Abstract
Abstract. River-water infiltration is of high relevance for hyporheic and riparian groundwater ecology as well as for drinking water supply by river-bank filtration. Heat has become a popular natural tracer to estimate exchange rates between rivers and groundwater. However, quantifying flow patterns and velocities is impeded by spatial and temporal variations of exchange fluxes, insufficient sensors spacing during field investigations, or simplifying assumptions for analysis or modeling such as uniform flow. The objective of this study is to investigate lateral shallow groundwater flow upon river-water infiltration at the shoreline of the riverbed and in the adjacent riparian zone of the River Thur in northeast Switzerland. Here we have applied distributed temperature sensing (DTS) along optical fibers wrapped around tubes to measure high-resolution vertical temperature profiles of the unsaturated zone and shallow riparian groundwater. Diurnal temperature oscillations were tracked in the subsurface and analyzed by means of dynamic harmonic regression to extract amplitudes and phase angles. Subsequent calculations of amplitude attenuation and time shift relative to the river signal show in detail vertical and temporal variations of heat transport in shallow riparian groundwater. In addition, we apply a numerical two-dimensional heat transport model for the unsaturated zone and shallow groundwater to obtain a better understanding of the observed heat transport processes in shallow riparian groundwater and to estimate the groundwater flow velocity. Our results show that the observed riparian groundwater temperature distribution cannot be described by uniform flow, but rather by horizontal groundwater flow velocities varying over depth. In addition, heat transfer of diurnal temperature oscillations from the losing river through shallow groundwater is influenced by thermal exchange with the unsaturated zone. Neglecting the influence of the unsaturated zone would cause biased interpretation and underestimation of groundwater flow velocities. The combination of high resolution field data and modeling shows the complex hydraulic and thermal processes occurring in shallow riparian groundwater close to losing river sections as well as potential errors sources for interpreting diurnal temperature oscillations in such environments.
Highlights
The biogeochemical and ecological processes at the river – groundwater interface depend on the exchange between both water bodies (Boulton et al, 1998; Hayashi and Rosenberry, 2002)
The temperature time series of each 5 mm depth interval are analyzed by dynamic harmonic regression for the frequency 1/d
We included the unsaturated zone into the dynamic harmonic regression analysis, to illustrate their temperature dynamics, even if the signal from the land surface is the main source of heat input for the top sediments of the unsaturated zone
Summary
The biogeochemical and ecological processes at the river – groundwater interface depend on the exchange between both water bodies (Boulton et al, 1998; Hayashi and Rosenberry, 2002). These exchange fluxes include water, solutes, and heat. Various studies have shown that hyporheic flow patterns are complex, with details depending on the exact pressure distribution at the riversediment interface, the (micro)morphology of the riverbed, the spatial distribution of hydraulic conductivity within the riverbed sediments, and the mean water exchange rate between the river and the aquifer From a strict groundwater perspective, some details of hyporheic flow, such as the return
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