A simple thermal mapping method for seasonal spatial patterns of groundwater–surface water interaction

Abstract

Summary A simple thermal mapping method for simulating seasonal and spatial patterns of groundwater–surface water interaction is developed and tested for a segment of the Aa River, Belgium. Spatially distributed temperature profiles in the hyporheic zone of the river are measured in winter and summer seasons of three consecutive years. Inverse modeling of the one-dimensional heat transport equation is applied to estimate vertical advective fluxes using the numerical STRIVE model and an analytical model. Results of the study show that seasonal flux estimates for summer and winter can be derived with a minimum data input and simulation effort. The estimated fluxes are analyzed via non-parametric statistical tests, while spatial interpolation techniques are used to generate maps of distributed flux exchange. The estimated seepage is compared with volumetric flux obtained from piezometer measurements and output of a groundwater model. The thermal method shows higher discharge rates in winter and that the relative contribution of exfiltration to the river discharge is higher in summer. A higher flux and a more heterogeneous flow pattern are observed in the upper reach of the river compared to the lower reach. This spatial difference shows the importance of the local geomorphology and to a lesser extent the hydrogeologic setting on hyporheic flux exchange in the river. A significantly higher flux is noted on the banks than in the center of the river, which is driven by the relatively high hydraulic conductivity of the river banks. It is concluded that bank flow in groundwater–surface water interaction deserves more attention. The main channel of the Aa River alone accounts for about 15% of the total river discharge at its outlet. As the developed thermal method is cost-effective, simple and fast, it is recommended for use in identifying zones of interest in initial stages of field investigations of groundwater–surface water interaction.