Abstract
Three hydrochemical types of CO2-rich water (i.e., Na-HCO3, Ca-Na-HCO3 and Ca-HCO3) occur together in the silicate bedrock (granite and gneiss) of Gangwon Province in South Korea. As a natural analogue of geological carbon storage (GCS), this can provide implications for the environmental impacts of the leakage of CO2 from deep GCS sites. By using hydrochemical and isotopic datasets that were collected for previous and current studies, this study aimed to carefully scrutinize the hydrochemical differences in the three water types with an emphasis on providing a better understanding of the impacts of long-term CO2 leakage on groundwater quality (especially the enrichments of minor and trace metals). As a result, the Na-HCO3 type CO2-rich water contained higher Li, Rb and Cs than the Ca-HCO3 type, whereas Fe, Mn and Sr were higher in the Ca-HCO3 type than in the Na-HCO3 type despite the similar geological setting, which indicate that the hydrochemical differences were caused during different geochemical evolutionary processes. The δ18O and δD values and tritium concentrations indicated that the Na-HCO3 type was circulated through a deep and long pathway for a relatively long residence time in the subsurface, while the Ca-HCO3 type was strongly influenced by mixing with recently recharged water. These results were supported by the results of principal component analysis (PCA), whose second component showed that the Na-HCO3 type had a significant relation with alkali metals such as Li, Rb and Cs as well as Na and K and also had a strong relationship with Al, F and U, indicating an extensive water-rock interaction, while the Ca-HCO3 type was highly correlated with Ca, Mg, Sr, Fe and Mn, indicating mixing and reverse cation exchange during its ascent with hydrogeochemical evolution. In particular, the concentrations of Fe, Mn, U and Al in the CO2-rich water, the result of long-term water-rock interaction and cation exchange that was enhanced by CO2 leakage into silicate bedrock, exceeded drinking water standards. The study results show that the leakage of CO2 gas and CO2-rich fluid into aquifers and the subsequent hydrogeochemical processes can degrade groundwater quality by mobilizing trace elements in rocks and consequently may pose a health risk.
Highlights
Carbon dioxide stored in geological carbon storage (GCS) sites can migrate upwards from a storage reservoir through various paths such as faults, fractures, small cracks in caprocks, andWater 2020, 12, 1457; doi:10.3390/w12051457 www.mdpi.com/journal/waterWater 2020, 12, 1457 borehole annulus [1,2,3,4,5]
Even though GCS is a promising technology for substantially reducing CO2 emissions [6,7], such migration of CO2 gas and CO2 -rich fluid into freshwater aquifers may lead to the degradation of potable groundwater by total dissolved solids and trace metals, and the leakage itself means the failure of net CO2 reduction [8,9,10,11]
To evaluate the potential impact of CO2 gas and CO2 −rich fluid leaked from GCS sites on groundwater quality, we investigated the hydrochemical and isotopic characteristics of naturally occurring CO2 −rich water and compared them with those of the adjacent shallow groundwater and surface water
Summary
Carbon dioxide stored in geological carbon storage (GCS) sites can migrate upwards from a storage reservoir through various paths such as faults, fractures, small cracks in caprocks, andWater 2020, 12, 1457; doi:10.3390/w12051457 www.mdpi.com/journal/waterWater 2020, 12, 1457 borehole annulus [1,2,3,4,5]. To find a geochemical index for CO2 leakage detection and to evaluate the impact of CO2 leakage on groundwater quality, the hydrochemical responses to the inflow of CO2 into aquifers have been studied in various ways such as in laboratory experiments [1,10,18,19], controlled CO2 injection field tests [9,10,11,12,20,21], and natural analogues [8,15,22,23,24]. A natural analogue study with CO2 -rich water is the best way to observe the hydrochemical changes caused by a long period of CO2 supply [8,12]
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