Quantifying groundwater-surface water interaction by using heat as a natural tracer

Groundwater-surface water interaction is an important process in riverine landscapes, wetlands and lakes. It links groundwater and surface water bodies and therefore controls the exchange of energy, solutes, particulate matter and organisms between the surface and the subsurface. Driven by potentials in hydraulic head, the magnitude and distribution of groundwater-surface water interaction is influenced by topography, hydrogeology and climate. The mixing and transition zone between groundwater and surface water, the so-called hyporheic zone is characterized by sharp environmental gradients. The hyporheic zone is a focal point of ecological transport, transformation and retention processes, which are influenced by the magnitude and direction of groundwater-surface water exchange. Hence, the estimation of the exchange of water is an important prerequisite for the determination of ecological transport processes. The natural temperature distribution in the subsurface is determined by a combination of temperature variations at the surface and groundwater flow. Therefore heat can serve as a natural tracer, known as the 'thermal method'. As an indirect method, the thermal method uses the measured temperatures in connection with inverse heat transport modeling to quantitatively estimate groundwater-surface water exchange fluxes. Since it also influences physical and biological processes, temperature is also an important parameter for ecological research. In this thesis I apply the thermal method to quantify groundwater-surface water interaction and its spatial and temporal distribution. Spatially distributed temperature profiles in the hyporheic zone were measured over several years in an acid mine lake in Germany, the Biebrza River, Poland and the Aa River, Belgium. I show that transient thermal influences are negligible in the summer and winter season where the temperature distribution in the subsurface can be seen as being in steady state. In such times the simple and fast thermal steady-state approximation for heat transport modeling can be applied. During transitional seasons (autumn and spring) the more elaborated transient mode is needed to model groundwater-surface water interaction fluxes. Seasonal and spatial patterns of groundwater-surface water interaction are investigated for a segment of the Aa River. Results from the thermal method and statistical tests indicate higher discharge rates in winter but a higher relative contribution of exfiltration to the river discharge in summer. Because of differing hydrogeology and hydromorphology the bank areas and the upper part of the examined section show higher exchange fluxes than the downstream part. The main channel of the Aa River alone accounts for about 15% of the total river discharge at its outlet. This thesis proves that the thermal method can be applied on rivers dominated by organic soils (Biebrza River case study). For such heterogeneous sites a hierarchical approach to investigate and interpret groundwater-surface water interaction is presented. Different field methods including piezometer Heat as a tracer for groundwater-surface water interaction nests and seepage meters are combined with the thermal method to overcome limitations of a single method. It was found that a classification of the varying thermal and physical properties of the subsurface is required to derive realistic flux estimates in such a heterogeneous environment. Results show