Combining isotopic tracers ( 222 Rn and δ 13 C) for improved modelling of groundwater discharge to small rivers K. Lefebvre, 1,2,3 * F. Barbecot, 3 M. Larocque 3 and M. Gillon 4 1 Université Paris Sud, Laboratoire GEOPS ‘Géosciences Paris Sud’, bâtiments 504 & 509, F-91405 Orsay cedex, France 2 CNRS, UMR 8148, Université Paris Sud, F-91405 Orsay cedex, France 3 GÉOTOP-UQAM, Département des sciences de la Terre et de l’atmosphère, Montréal (Québec), Canada 4 UMR UAPV-INRA EMMAH, Université d’Avignon et des Pays du Vaucluse, France Abstract: In regions where aquifers sustain rivers, the location and quantification of groundwater discharge to surface water are important to prevent pollution hazards, to quantify and predict low flows and to manage water supplies. 222 Rn is commonly used to determine groundwater discharge to rivers. However, using this isotopic tracer is challenging because of the high diffusion capacity of 222 Rn in open water. This study illustrates how a combination of isotopic tracers can contribute to an enhanced understanding of groundwater discharge patterns in small rivers. The aim of this paper is to combine 222 Rn and δ 13 C DIC to better constrain the physical parameters related to the degassing process of these tracers in rivers. The Hallue River (northern France) was targeted for this study because it is sustained almost exclusively by a fractured chalk aquifer. The isotopes 222 Rn, δ 13 C DIC , δ 2 H and δ 18 O were analysed along with other natural geochemical tracers. A mass balance model was used to simulate 222 Rn and δ 13 C DIC . The results of δ 2 H and δ 18 O analyses prove that evaporation did not occur in the river. The calibration of a numerical model to reproduce 222 Rn and δ 13 C DIC provides a best-fit diffusive layer thickness of 3.21 × 10 5 m. This approach is particularly useful for small rivers flowing over carbonate aquifers with high groundwater DIC where the evolution of river DIC reflects the competing processes of groundwater inflow and CO 2 degassing. This approach provides a means to evaluate groundwater discharge in small ungauged rivers. Copyright © 2014 John Wiley & Sons, Ltd. KEY WORDS radon-222; groundwater–surface water interactions; mass balance; modelling; carbon-13; small river Received 10 January 2014; Accepted 15 November 2014 INTRODUCTION Surface water and groundwater were increasingly con- sidered as a single resource (Cook, 2013). However, water exchanges between groundwater and surface water are still difficult to quantify with field methods and difficult to model (Kalbus et al., 2006; Brunner et al., 2011). In regions where aquifers sustain rivers, the location and quantification of groundwater discharge to surface water are important to prevent pollution hazards, to quantify and predict low flows and to manage water supplies. In the past decades, many studies have focused on groundwater discharge in river systems (Corbett et al., 1997; Kimball et al., 2001; Cook et al., 2003; Becker et al., 2004; Holtzman et al., 2005; Mencio and Mas-Pla, 2008; Gleason et al., 2009; Meredith et al., 2009; Meredith and Kuzara, 2012; Atkinson et al., 2013; Unland et al., 2013; Battle-Aguilar et al., 2014). A wide range of methods have been reported in the literature for the study of groundwater–surface water interactions (see Cook, 2013 for review), such as heat tracer methods (Becker et al., 2004; Constantz, 2008), methods based on Darcy’s law (Freeze and Cherry, 1979; Landon et al., 2001) or approaches using tracer mass balance (Zellweger, 1994; Harvey and Wagner, 2000). The most common tools used to determine groundwater discharge with a mass balance approach are geochemical and isotopic tracers, such as 222 Rn (Gleason et al., 2009; Cartwright et al., 2011; McCallum et al., 2012; Batlle- Aguilar et al., 2014), CFCs (Cook et al., 2003), SF 6 (Cook et al., 2006), δ 13 C (Meredith and Kuzara, 2012), 87 / 86 Sr (Katz et al., 1997), δ 18 O and δ 2 H (Sklash and Farvolden, 1979; Meredith et al., 2009) and major ions (Mencio and Mas-Pla, 2008). Among these tracers, 222 Rn is commonly used to locate groundwater inflow in surface water because it is produced by the radioactive decay of the 226 Ra in almost all aquifers, it is a non-reactive tracer and its activity in groundwater is usually significantly higher than in river water (Genereux and Hemond, 1990; Cable et al., 1996; Cook, 2013). 222 Rn analysis can be measured by liquid scintillation counting in the laboratory *Correspondence to: K. Lefebvre, CNRS, UMR 8148, Université Paris Sud, F-91405 Orsay cedex, France. E-mail: karine.lefebvre@u-psud.fr HYDROLOGICAL PROCESSES Hydrol. Process. 29, 2814–2822 (2015) Published online 21 December 2014 in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/hyp.10405 Copyright © 2014 John Wiley & Sons, Ltd.