Soil gas radon emanometry: A tool for delineation of fractures for groundwater in granitic terrains D.V. Reddy a, * , B.S. Sukhija a , P. Nagabhushanam a , G.K. Reddy a , Devender Kumar a , P. Lachassagne b a National Geophysical Research Institute (NGRI), Tritium Lab, Uppal Road, Hyderabad, Andhra Pradesh 500007, India b BRGM/Water Division/Resource Evaluation, Discontinuous Aquifers Unit-1039, rue de Pinville, 34000 Montpellier, Franc Received 12 April 2005; received in revised form 7 February 2006; accepted 10 February 2006 Summary In severalarid 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 soilgas 222 radon (Rn) and helium ( 4 He) have been explored but not investigated sufficiently. This paper reports the results of a systematic survey of 222 Rn 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 222 Rn anomalies, which were supplemented by 4 He measure- ments. Drilling was carried out on the sites of these high 222 Rn anomalies as well as on low 222 Rn anomalies. The bore wells drilled at the site of high 222 Rn concentration were found to be quite productive and those drilled on low 222 Rn concentration sites were dry; those sites of the intermediate 222 Rn concentrations had intermediate yields. These preliminary results suggest that 222 Rn 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 222 Rn anomalies. Our observations of 222 Rn concentration in the soil gas and of 222 Rn emanation rates from the soil and uranium concentrations in the soil indicate that fracturing of the rock, rather than advection of 222 Rn from deeper strata may be responsible for such 222 Rn anomalies. c 2006 Elsevier B.V. All rights reserved. KEYWORDS Radon; Helium; Soil gas; Groundwater; Hard rock; Fracture delineation 0022-1694/$ - see front matter c 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jhydrol.2006.02.033 * Corresponding author. Tel.: +91 40 23434602; fax: +91 40 23434651. E-mail address: dvr_ngri@yahoo.co.in (D.V. Reddy). Journal of Hydrology (2006) 329, 186– 195 a v a i l a b l e a t w w w . s c i e n c e d i r e c t . c o m j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / j h y d r o l