Hydrogeological and geochemical control of the variations of 222Rn concentrations in a hard rock aquifer: Insights into the possible role of fracture-matrix exchanges

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 Bq L −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 mBq m −2 s −1 in granite and from 0.5 to 0.9 mBq m −2 s −1 in micaschists. 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 −3 m 2 s −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.