Principal Research Results

Validation of Groundwater Dating Method Using 36Cl for
Low Permeable RockBackgroundOn safety assessment of high level waste disposal, groundwater flow is important for evaluation of radionuclide transport(Fig.1). The groundwater flow in candidate rock for waste disposal is expected to be stagnant, thus collecting water for chemistry
analysis would be difficult. Therefore, we have researched groundwater dating methods using 36Cl and 4He which are applicable fromtens of thousands to million of years (Fig.2), and developed these methods for low permeable rock. We have already established the4He method for low permeable rock. It is necessary to establish the 36Cl method.

Objectives
The purpose of this study is to validate the developed 36Cl dating method for application to low permeable layers in theGreat Artesian Basin.

Principal Results
The Great Artesian Basin is suitable for validating groundwater dating methods because it has relatively simple geologicalformation and is stable over the long term. We conducted borehole investigation at Richmond and Marree in the Great Artesian Basin(Fig.3). We collected core samples and applied 36Cl method that we developed for low permeable rock.
1. 36Cl sample collecting method from low permeable rock
Squeezing and leaching were conducted for collecting 36Cl sample from low permeable rock. The 36Cl concentrations ofsqueezing and leaching are consistent with each other. Therefore, it is confirmed that water sample could be collected by squeezing
and leaching (Fig.4).
2. Profile of 36Cl and transport mechanism
The Profile of 36Cl was investigated. 36Cl concentration decreases with depth exponentially. It was presumed that 36Cl wassupplied by rain and transported by diffusion with radioactive decay (Fig.5). The profile of 36Cl was expressed by exponential function
using diffusion-equation under steady state. Moreover, the profile of 36Cl could be reproduced by diffusion equation using diffusion
coefficient and porosity measured in laboratory test (Fig.6).
3. Characterizing groundwater velocity through low permeable rock using 36Cl
The validity of 36Cl dating developed for low permeable rock was confirmed by no difference between colleting methods
and reproducibility of the 36Cl profile. From Peclet Number with diffusion dominant condition, the groundwater velocity is quite
smaller than 7× 10-14 m/s at Richmond and 2× 10-13 m/s at Marree, respectively. From these results, the very slow groundwater flow
was proved in low permeable layer.
This study entitled Research and development on groundwater dating technique was done under contracts awarded fromMETI (Ministry of Economy, Trade and Industry).

Future Developments
Developed 36Cl dating method for low permeable rock will be conducted in Japan to confirm applicability and usefulness ofcharacterization on very slow groundwater flow.
Main Researcher: Takuma Hasegawa, Ph. D.,
Research Scientist, Nuclear Fuel Cycle Backend Research Center, Civil Engineering Research Laboratory
Reference
Research and development of groundwater dating (Part 8) - Validation on 36Cl dating method to characterize low permeable layer-,
Civil Engineering Research Laboratory Rep. No.N07038. (in Japanese)945. Nuclear

Groundwater flow

over300m

Leaking radionuclide
Rnu
io
ad

declimntio
raig

Waste disposal facility

Fig.2 Concept of groundwater dating method

Fig.1 Concept of waste disposal in deep geologicalformation

5030

X f it )Y= ectfer(P4036-15Cl/Cl(x10 )Very slow groundwater flow is expected in deep geological formation,
therefore waste facility plan to construct over 300 m in depth

By leaching

36
Cl has half-lives with 301,000 year, groundwater age aredetermined by concentration change due to radioactive decay

Correlation factor
R=0.90201000102030

By squeezing

Fig.3 Location of borehole in Great Artesian Basin

540
50-15Cl/Cl(x10 )36

Fig.4 Comparison of sampling method for 36Cl/Cl
Squeezing: water depleted by compressing
Leaching: exchange water by diffusion

Rain fall (almostconstant 36Cl
supply )3636ClCl (atm/ l)1012 1013 1014

11100−Near surface−surface
Secular equilibrium
due to supply andradioactive decay

Depth (m)50

=Underground=Transport bydiffusion withradioactive decay

01010141015

Squeezing
Leaching

50

100
150200250

Low permeable rock(k<1x10-11 m/s)

Cl (atm/ l)

Squeezing
Leaching

Depth (m)Radioactivedecay

36
13300

Laboratory test
D=7x10-12m2/s
n=20%

100

Laboratory test
D=3x10 -11m 2/s
n=35%
Theoretical equation

150

C = Co e−nλz
D

D:Diffusion Coefficient.
n: porosityn
L:Decay Constant
Z DepthZ:200(1) Richmond(2) MarreeFig.5 Transport mechanism of Cl
36

Cl concentration is almost constant near surface, andis transported by diffusion with radioacitive decay inunderground.

Fig.6 Profile of 36Cl concentration

36

Measured data was reproduced by diffusion equation using diffusion coefficientand porosity measured in laboratory test (Red line).

95