Advection vs. conduction - Heat and water transport in a lowland river under groundwater up-welling conditions

Groundwater-surface water interactions and the resulting variability in temperature gradients and dynamics have an influence on both physicochemical processes (such as reactive transport of nutrients or contaminants) and habitat conditions. This study uses high resolution observations of vertical hydraulic gradients and interstitial pore water temperatures to investigate spatial patterns and temporal dynamics of aquifer-river exchange fluxes and streambed thermal properties of a pool-riffle-pool sequence of a UK lowland river. Process hypotheses gained from this dataset are tested with the help of physically based heat transport modelling. The results of our investigations indicate that, although groundwater is dominantly up-welling in the research area, exchange flow patterns are strongly influenced by the streambed geomorphology. Advective heat flux caused by groundwater up-welling is shown to have a moderating impact on interstitial temperature patterns and partly compensates the impact of conduction of diurnal surface water temperature fluctuations into the streambed. Consequently, diurnal temperature oscillations, which are clearly pronounced in the top 10 cm of the streambed (up to 2 °C), are reduced by 90% at depths below 20 cm. Modelling results confirm the importance of heat conduction despite the up-welling conditions. As up-welling is found to be highly variable in space this creates a mosaic of temperature gradients with depth and at the same time results in spatially heterogeneous propagation of diurnal temperature dynamics into the sediment.