Heat transport of diurnal temperature oscillations upon river-water infiltration investigated by fiber-optic high-resolution temperature profiling

Abstract

River-water infiltration is of high relevance for hyporheic and ripariangroundwater ecology as well as for drinking water supply by river-bankfiltration. Heat has become a popular natural tracer to estimateexchange rates between rivers and groundwater. However, quantifyingflow patterns and velocities is impeded by spatial and temporal variationsof exchange fluxes, insufficient sensors spacing during field investigations,or simplifying assumptions for analysis or modeling such as uniformflow. The objective of this study is to investigate local heat transportupon river-water infiltration in the riverbed and the adjacent riparianzone of the losing River Thur in northeast Switzerland. Here we haveapplied distributed temperature sensing (DTS) along optical fiberswrapped around three tubes to measure high-resolution temperatureprofiles of the unsaturated zone and shallow groundwater. Diurnaltemperature oscillations were tracked in the subsurface and analyzedby means of dynamic harmonic regression to extract amplitudes andphase angles. Subsequent calculations of amplitude attenuation andtime shift relative to the river signal show in detail vertical andtemporal variations of heat transport. In addition, we apply a numericaltwo-dimensional heat transport model for the unsaturated zone andshallow groundwater to get a better understanding of the observedheat transport processes in the riparian zone. Our results show thatheat transfer of diurnal temperature oscillations from the losingriver through groundwater is influenced by thermal exchange withthe unsaturated zone. Neglecting the influence of the unsaturatedzone would cause biased interpretation and underestimation of groundwaterflow velocities. In addition, the observed riparian groundwater temperaturedistribution cannot be described by uniform flow, but rather by horizontalgroundwater flow velocities varying over depth. © 2011 Author(s).