River-aquifer exchange fluxes under monsoonal climate conditions Svenja Bartsch a , Sven Frei a , Marianne Ruidisch b , Christopher L. Shope a,c , Stefan Peiffer a , Bomchul Kim d , Jan H. Fleckenstein a,e,⇑ a Department of Hydrology, Bayreuth Center of Ecology and Environmental Sciences – BayCEER, University of Bayreuth, Bayreuth, Germany b Department of Soil Physics, Bayreuth Center of Ecology and Environmental Sciences – BayCEER, University of Bayreuth, Bayreuth, Germany c U.S. Geological Survey, Utah Water Science Center, Salt Lake City, UT, USA d Department of Environmental Science, Kangwon National University, Chuncheon, Republic of Korea e Department of Hydrogeology, Helmholtz Centre for Environmental Research – UFZ, Leipzig, Germany article info Article history: Received 26 August 2013 Received in revised form 3 December 2013 Accepted 5 December 2013 Available online 12 December 2013 This manuscript was handled by Peter K. Kitanidis, Editor-in-Chief, with the assistance of Philippe Negrel, Associate Editor Keywords: River-aquifer exchange fluxes Heat as a natural tracer Monsoonal-type climate Hydraulic gradient reversals HydroGeoSphere Natural attenuation of nitrate summary An important prerequisite to better understand the transport of nutrients and contaminants across the river-aquifer interface and possible implications for biogeochemical transformations is to accurately characterize and asses the exchange fluxes. In this study we investigate how monsoonal precipitation events and the resulting variability in river discharge affect the dynamics of river-aquifer exchange and the corresponding flux rates. We evaluate potential impacts of the investigated exchange fluxes on local water quality. Hydraulic gradients along a piezometer transect were monitored at a river reach in a small catchment in South Korea, where the hydrologic dynamics are driven by the East-Asian Monsoon. We used heat as a tracer to constrain river-aquifer exchange fluxes in a two-dimensional flow and heat transport model implemented in the numerical code HydroGeoSphere, which was calibrated to the mea- sured temperature and total head data. To elucidate potential effects of river-aquifer exchange dynamics on biogeochemical transformations at the river-aquifer interface, river water and groundwater samples were collected and analyzed for dissolved organic carbon (DOC), nitrate (NO 3 ) and dissolved oxygen sat- uration (DO sat ). Our results illustrate highly variable hydrologic conditions during the monsoon season characterized by temporal and spatial variability in river-aquifer exchange fluxes with frequent flow reversals (changes between gaining and losing conditions). Intense monsoonal precipitation events and the associated rapid changes in river stage are the dominant driver for the observed riverbed flow rever- sals. The chemical data suggest that the flow reversals, when river water high in DOC is pushed into the nitrate-rich groundwater below the stream and subsequently returns to the stream may facilitate and enhance the natural attenuation of nitrate in the shallow groundwater. Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction The dynamic exchange of water, energy and solutes across the river-aquifer interface affects the ecology of river systems (Brunke and Gonser, 1997), pathways of nutrient cycling (Krause et al., 2009) as well as the transformation and attenuation of nutrients and contaminants (Smith et al., 2009; Zarnetske et al., 2011a,b). Across scientific disciplines interest in the dynamics of river-aqui- fer exchange and the transition zone between ground- and surface water where differences in chemical, biological and physical properties of the two adjoining compartments result in steep bio- geochemical gradients has steadily grown in recent years (Flecken- stein et al., 2010; Krause et al., 2013). River-aquifer interactions were found to have positive as well as negative effects on ground- water and stream water quality (Grasby and Betcher, 2002; Schmidt et al., 2011). High concentrations of contaminants in groundwater can significantly impact surface water quality and vice versa (Kalbus et al., 2007). Groundwater ecosystems often de- pend on infiltrating surface water that is rich in organic matter as an energy source (Madsen et al., 1991) for biogeochemical reactions. An important prerequisite to understand the transport of nutri- ents and contaminants across the river-aquifer interface and the resulting biogeochemical processes in the transition zone is to accurately characterize and asses the exchange fluxes at the riv- er-aquifer interface (Conant, 2004; Greenberg et al., 2002). A broad range of methods exists to quantify groundwater–surface water exchange fluxes (Kalbus et al., 2006). However, the extreme vari- ability of hydrologic conditions in monsoonal systems can make the use of many of them quite challenging. For example direct 0022-1694/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jhydrol.2013.12.005 ⇑ Corresponding author. Address: Department of Hydrogeology, Helmholtz Centre for Environmental Research – UFZ, Permoserstr. 15, 04318 Leipzig, Germany. Tel.: +49 (0) 341 235 1207. E-mail address: jan.fleckenstein@ufz.de (J.H. Fleckenstein). Journal of Hydrology 509 (2014) 601–614 Contents lists available at ScienceDirect Journal of Hydrology journal homepage: www.elsevier.com/locate/jhydrol