves with narrow distribution and is advantageous for the tracer data to reflect multiple pathways. By contrast, once the CO2 sweeps the formation, the tracer moves much faster with a wider dispersion pattern. Key Points: We present experimental observations with numerical results for characterizing the tracer transport behavior in two‐phase flow systemThe key features of breakthrough curves and tracer velocity highly depend on whether the formation was previously swept by CO2A series of tracer pulse can be used to identify whether new pathways have been developed during CO2 injection [ABSTRACT FROM AUTHOR]

Abstract: Geochemical monitoring was performed on dilute CO2-rich water as a natural analogue study of CO2 geologic storage. Field measurement and sampling for dissolved ions analyses and isotope analysis were carried out from July of 2009 to November of 2010. Spring and groundwater samples were classified into diluted CO2-rich water (average pH=4.6, EC=151 S/cm), concentrated CO2-rich water (average pH=6.1, EC=1,301), and ordinary groundwater (average pH=6.6, EC=201). pH, EC and dissolved major ions were not changed noticeably during monitoring period which indicates that reservoir of CO2-rich water was not affected by changes of surface condition and recharged groundwater. pH and EC of diluted CO2-rich water from this study area is lowest value among the CO2-rich waters from South Korea. On the other hand, concentrated CO2-rich water was also observed in 5 km north of diluted CO2 - rich water of Daepyeong area. Carbon isotope ratio (δ13 C) of concentrated CO2-rich water is -7.4% and average δ13 C of diluted CO2 rich water is -7.2% which indicates that the CO2 source of concentrated and diluted CO2-rich water could be closely related. The result of simple mixing model revealed that low pH and high TDIC (Total Dissolved Inorganic Carbon) concentration of diluted CO2-rich water was not explained by simple mixing between groundwater and concentrated CO2-rich water. Therefore, inflow of excess CO2 and/or secondary accumulated CO2 should be applied CO2-rich water system in Daepyeong area and reacted with host rock. A batch kinetic model was applied for simulating the ion concentration, pH and TDIC of diluted CO2-rich water after reacting with groundwater, rock and high CO2 condition (50.2 mmol/L). As a result of batch kinetic model, Na+ and K+ concentration of DPw-4 sample was reached in about 20 years. Conclusively, low EC, dissolved ions and high TDIC contents of dilute CO2-rich water indicate that water-rock interaction has been took place in short period of time after input of CO2. The result of this study implies that shallow groundwater quality except pH could not change significantly in the early stage of leakage of CO2. The water-rock reaction rate might not be fast enough in surface environment. [Copyright &y& Elsevier]

High fluoride concentrations (median=4.4 mg/L) in deep bedrock groundwater of South Korea prevent the usage of it as a drinking water source. The hydrogeochemistry of deep thermal groundwaters (N =377) in diverse bedrocks has been studied in order to evaluate the geologic and geochemical controls on fluoride concentrations in groundwater. The groundwater samples were clustered geologically, and the average and median concentrations of fluoride were compared by the Mann–Whitney U test. The order of median fluoride concentration with respect to geology is as follows: metamorphic rocks≥granitoids≥complex rock≫volcanic rocks≥sedimentary rocks. This result indicates that the geological source of fluoride in groundwater is related to the mineral composition of metamorphic rocks and granitoids. With respect to groundwater chemistry, the fluoride concentration was highest in Na-HCO3 type groundwater and lowest in Ca-HCO3 type groundwater. Ionic relationships also imply that the geochemical behavior of fluoride in groundwater is related to the geochemical process releasing Na and removing Ca ions. The thermodynamic relationship between the activities of Ca and F indicates that fluoride concentration is controlled by the equilibrium of fluorite (CaF2). In other words, the upper limits of fluoride concentration are determined by the Ca ion; i.e., Ca concentrations play a crucial role in fluoride behavior in deep thermal groundwater. The result of this study suggests that the high fluoride in groundwater originates from geological sources and fluoride can be removed by fluorite precipitation when high Ca concentration is maintained. This provides a basis for a proper management plan to develop the deep thermal groundwater and for treatment of high fluoride groundwater frequently found in South Korea. [Copyright &y& Elsevier]

The ionic and isotopic compositions (δD, δ18O, and 3H) of urban groundwaters have been monitored in Seoul to examine the water quality in relation to land-use. High tritium contents (6.1–12.0 TU) and the absence of spatial/seasonal change of O–H isotope data indicate that groundwaters are well mixed within aquifers with recently recharged waters of high contamination susceptibility. Statistical analyses show a spatial variation of major ions in relation to land-use type. The major ion concentrations tend to increase with anthropogenic contamination, due to the local pollutants recharge. The TDS concentration appears to be a useful contamination indicator, as it generally increases by the order of forested green zone (average 151 mg/l), agricultural area, residential area, traffic area, and industrialized area (average 585 mg/l). With the increased anthropogenic contamination, the groundwater chemistry changes from a Ca–HCO3 type toward a Ca–Cl(+NO3) type. The source and behavior of major ions are discussed and the hydrochemical backgrounds are proposed as the basis of a groundwater management plan. [ABSTRACT FROM AUTHOR]

Microorganisms influence the chemical and physical properties of subsurface environments and thus represent an important control on the fate and environmental impact of CO 2 that leaks into aquifers from deep storage reservoirs. How leakage will influence microbial populations over long time scales is largely unknown. This study uses natural analog sites to investigate the long-term impact of CO 2 leakage from underground storage sites on subsurface biogeochemistry. We considered two sites with elevated CO 2 levels (sample groups I and II) and one control site with low CO 2 content (group III). Samples from sites with elevated CO 2 had pH ranging from 6.2 to 4.5 and samples from the low-CO 2 control group had pH ranging from 7.3 to 6.2. Solute concentrations were relatively low for samples from the control group and group I but high for samples from group II, reflecting varying degrees of water-rock interaction. Microbial communities were analyzed through clone library and MiSeq sequencing. Each 16S rRNA analysis identified various bacteria, methane-producing archaea, and ammonia-oxidizing archaea. Both bacterial and archaeal diversities were low in groundwater with high CO 2 content and community compositions between the groups were also clearly different. In group II samples, sequences classified in groups capable of methanogenesis, metal reduction, and nitrate reduction had higher relative abundance in samples with relative high methane, iron, and manganese concentrations and low nitrate levels. Sequences close to Comamonadaceae were abundant in group I, while the taxa related to methanogens, Nitrospirae , and Anaerolineaceae were predominant in group II. Our findings provide insight into subsurface biogeochemical reactions that influence the carbon budget of the system including carbon fixation, carbon trapping, and CO 2 conversion to methane. The results also suggest that monitoring groundwater microbial community can be a potential tool for tracking CO 2 leakage from geologic storage sites.