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.