Soil gas radon emanometry: A tool for delineation of fractures for groundwater in granitic terrains
Résumé
In several arid and semi-arid hard rock areas of the world, overexploitation of groundwater has caused de-saturation of the phreatic weathered zone. Attempts are now underway to draw supplies from deeper fracture zones. But random drilling often fails to encounter such productive zones. Further, currently available hydrogeological and geophysical methods are of limited applicability. Alternative geochemical methods, employing soil gas 222radon (Rn) and helium (4He) have been explored but not investigated sufficiently.
This paper reports the results of a systematic survey of 222Rn concentration in soil gas (at a depth of 60 and 160 cm) in a watershed located in a semi-arid granitic area in India. We were able to demarcate three high 222Rn anomalies, which were supplemented by 4He measurements. Drilling was carried out on the sites of these high 222Rn anomalies as well as on low 222Rn anomalies. The bore wells drilled at the site of high 222Rn concentration were found to be quite productive and those drilled on low 222Rn concentration sites were dry; those sites of the intermediate 222Rn concentrations had intermediate yields. These preliminary results suggest that 222Rn emanometry method may turn out to be useful. The method still needs to be investigated in detail and its usefulness in various settings still needs to be established on a statistical basis. We have made an attempt to understand the process that may cause such 222Rn anomalies. Our observations of 222Rn concentration in the soil gas and of 222Rn emanation rates from the soil and uranium concentrations in the soil indicate that fracturing of the rock, rather than advection of 222Rn from deeper strata may be responsible for such 222Rn anomalies.