Radon-222 measurements in groundwaters and rocks from fractured aquifer (Ploemeur, Britanny)
Titre du congrès :European Geosciences Union General Assembly 2005
Ville du congrès :Vienne
Date du congrès :24/04/2005
This paper deals with Radon-222 (222Rn) and Radium-226 (226Ra) contents of
groundwater in Brittany (France) during the exploitation within a fractured aquifer
in a nitrate-rich agricultural environment (Ploemeur site).
In the last years, many studies have been focused on the different parameters and
physical phenomena that influence radon-emanation and -transport in soils, particularly
for radon-mapping and -risk management.
Radon can help in the understanding of the transport and exchange mechanisms in the lithosphere , the atmosphere and the hydrosphere.
Radon-222, a radioactive inert noble gas with a half-life time of 3.8 days, is produced
from 226Ra in the radioactive decay chain of 238U. This very mobile and practically
inert element is affected by physical phenomena, e.g. adsorption, dissolution E, but not by chemical ones. It emanates from mineral grains by alpha recoil or diffusion, and
the rate depends on the microstructure of rocks and environmental conditions. Studies
in hydrogeology often rely on the assessment of differences in 222Rn concentrations
between deep and shallow waters. Indeed, in fractured aquifers 222Rn concentrations
in deep water are strongly dependent upon the hydrological settings and 226Ra concentrations
in rocks. On the other hand, in shallow waters 222Rn concentrations fall within the concentration value in groundwater and that of rainwater in equilibrium with the atmosphere. Groundwater-dissolved 222Rn can, thus, be used as a tracer to investigate and estimate flow-rates, get insight into shallow and deep water mixing or calculate flow velocities in homogeneous aquifers.
The Ploemeur fractured aquifer is located within a granite/schist shear zone, at a depth
of 40-100 m. The 21 boreholes of the site allow the sampling of different surroundings
(e.g. shallow and deep waters). During a 10-year period, production rate and chemical
parameters were continuously measured in the tap water from a deep-water plant. A
SO4-NO3 evolution was observed over the time after pumping initiation. This evolution
has been attributed to an equilibrium between the physical parameters of the flow and the chemical kinetics of an autotrophic denitrification process which occurs in the pyrite bearing-fractures. The chemical characteristics of the groundwater collected
in 18 wells located around the site allowed us to identify three different groups:
(i) Group I is directly related to the main pumped well and characterized by reduced
NO3- levels combined with an increased SO42- production resulting from the denitrification
process occurring in pyrite bearing fractures; (ii) Group II, weakly influenced
by pumping is characterized by high NO3- concentrations and a short residence time;
(iii) Group III, poorly related to pumping, presents very stable concentrations through
time and a short residence time.
During 1-week field seasons held in March, June and December 2004, 54 water samples
were collected in 21 boreholes. The 222Rn volume activity was measured by using an ionization chamber coupled with a degassing unit (whole system closed).
The 222Rn dissolved in water sample was extracted from the liquid phase using a
pump. The 222Rn volume activity is then measured in the air of the system by using
the ionization chamber. The measurement uncertainty is 10-20 %. The 226Ra volume
activity was indirectly measured in 15 water samples by measuring the 222Rn volume
activity in air after degassing at the equilibrium state. This measurement is realized by using scintillation flasks and a counting chamber. The measurement uncertainty is 10- 15 %. The 226Ra concentration in rocks was also measured in 10 samples (granites
and micaschists) by gamma spectrometry (HP Germanium detector).The measurement
uncertainty is 10 %.
These first results allow us to highlight: (i) the spatial distribution of 222Rn activity
in groundwater, (ii) a relationship between variations in 222Rn concentrations and
hydrogeological settings, (iii) a relation 222Rn vs NO3/SO4 in groundwater and (iiii)
the variability of 226Ra contents in rocks and in groundwaters.