9:15 AM - 9:30 AM
[MGI29-02] Assessment of formation of groundwater hydrochemistry by using multivariate analysis: a study on fossil seawater in Japanese Neogene sedimentary rocks
Keywords:Principle Component Analysis (PCA), Hierarchical Clustering Analysis (HCA), Neogene sedimentary rocks, Fossil seawater, Burial diagenesis
At repository site selection for radioactive waste disposal, it is important to assess and model groundwater chemistry. Multivariate analysis of groundwater has already been used (Sasamoto et al., 1999), and it enables to identify the end-members, calculate their ideal mixing and the deviations from ideal mixing model (Laaksoharju et al., 1999). In order to model groundwater chemistry, geochemical reactions relating to the deviations have to be interpreted. In this study, groundwater and whole-rock chemistry were investigated by multivariate analyses, and geochemical characteristics relating to water-rock interaction were assessed.
The samples are groundwater in Wakkanai and Koetoi formations occurring in Horonobe area, northern Hokkaido, Japan. The Wakkanai and Koetoi formations are Neogene sedimentary rocks composed of siliceous (diatomaceous) mudstone with SiO2 contents of about 65~80 wt%. X-ray diffraction analyses indicate that the Wakkanai and Koetoi formations consist mainly of opal-A and/or -CT plus trace amounts of smectite, illite, kaolinite, chlorite, pyrite and siderite. Hydrochemistry of saline groundwater (called as the fossil seawater) in the Wakkanai and Koetoi formations would be formed mainly during the diagenesis (Miyakawa et al., 2023). The fossil seawater has a heavy δD-δ18O characteristics and indicates NaCl-type composition with salinity of about 1/3~1/2 that of seawater.
A dataset for multivariate analysis was compiled from geochemical data of groundwater and rock cores obtained by JAEA in the Horonobe Underground Research Laboratory Project (Kunimaru et al., 2007; Sasamoto et al., 2011; Hiraga and Ishii, 2008). Principal Component Analysis (PCA) and Hierarchical Clustering Analysis (HCA) were performed. The normalized concentrations of Na2O, K2O, CaO, MgO, FeO, Fe2O3, and LOI (loss of ignition) versus Al2O3 were used as variables in PCA of whole-rock chemistry. The concentrations of Na+, K+, Ca2+, Mg2+, Cl-, HCO3-, and SO42- were used in PCA of groundwater hydrochemistry. HCA was performed on the PCA results using the principal components as variables until the cumulative contribution exceeds 90%. The Ward's method was used for clustering.
Figures 1a and 1b show the results of PCA and HCA for the major constituents of groundwater, respectively. The siliceous mudstone obtained from the Horonobe area can be classified into two groups based on LOI/Al2O3 and K2O/Al2O3. Clusters with high LOI/Al2O3 and K2O/Al2O3 correspond to the Koetoi Formation (“Kt”), and those with low LOI/Al2O3 correspond to the Wakkanai Formation. The Wakkanai formation is further divided into two types according to CaO/Al2O3 and Na2O/Al2O3.
Assuming that the volatile component of the samples is mainly H2O, LOI/Al2O3 can be considered as an indicator of the amount of H2O released during the phase transition of silica minerals and/or smectite. The average values of LOI/Al2O3 were 1.62 for Kt, 0.96 for Wk-1, and 0.85 for Wk-2. Assuming that all H2O contributed to groundwater dilution, the dilution factor could be estimated as 0.59 from Kt to Wk-1, and 0.52 from Kt to Wk-2. These values are consistent with Cl- concentration.
Assuming simple dilution of fossil seawater of Kt, mass balance calculation suggested that Na+ and Ca2+ leached from Wk-1 while only Na+ leached from Wk-2. Figure 2 shows quaternary plots of the groundwater Na-K-Ca-Mg and the whole-rock Na2O-K2O-CaO-MgO systems. The composition of groundwater gradually enriched in Na+ according to burial depth. The whole-rock chemistry showed complex changes. Comparing Kt and Wk-1, Na2O/Al2O3 decreased. On the other hand, comparing Wk-1 and Wk-2, Na2O/Al2O3 rather increased and the ratio of K2O/CaO decreased due to a K2O/Al2O3 loss and a CaO/Al2O3 gain. It is generally known that exchangeable Na and Ca decrease and non-exchangeable K increases with the phase transition from smectite to illite (e.g., Inoue, 1986). The changes in the whole-rock chemistry would be relating to the illitization, but additional analyses such as exchangeable cationic composition of the mudstone and mineral chemistry of smectite and illite are required for further quantitative discussion.
The samples are groundwater in Wakkanai and Koetoi formations occurring in Horonobe area, northern Hokkaido, Japan. The Wakkanai and Koetoi formations are Neogene sedimentary rocks composed of siliceous (diatomaceous) mudstone with SiO2 contents of about 65~80 wt%. X-ray diffraction analyses indicate that the Wakkanai and Koetoi formations consist mainly of opal-A and/or -CT plus trace amounts of smectite, illite, kaolinite, chlorite, pyrite and siderite. Hydrochemistry of saline groundwater (called as the fossil seawater) in the Wakkanai and Koetoi formations would be formed mainly during the diagenesis (Miyakawa et al., 2023). The fossil seawater has a heavy δD-δ18O characteristics and indicates NaCl-type composition with salinity of about 1/3~1/2 that of seawater.
A dataset for multivariate analysis was compiled from geochemical data of groundwater and rock cores obtained by JAEA in the Horonobe Underground Research Laboratory Project (Kunimaru et al., 2007; Sasamoto et al., 2011; Hiraga and Ishii, 2008). Principal Component Analysis (PCA) and Hierarchical Clustering Analysis (HCA) were performed. The normalized concentrations of Na2O, K2O, CaO, MgO, FeO, Fe2O3, and LOI (loss of ignition) versus Al2O3 were used as variables in PCA of whole-rock chemistry. The concentrations of Na+, K+, Ca2+, Mg2+, Cl-, HCO3-, and SO42- were used in PCA of groundwater hydrochemistry. HCA was performed on the PCA results using the principal components as variables until the cumulative contribution exceeds 90%. The Ward's method was used for clustering.
Figures 1a and 1b show the results of PCA and HCA for the major constituents of groundwater, respectively. The siliceous mudstone obtained from the Horonobe area can be classified into two groups based on LOI/Al2O3 and K2O/Al2O3. Clusters with high LOI/Al2O3 and K2O/Al2O3 correspond to the Koetoi Formation (“Kt”), and those with low LOI/Al2O3 correspond to the Wakkanai Formation. The Wakkanai formation is further divided into two types according to CaO/Al2O3 and Na2O/Al2O3.
Assuming that the volatile component of the samples is mainly H2O, LOI/Al2O3 can be considered as an indicator of the amount of H2O released during the phase transition of silica minerals and/or smectite. The average values of LOI/Al2O3 were 1.62 for Kt, 0.96 for Wk-1, and 0.85 for Wk-2. Assuming that all H2O contributed to groundwater dilution, the dilution factor could be estimated as 0.59 from Kt to Wk-1, and 0.52 from Kt to Wk-2. These values are consistent with Cl- concentration.
Assuming simple dilution of fossil seawater of Kt, mass balance calculation suggested that Na+ and Ca2+ leached from Wk-1 while only Na+ leached from Wk-2. Figure 2 shows quaternary plots of the groundwater Na-K-Ca-Mg and the whole-rock Na2O-K2O-CaO-MgO systems. The composition of groundwater gradually enriched in Na+ according to burial depth. The whole-rock chemistry showed complex changes. Comparing Kt and Wk-1, Na2O/Al2O3 decreased. On the other hand, comparing Wk-1 and Wk-2, Na2O/Al2O3 rather increased and the ratio of K2O/CaO decreased due to a K2O/Al2O3 loss and a CaO/Al2O3 gain. It is generally known that exchangeable Na and Ca decrease and non-exchangeable K increases with the phase transition from smectite to illite (e.g., Inoue, 1986). The changes in the whole-rock chemistry would be relating to the illitization, but additional analyses such as exchangeable cationic composition of the mudstone and mineral chemistry of smectite and illite are required for further quantitative discussion.