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Status of Geochemical Modeling of Groundwater Evolution at the Tono In-Situ Tests Site, Japan

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Status of Geochemical Modeling of Groundwater Evolution at the Tono In-Situ Tests Site, Japan JNC TN8400 99-074 JP0055254 Status of Geochemical Modeling of Groundwater Evolution at the Tono In-situ Tests Site, Japan December 1999 33003059 Tokai Works Japan Nuclear Cycle Development Institute \ 3 1 / 4 8 319-1194 lii'l Inquiries about copyright and reproduction should be addressed to: Technical Information Section, Administration Division, Tokai Works, Japan Nuclear Cycle Development Institute 4-33 Muramatsu, Tokai-mura, Naka-gun, Ibaraki-ken, 319-1194 Japan PLEASE BE AWARE THAT ALL OF THE MISSING PAGES IN THIS DOCUMENT WERE ORIGINALLY BLANK JNC TN8400 99-074 December, 1999 Status of Geochemical Modeling of Groundwater Evolution at the Tono In-situ Tests Site, Japan Hiroshi Sasamoto", Mikazu Yui°, Randolph C Arthur2* Abstract Hydrochemical investigation of Tertiary sedimentary rocks at JNC's Tono in-situ tests site indicate the groundwaters are: • meteoric in origin, • chemically reducing at depths greater than a few tens of meters in the sedimentary rock, • relatively old [carbon-14 ages of groundwaters collected from the lower part of the sedimentary sequence range from 13,000 to 15,000 years BP (before present)] • Ca-Na-HCO3 type solutions near the surface, changing to Na-HCO3 type groundwaters with increasing depth. The chemical evolution of the groundwaters is modeled assuming local equilibrium for selected mineral-fluid reactions, taking into account the rainwater origin of these solutions. Results suggest it is possible to interpret approximately the "real" groundwater chemistry {i.e., pH, Eh, total dissolved concentrations of Si, Na, Ca, K, Al, carbonate and sulfate) if the following assumptions are adopted: • CO2 concentration in the gas phase contacting pore solutions in the overlying soil zone = 10'1 bar, • minerals in the rock zone that control the solubility of respective elements in the groundwater include; chalcedony (Si), albite (Na), kaolinite (Al), calcite (Ca and carbonate), muscovite (K) and pyrite (Eh and sulfate). It is noted, however, that the available field data may not be sufficient to adequately constrain parameters in the groundwater evolution model. In particular, more detailed information characterizing certain site properties {e.g., the actual mineralogy of "plagioclase", "clay" and "zeolite") are needed to improve the model. Alternative conceptual models of key reactions may also be necessary. For this reason, a model that accounts for ion-exchange reactions among clay minerals, and which is based on the results of laboratory experiments, has also been evaluated in the present study. Further improvements of model considering ion-exchange reactions are needed in future, however. 1) Japan Nuclear Cycle Development Institute, Tokai Works, Japan 2) Monitor Scientific, L.L.C., Denver, Colorado USA (formerly QuantiSci Inc.) JNC TN8400 99-074 » M#=ft=ftl "", Randolph C Arthur2' fcS. 13,000 ^~ 15,000 Ca-Na-HCO3 Na-HCO3 pH, Eh iJ (Si, Na, Ca, K, Al, fltBliS <£ : iogpc02=-i.o h (AI f: (K«Jg) , , Eh) £) It, -^tLX, t\z 2) Monitor Scientific, L.L.C., Denver, Colorado USA JNC TN8400 99-074 Table of Contents 1. BACKGROUND 1 2. GEOCHEMICAL MODELS OF GROUNDWATER EVOLUTION 2 2.1 INTRODUCTION 2 2.2 GEOLOGICAL SETTING 3 2.3 GROUNDWATER CHEMISTRY 3 2.3.1 Origin and Age of Groundwater 5 2.3.2 Variations in Groundwater Chemistry with Increasing Depth 5 2.3.3 State of Groundwater Equilibrium in the Tono Sedimentary Rocks 5 2.3.3.1 Stability Diagrams 5 2.3.3.2 Saturation Indices of Minerals 8 2.3.3.3 Saturation Indices for Reactions Involving Two Aluminosilicates 14 2.3.4 Redox Conditions of the Groundwater in the Sedimentary Rocks 20 2.4 GEOCHEMICAL MODELING OF WATER-ROCK INTERACTION 22 2.5 GROUNDWATER EVOLUTION MODELING 22 2.5.1 Reference Chemistry of Groundwater in the Tono Sedimentary Rocks 23 2.5.2 Conceptual Groundwater Evolution Model in the Tono Sedimentary Rock .....23 2.5.2.1 Single Point Model 26 2.5.2.2 Reaction Path Model 35 2.5 SUMMARY ...40 3. DISCUSSION ON GROUNDWATER EVOLUTION MODELS 42 3.1 EXECUTIVE SUMMARY OF DISCUSSION 42 3.2 DISCUSSION POINTS 43 3.3 DISCUSSION 44 3.3.1 Model Constraints 44 3.3.1.1 Groundwater Chemistry 44 • 3.3.1.2 Mineralogy 45 3.3.1.3 Open- vs. Closed-System Behavior 46 3.3.1.4 Consistency of Model Results and Field Data 46 3.5 RECOMMENDATIONS 50 3.5.1 Conceptual Model 50 3.5.2 Model Constraints: Groundwater Chemistry 50 3.5.3 Model Constraints: Mineralogy 50 in JNC TN8400 99-074 3.5.4 Model Constraints with Field Data: Groundwater Chemistry 50 3.5.5 Model Constraints with Field Data: Mineralogy 51 4. CONCLUSIONS 52 5. ACKNOWLEDGEMENTS 53 6. REFERENCES 54 IV JNC TN8400 99-074 List of Tables Table 1: Comparison of model results with measured values of groundwater chemistry (KNA- 6 borehole) in sedimentary rocks at theTono site 30 Table 2 : Calculated compositions of Tono groundwaters for a range of assumed porosities and log PC02 values 32 List of Figures Figure 1 : Location and geology of the Tono area, where Tertiary sedimentary rocks overlie Cretaceous granitic rocks 4 Figrue 2 : Vertical variation of groundwater chemistry in sedimentary rocks at the Tono area (from Iwatsuki et al., 1995). The lower permeable layer divides the groundwater system into two aquifers 6 Figure 3 : Piper plot of the compositions of surface waters and groundwaters in sedimentary rocks at the Tono site 7 Figure 4-1 : Stability diagrams for the Nap-AljC^-SiOj-HjO system (upper figure) and CaO- Al2O3-SiO2-H2O system (lower figure) at 18*0, 1 bar. Surface waters and groundwaters at the Tono site are represented by green triangles (surface waters), red squares (Oidawara formation), red circles (Akeyo formation) and blue diamonds (Toki lignite-bearing formation) 9 Figure 4-2 : Stability diagrams for the Nap-CaO-AljOj-SiOj-HjO system (upper figure) and Kfl- Al2O3-SiO2-HjO system (lower figure) at 18*0, 1 bar. Surface waters and groundwaters at the Tono site are represented by green triangles (surface waters), red squares (Oidawara formation), red circles (Akeyo formation) and blue diamonds (Toki lignite-bearing formation). The average 3 05 SiO2(aq) activity of groundwaters at the Tono site, 10" ' , is used to construct the upper figure. 10 Figure 5-1 : Saturation indices as a function of pH of hydrolysis reactions involving albite, analcime, Ca-beidellite and Na-beidellite. Estimated uncertainties in the saturation index (0± 0.5) are indicated by red bands 11 Figure 5-2 : Saturation indices as a function of pH of reactions involving calcite, amorphous ferric hydroxide [Fe(OH)3ppd], chalcedony and quartz. Estimated uncertainties in the saturation index (0+0.5) are indicated by red bands 12 Figure 5-3 : Saturation indices as a function of pH of reactions involving laumontite, maximum microcline, and "high" and "low" Fe-Mg smectite. Estimated uncertainties in the saturation index (0±0.5) are indicated by red bands 13 JNC TN8400 99-074 Figure 6-1 : Frequency distribution of saturation indices for the albite-kaolinite equilibrium reaction. Values within the estimated uncertainty in the saturation index (0+0.5) are indicated by red bars 15 Figure 6-2 : Frequency distribution of saturation indices for the Na-clinoptilolite-kaolinite equilibrium reaction. Values within the estimated uncertainty in the saturation index (0±0.5) are indicated by red bars 15 Figure 6-3 : Frequency distribution of saturation indices for the Na-beidellite-kaolinite equilibrium reaction. Values within the estimated uncertainty in the saturation index (0±0.5) are indicated by red bars 16 Figure 6-4 : Frequency distribution of saturation indices for the anorthite-kaolinite equilibrium reaction. Values of the saturation index (0+0.5) are estimated as uncertainty. 16 Figure 6-5 : Frequency distribution of saturation indices for the Ca-beidellite-kaolinite equilibrium reaction. Values within the estimated uncertainty in the saturation index (0±0.5) are indicated by red bars 17 Figure 6-6 : Frequency distribution of saturation indices for the heulandite-kaolinite equilibrium reaction. Values within the estimated uncertainty in the saturation index (0+.0.5) are indicated by red bars 17 Figure 6-7 : Frequency distribution of saturation indices for the muscovite-kaolinite equilibrium reaction. Values within the estimated uncertainty in the saturation index (0+0.5) are indicated by red bars 18 Figure 6-8 : Frequency distribution of saturation indices for the "maximum microcline"-kaolinite equilibrium reaction. Values of the saturation index (0+.0.5) are estimated as uncertainty 18 Figure 6-9 : Frequency distribution of saturation indices for the K-clinoptilolite-kaolinite equilibrium reaction. Values within the estimated uncertainty in the saturation index (0+0.5) are indicated by red bars 19 Figure 6-10 : Frequency distribution of saturation indices for the illite-kaolinite equilibrium reaction. Values within the estimated uncertainty in the saturation index (0+0.5) are indicated by red bars 19 Figure 7 : Schematic view of the Tono area. A cross section of the Tono mine is shown in the upper figure. The lower figure shows a view of the groundwater monitoring system at the KNA-6 borehole (Iwatsuki andYoshida, 1999) 21 Figure 8 : Ranges in groundwater compositions and pH in sedimentary rocks at the Tono site based on analyses reported by Yoshida et al. (1994) 24 Figure 9: Conceptual model of groundwater evolution in sedimentary rocks at the Tono site. 25 VI JNC TN8400 99-074 Figure 10 : Comparison of groundwater compositions predicted using the groundwater evolution model (blue circles) with ranges in pH and cation and anion concentrations
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