『Abstract
To investigate the possible variations of Rn concentration in
crystalline rocks as a function of flow conditions, a field study
was carried out of a fractured aquifer in granite. The method
is based on the in situ measurement of Rn in groundwater, aquifer
tests for the determination of hydraulic characteristics of the
aquifer and laboratory measurement of Rn exhalation rate from
rocks. A simple crack model that simulates the Rn concentration
in waters circulating in a fracture intersecting a borehole was
also tested. The Rn concentrations in groundwaters from boreholes
of the study site ranged from 192 to 1597 BqL-1. The
Rn exhalation rates of selected samples of granite and micaschist
were determined from laboratory experiments. The results yielded
fluxes varying from 0.5 to 1.3 mBqm-2s-1
in granite and from 0.5 to 0.9 mBqm-2s-1
in micaschist. Pumping tests were performed in the studied boreholes
to estimate the transmissivity and calculate the equivalent hydraulic
aperture of the fractures. Transmissivities ranged from 10-5
to 10-3m2s-1. Using the cubic
law, hydraulic equivalent fracture apertures were calculated to
be in the range of 0.5-2.3 mm.
To gain a better insight into the spatial variability of Rn contents
in groundwater, theoretical Rn concentrations were calculated
from an available simple crack model using results from field
and laboratory experiments. This model gave satisfactory results
for boreholes characterized by low-flow conditions, in which case,
the calculated Rn contents were in the range of Rn concentrations
set by the analytical uncertainty of concentrations measured in
water. However, for boreholes characterized by high-flow conditions,
the model underestimated the Rn concentration in groundwater.
The higher the flow in the fracture, the larger the difference
between calculated and measured Rn concentrations in water. These
observations led to performing pumping tests to obtain a better
understanding of the hydrogeological control of Rn content in
water. The results clearly show an increase of Rn content in groundwater
after the pumping test, which could be explained by the input
of Rn-rich waters from the host matrix.』
1. Introduction
2. Environmental setting of the study site: Stang-er-Brune
2.1. Geology
2.2. Hydrogeology
3. Data acquisition
3.1. Field measurements of radon volume activity in water
3.2. Field measurements of physical properties of the aquifer
3.3. Laboratory tests: 222Rn exhalation rate and 226Ra
activity of rock samples
4. Modelling of radon concentration in water
5. Results and discussion
5.1. 222Rn exhalation rate and 226Ra
activity of rock samples
5.2. Hydrogeological setting and hydraulic properties
5.3. Radon volume activity in water at ambient conditions
5.3.1. Correlations between radon concentrations in water and
lithology
5.3.2. Variations of Radon concentrations in water related to
hydrology
5.3.3. Comparison with model predictions
5.3.4. Estimation of uncertainty
5.3.4.1. Influence of the 222Rn source disc radius
variation
5.3.4.2. Influence of variation in water inflow
5.3.4.3. Influence of variation in radon exhalation rate
5.3.4.4. Influence of variation in fracture aperture
5.4. Radon volume activity in water during pumping test conditions
5.4.1. Evolution of water concentrations during pumping in boreholes
B1, B2 and B3
5.4.2. Overall changes in water concentration with time during
the field experiment
5.4.3. Enhancement of fracture-matrix exchanges as a possible
source of radon
5.4.4. Estimation of radon concentration in the host rock
6. Conclusions
Acknowledgements
References