Geochemical modeling of water-rock interactions in granitic rocks, eastern Taiwan
Yu-Te Chang
Material and Chemical Research Laboratories, Industrial Technology Research Institute
Address: 8F, -4, No.9, Sec. 2, Roosevelt Rd. Zhongzheng Dist., Taipei City 10093, Taiwan
Key words: granitic rocks, groundwater, reaction-path modeling

Abstract
Crystalline rocks such as granite have been considered as potential host rocks of radioactive waste repository. As
groundwater flows through fractured-rock aquifers, the water-rock interactions could induce dissolution andprecipitation of minerals and change the geochemical, structural, and mineral characteristics of the host rocks.
Therefore, study of possible water-rock reaction processes and mechanisms is crucial for suitability evaluation ofpotential host rocks. The granitic rocks in eastern Taiwan are mainly surrounded by marble, which could bepredicted that the groundwater quality (especially pH value) will mainly controlled by carbonate system. In thisstudy, the groundwater and rock compositions in eastern Taiwan were used for reaction-path modeling by 3
scenario cases (shallow open fracture, deep open fracture and deep closed fracture), which can be used tounderstand the conditions of rock composition and the surrounding water quality, and their chemical changes overtime, e.g., the variation of pH and Eh and the formation of secondary minerals. The simulation results show thatthe calcite and sericite are the major secondary minerals under the final equilibrium state. Besides calcite andsericite, the rest of the main secondary minerals will change gradually from andradite, saponite-Ca to dolomite,
nontronite-Na, and dawsonite when the fugacity of carbon dioxide increases. For the fractured-rock aquifers ineastern Taiwan granitic rocks, the simulated pH and Eh ranges (pH = 7.12 to 8.93 and Eh = -0.37 volts to -0.15volts) should be considered in the sensitivity analysis of the nuclide solubility and/or adsorption characteristics.
Compared to the similar simulation results in Taiwan’s granitic offshore island (pH = 6.99 to 9.75 and Eh = -0.45
volts to -0.17 volts), it shows that the groundwater characteristics (especially pH value) of granitic rocks in eastern
Taiwan are obviously affected by carbonate system and with less pH fluctuations.

1. Introduction
Crystalline rocks such as granite have been
considered as suitable host rocks of radioactive waste
repository. As groundwater flows through fracturedrock aquifers, the water-rock interactions could induce
dissolution and precipitation of minerals and changethegeochemical,structural,andmineralcharacteristics of the host rocks. Therefore, study ofpossible water-rock reaction processes andmechanisms is crucial for suitability evaluation ofpotential disposal host rocks.
In Taiwan, granitic rocks are mainly distributed ineastern Taiwan and western offshore islands. Thegranitic rocks in eastern Taiwan are mainly
surrounded by marble. Outcrop and core observations
also found fractures in the granitic rocks would be
easily filled and sealed by carbonate precipitation (seeFig. 1).
Petrographic analysis of core samples [1] indicated
the granitic rock in eastern Taiwan is mainlycomposed of quartz, plagioclase, biotite, potassium
feldspar, and muscovite. Second and accessory
minerals include epidote, calcite, chlorite, sphene,

apatite, zircon, garnet, allanite, and opaque minerals(e.g., pyrite and magnetite) [2].
As mentioned previously, the granitic rocks ineastern Taiwan are mainly surrounded by marble,
therefore, the groundwater quality (especially, pHvalue) is expected to be mainly controlled bycarbonate system. In other words, pH fluctuations areexpected to be small for both surface water andgroundwater in this area. Table 1 showed the averagewater quality of 11 shallow groundwater samples ineastern Taiwan [3], which called SGWCGS in thisstudy. The average temperature and pH of SGWCGS
are approximately 23.5℃ and 8.1 respectively, andbelongs to Ca-HCO3 type groundwater.
The purpose of this study is to estimate the possible
variations of pH and Eh of groundwater in granitic
rocks, and the possible secondary minerals formed bywater-rock interaction, which should be considered in
the sensitivity analysis of the nuclide solubility and/oradsorption characteristics in the safety assessment.
Reaction path modeling is used to estimate above
possible variations in this study.

6th East Asia Forum on Radwaste Management Conference
November 27-29, 2017, Osaka, Japan

feldspar (20 vol.%), plagioclase (20 vol.%), biotite(10 vol.%), muscovite (10 vol.%), calcite (4 vol.%),epidote (5 vol.%) and opaque minerals (1 vol.%).

Due to the different ways of naming or the lackof thermodynamic information, the mineralname must be properly classified or assumed
based on the thermodynamics database of thesoftware before modeling. In this study, K-Fig. 1. The geological background of the study area.
Table 1: The average shallow groundwater quality inthe study area
Itemunit

SGWCGS

DepthTemp.
ECpHEhCl_tot
C_tot
S_tot
N_tot
Na_tot
K_tot
Ca_tot
Mg_totFe_tot
Mn_tot
As_totm℃mS/cm*Vmole/Lmole/Lmole/Lmole/Lmole/Lmole/Lmole/Lmole/Lmole/Lmole/Lmole/L

20~100
23.5
0.320
8.1
0.281
1.24E-04
2.82E-03
2.64E-04
1.60E-05
5.45E-04
3.38E-05
9.72E-04
3.78E-04
7.16E-07
9.02E-07
1.84E-07

2. Methods
The GWB (Geochemist’s Workbench®, Version 9)[4], a commercial software package, and its
thermodynamic database - thermo.com.V8.R6+.dat
was used in this study to perform reaction pathmodeling of 3 different cases (see Fig. 2). The purpose
and assumptions of each case were listed in the Table
2. Some common assumptions for the simplicity
reason were as follows: The initial rock compositions were assumed to be
the same for all locations before weathering. The
minerals of a fresh core sample (sampling depth =499 m to 500 m) was used as the initial rockcomposition. According to petrographic analysis,
it is mainly composed of quartz (30 vol.%), K-feldspar is represented by microcline. Plagioclase
is assumed to be composed of 85% albite and 21%anorthite [5]. Biotite is assumed to be composed of60% annite and 40% phogopite [5].
 The initial deep groundwater compositions wereassumed to be the same for all locations. Thegroundwater property of DGWb (= DGWa @ t =0, DGWa = SGWCGS equilibrate with calcite @ T= 32.6 ℃ ) was used as the initial deep
groundwater composition, and the 32.6 ℃ is ahypothetical temperature based on geophysicalwell logging at 500 m depth [6].
 The initial shallow groundwater compositions
were assumed to be the same for all locations. The
groundwater property of SGWb (= SGWa @ t =0, SGWa = SGWCGS equilibrate with calcite @ T= 23.5 ℃ ) was used as the initial shallow
groundwater composition.
 The theoretical fugacity values of carbon dioxide
and oxygen of the initial deep and shallow
groundwater were assumed equal to their partialpressure (PCO2 and PO2 ), and estimated by GWB
in this study.
Theoretically, there could be many different paths(processes) between any two different thermodynamic(equilibrium) states. One of these (reaction) paths, thereasonable one, must involve the formation of thosesecondary minerals found in the weathering rocksamples. This provide a chance to constrain areasonable reaction path.
In reaction path modeling, especially for natural
reaction systems, it is impossible to calculate thereaction rates of all possible reactions by kinetic
approaches because of the kinetic information is very
limited in reality, and the numerical calculation will bea big challenge if there are huge difference innumerical values of reaction rates for reactions
occurred at same time. By coupling kinetic andequilibrium approaches, the GWB can simulate thereaction path and express results as a function ofreaction time. In this study, our interested timescale isup to million years in order to understand the longterm chemical evolution for the concerns of deep
geological disposal of radioactive waste, therefore,
only the precipitation and dissolution reactions of theinitial rock composition (expect calcite) were treatedby kinetic approach, and the mathematical functions
and their associated parameters compiled by Palandriand Kharaka [7] were used to describe their reaction

6th East Asia Forum on Radwaste Management Conference
November 27-29, 2017, Osaka, Japanrates. The rest of all possible chemical reactions,
including the precipitation and dissolution of the rest
minerals, ion association reactions in aqueous phase,etc., were treated by equilibrium approach.
3. Results
The variations of pH and Eh as function of reaction
progress during the reaction path modeling of ourstudy cases were shown in Fig. 3 to Fig. 5. Amongthem, the higher pH value (10.31) in the deep closed
fracture scenario (case1) is due to the gradualdepletion of CO2 in the reaction system. On the otherhands, the lower pH value (7.12) in the open fracture
scenario (case4) is caused by the higher CO2 value(equal to the CO2 partial pressure in the soil layer).
In addition, the case2 and case4 were designed tounderstand the chemical evolutions of groundwater indeep and shallow open fractures, the simulated pH and
Eh ranges of these two cases (pH = 7.12 to 8.93 andEh = -0.37 volts to -0.15 volts) could be used toconstraint the possible pH and Eh ranges in the water
conductive features in our study area. Compared to thesimilar simulation results in Taiwan’s granitic offshoreisland (pH = 6.99 to 9.75 and Eh = -0.45 volts to -0.17volts) [8], it shows that the groundwater characteristics(especially pH value) of granitic rocks in eastern
Taiwan is obviously affected by carbonate system andwith less pH fluctuations. As for Eh variation, there isno obvious difference between them.
Fig. 6 to Fig. 8 showed the variations of mineralphases as function of time during the reaction pathmodeling of our study cases. In these figures, the graylines represent the initial reactants (minerals), such asquartz, microcline, anorthite, albite, annite, phlogopite,muscovite, epidote and calcite. The other colors
represent secondary minerals that occur or disappear
with the time during the reaction path modeling: Case1 (deep closed fracture): In the absence ofcontinuous supply of CO2 , the minerals such as
calcite, muscovite group (i.e., sericite), andraditeand saponite-Ca are the major secondary minerals
in the final equilibrium state.
 Case2 (deep open fracture): When the supply of
CO2 is continuous, the minerals such as calcite,
dolomite, nontronite-Na and muscovite group (i.e.,sericite) are the major secondary minerals in thefinal equilibrium state.
 Case4 (shallow open fracture): When CO2
content is higher and the supply of CO2 iscontinuous, the minerals such as quartz, dawsonite,
calcite, dolomite, nontronite-Na and muscovite
group (i.e., sericite) are the major secondary
minerals in the final equilibrium state.

rest of the main secondary minerals will change
gradually from andradite, saponite-Ca to dolomite,
nontronite-Na, dawsonite when the amount of CO2
increase, which could be related to the increase in theamount of albite dissolved.
4. Conclusions
The following important conclusions were drawn:
 The calcite and sericite are the major secondary
minerals in the final equilibrium state in this study.
 For the fractured-rock aquifers in eastern Taiwan
granitic rocks, the simulated pH and Eh ranges (pH= 7.12 to 8.93 and Eh = -0.37 volts to -0.15 volts)
should be considered in the sensitivity analysis of
the nuclide solubility and/or adsorption
characteristics.
 Compared to the similar simulation results inTaiwan’s granitic offshore island (pH = 6.99 to9.75 and Eh = -0.45 volts to -0.17 volts), it showsthat the groundwater characteristics (especially pHvalue) of granitic rocks in eastern Taiwan are
obviously affected by carbonate system and withless pH fluctuations.
ACKNOWLEDGEMENTS
This work was financially supported by the Taiwan
Power Company for which the author is very grateful.
REFERENCES[1] Chen, W.S., Hsieh, P.S., Lin, W., Lin, C.K., Huang, S.Y.,
and Yu, J.C. (2012) HCBH01 Drill-hole report. Plan
Entrusted by Taipower Company, SNFD-EEL-90-284,Industrial Technology Research Institute.
[2] Hsieh, P.S. (2015) Rock and Mineral Characteristics
Analysis. Plan Entrusted by Taipower Company, SNFDEEL-90-306, Industrial Technology Research Institute.
[3] CGS (2013) Hydro-geological survey and compilation ofhydro-geological map in central flow basin, Taiwan (4/4),
Central Geology Survey.
[4] Bethke, C.M. (2008) Geochemical and Biogeochemical
Reaction Modeling, 2nd ed., Cambridge University Press.
[5] Lan, C.Y. (1989) Geochronology and rock chemistry of
Taiwan gneisses: Ph. D. thesis, National TaiwanUniversity, 211p (in Chinese).
[6] Chen, W.S., Huang, S.Y., Liu, Z.Y., Li, Y.H., Hsieh, P.S.,
Lin, W., and Lin, C.K. (2015) Fracture Characterizationof HCBH01 Borehole. Plan Entrusted by Taipower
Company, SNFD-ITRI-90-308, Industrial Technology
Research Institute.
[7] Palandri, J.L., and Kharaka, Y.K. (2004) A compilationof rate parameters of water-mineral interaction kinetics
for application to geochemical modeling: U.S. GeologicalSurvey Open File Report 2004-1068.
[8] Chang, Y.T., Lin, C.K., Hsieh, P.S., and Lin, W. (2015)
Reaction path modeling of K Area. Plan Entrusted byTaipower Company, SNFD-ITRI-90-305, Industrial
Technology Research Institute.

Overall, the simulation results of 3 cases show thatcalcite and muscovite group (i.e., sericite) are themajor secondary minerals in the final equilibrium state.
Besides calcite and muscovite group (i.e., sericite), the

6th East Asia Forum on Radwaste Management Conference
November 27-29, 2017, Osaka, Japan

(T=23.5℃)
Case 4 : shallow open fracture

MarbleGranite

Marble(T=32.6℃)
Case 2 : deep open fracture

Case 1 : deep close fractureFig. 2. Illustration of simulation cases used in this study

Table 2: Purpose and assumptions of simulation cases used in this study
Case ID & namePurpose

Initial condition

Deep close
fracture(case 1)

To estimate the final
equilibrium state(chemical condition) in a
deep and well isolated
environment.

Deep open
fracture(case 2)

To estimate the possible
chemical condition in thewater conductive fracture
at the expected disposal
depth.
To estimate the possible
chemical condition in thewater conductive fracture
at the shallow depth.

Shallow open
fracture(case 4)

Major Assumptions

 The initial rock composition = H-20 The initial groundwater composition= 𝐷𝐺𝑊𝑏 (=𝐷𝐺𝑊𝑎 @ t = 0)
 The initial value of 𝑃𝐶𝑂2 = eq. with
𝐷𝐺𝑊𝑏
 The initial value of 𝑃𝑂2 = eq. with
𝐷𝐺𝑊𝑏
 The initial rock composition = H-20
 The initial groundwater composition= 𝐷𝐺𝑊𝑏 (=𝐷𝐺𝑊𝑎 @ t = 0)
 The initial value of 𝑃𝑂2 = eq. with
𝐷𝐺𝑊𝑏
 The initial rock composition = H-20
 The initial groundwater composition= 𝑆𝐺𝑊𝑏 (=𝑆𝐺𝑊𝑎 @ t = 0)
 The initial value of 𝑃𝑂2 = eq. with
𝑆𝐺𝑊𝑏

Boundarycondition
No mass transfer
and no energy
transfer across thesystem boundaryThe value of 𝑃𝐶𝑂2= constant = eq.
with 𝐷𝐺𝑊𝑏
The value of 𝑃𝐶𝑂2= constant = 0.01
atm (eq. with soil)

6th East Asia Forum on Radwaste Management Conference
November 27-29, 2017, Osaka, Japan

1110.5

pH = 10.31

109.5
9Eh (V)pH

8.5

7.56 ≦ pH ≦ 10.31

8
7.5
7
6.5
6
5.5
5

0.1.2.3.4.5.6.7.8

.9

1

1.9.8
.7
.6
.5
.4
.3.2.10-.1-.2-.3-.4-.5-.6-.7-.8-.9-1-0.51 V ≦ Eh ≦ -0.20 V

Eh = -0.51 V0.1.2.3.4.5.6.7.8.9

1

Rxn progress

Rxn progressFig. 3: The simulated pH and Eh as function of reaction progress (case 1)

1110.5
109.5

pH = 8.93

9Eh (V)pH

8.5

7.56 ≦ pH ≦ 8.93

8
7.5
7
6.5
6
5.5
5

0.1.2.3.4.5.6.7.8.9

1

1.9.8
.7.6
.5
.4.3
.2
.1
0
-.1-.2-.3-.4-.5-.6-.7-.8-.9-1-0.37 V ≦ Eh ≦ -0.20 V
Eh = -0.37 V

0.1.2.3

Rxn progress.4.5.6.7.8.9

1

Rxn progressFig. 4: The simulated pH and Eh as function of reaction progress (case 2)1110.5109.5
9pH = 8.49
7.12 ≦ pH ≦ 8.49

8Eh (V)

pH

8.5

7.5
7
6.5
6
5.5
5

0.1.2.3.4.5.6.7.8.9

1

1.9.8.7.6.5.4.3.2.10-.1-.2-.3-.4-.5-.6-.7-.8-.9-1-0.32 V ≦ Eh ≦ -0.15 V
Eh = -0.32 V0.1.2.3.4.5.6.7.8.9

1

Rxn progress

Rxn progressFig. 5: The simulated pH and Eh as function of reaction progress (case 4)

6th East Asia Forum on Radwaste Management Conference
November 27-29, 2017, Osaka, Japan

2010815Muscovite

Calcite10Andradite

4Saponite-Ca

2 Kaolinite

C-

Daphnite
14A Grossular
ZoisiteDolomite
Magnetite
Nontronite-Ca
0 Albite_low

Pyrite Calcite
Annite-2
Maximum_Microcline

Phlogopite

Anorthite-4

Minerals (delta mol)

Minerals (delta mol)

6Muscovite
AndraditeKaolinite GrossularDaphnite
14A C
ZoisiteDolomite
Magnetite
Nontronite-Ca
0 Albite_low

5Saponite-CaPyrite

AnniteMaximum_MicroclineAnorthitePhlogopite
Epidote-5 Quartz-10

EpidoteQuartz-15-6-8(a)0(b)+1e4+2e4+3e4+4e4+5e4+6e4+7e4+8e4+9e4-20+1e50+1e5+2e5+3e5+4e5+5e5+6e5+7e5+8e5+9e5+1e6Time (yr)Time (yr)Fig. 6: The simulated mineral phase assemblage as function of reaction time (case 1)

20

10
Nontronite-Na
8 Calcite

DolomiteMuscovite

62
Nontronite-CaDaphnite
Magnetite14A Pyrite

Maximum_Microcline
Phlogopite Albite_lowAnorthite-4

EpidoteAnniteMuscovite
Kaolinite

5C
Nontronite-CaPyrite
Magnetite-14ADaphnite
Dawsonite

0

Maximum_Microcline
Anorthite-5

Albite_low Phlogopite
QuartzAnnite Epidote-10

Quartz-15-6-8

Dawsonite

Minerals (delta mol)

Minerals (delta mol)

C-2

DolomiteNontronite-Na

10

Kaolinite
40

Calcite

15(a)

(b)
-20

0+1e4+2e4+3e4+4e4+5e4+6e4+7e4+8e4+9e4+1e50+1e5+2e5+3e5+4e5+5e5+6e5+7e5+8e5+9e5+1e6Time (yr)Time (yr)Fig. 7: The simulated mineral phase assemblage as function of reaction time (case 2)

1050

Nontronite-Na
8Dolomite

QuartzMuscovite

40Calcite
Quartz4 Kaolinite

Dawsonite C2-0

Nontronite-Ca
Daphnite
14A Pyrite
Magnetite

Maximum_Microcline-2-4Anorthite
Epidote3020
Dawsonite Calcite

0Dolomite

Nontronite-Na

10 Muscovite

CKaolinite
Nontronite-CaPyrite
Daphnite
Magnetite14A
Maximum_Microcline

AnniteAnorthite

Phlogopite

Annite Epidote+3e5+5e5+7e5-10-6-8Phlogopite

Minerals (delta mol)

Minerals (delta mol)

6Albite_low(a)0+1e4+2e4+3e4+4e4+5e4+6e4Time (yr)+7e4+8e4+9e4+1e5(b)-200+1e5+2e5+4e5+6e5+8e5

Albite_low+9e5 +1e6

Time (yr)Fig. 8: The simulated mineral phase assemblage as function of reaction time (case 4)

6th East Asia Forum on Radwaste Management Conference
November 27-29, 2017, Osaka, Japan