Propagation of Seasonal Temperature Signals into an Aquifer upon Bank Infiltration by Nelson Molina-Giraldo 1 , Peter Bayer 2 , Philipp Blum 3 , and Olaf A. Cirpka 4 Abstract Infiltrating river water carries the temperature signal of the river into the adjacent aquifer. While the diurnal temperature fluctuations are strongly dampened, the seasonal fluctuations are much less attenuated and can be followed into the aquifer over longer distances. In one-dimensional model with uniform properties, this signal is propagated with a retarded velocity, and its amplitude decreases exponentially with distance. Therefore, time shifts in seasonal temperature signals between rivers and groundwater observation points may be used to estimate infiltration rates and near-river groundwater velocities. As demonstrated in this study, however, the interpretation is nonunique under realistic conditions. We analyze a synthetic test case of a two-dimensional cross section perpendicular to a losing stream, accounting for multi-dimensional flow due to a partially penetrating channel, convective-conductive heat transport within the aquifer, and heat exchange with the underlying aquitard and the land surface. We compare different conceptual simplifications of the domain in order to elaborate on the importance of different system elements. We find that temperature propagation within the shallow aquifer can be highly influenced by conduction through the unsaturated zone and into the underlying aquitard. In contrast, regional groundwater recharge has no major effect on the simulated results. In our setup, multi-dimensionality of the flow field is important only close to the river. We conclude that over-simplistic analytical models can introduce substantial errors if vertical heat exchange at the aquifer boundaries is not accounted for. This has to be considered when using seasonal temperature fluctuations as a natural tracer for bank infiltration. Introduction A surface-water body that infiltrates into an aquifer also influences the subsurface temperature regime. Tem- poral fluctuations of temperature have been recognized as a natural tracer that may be used for hydrogeological 1 Corresponding author: Center for Applied Geoscience (ZAG), University of T¨ ubingen, Sigwartstraße 10, 72076 T ¨ ubingen, Ger- many; (49) 7071 2973172; fax: (49) 7071 295059; nelson.molina- giraldo@uni-tuebingen.de 2 Engineering Geology, ETH Z ¨ urich, Sonneggstraße 5, 8092 Zurich, Switzerland. 3 Institute for Applied Geosciences (AGW), Karlsruhe Institute of Technology (KIT), Kaiserstraße 12, 76131 Karlsruhe, Germany. 4 Center for Applied Geoscience (ZAG), University of T ¨ ubingen, Sigwartstraße 10, 72076 T ¨ ubingen, Germany. Received March 2010, accepted July 2010. Copyright  2010 The Author(s) Journal compilation  2010 National Ground Water Association. doi: 10.1111/j.1745-6584.2010.00745.x interpretation (Suzuki 1960; Stallman 1965; Lapham 1989; Constantz 1998; Conant 2004; Anderson 2005; Blasch et al. 2006; Hatch et al. 2006; Hoehn and Cirpka 2006; Keery et al. 2007; Duque et al. 2010; Vogt et al. 2010, among others). Natural tempera- ture variations are attractive tracers, because they are intrinsic to the system. In contrast to synthetic tracers in well-to-well or river-to-well applications, no addi- tional injection campaigns are necessary, thus avoiding alterations of the natural flow regime and groundwa- ter chemistry (Constantz et al. 2003). Furthermore, none of the regulative constraints that may apply for con- ventional tracers, such as fluorescent dyes, have to be considered. Finally, groundwater temperature can easily be measured over long time periods in monitoring wells by means of relatively inexpensive thermometers, com- monly included in water-level loggers (Stonestrom and Blasch 2003; Anderson 2005; Ma and Zheng 2010). NGWA.org Vol. 49, No. 4–GROUND WATER–July-August 2011 (pages 491–502) 491