Lars Bäthke

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The cumulative and bidirectional groundwater-surface water (GW-SW) interaction along a stream is defined as hydrological turnover (HT) influencing solute transport and source water composition. However, HT proves to be highly variable, producing spatial exchange patterns influenced by local surface- and groundwater levels, geology and topography. Hence, identifying factors controlling HT in streams poses a challenge. We studied the spatiotemporal variability of HT processes at a third order tributary of the river Mosel, Germany at two different stream reaches over a period of two years. Additionally, we sampled for silicate concentrations in the stream as well as in the near-stream groundwater. Thus, creating snapshots of the boundary layer between ground- and surface water where turnover induced mixing occurs. We characterize reach specific drainage behavior by utilizing a delayed/base flow separation analysis for both reaches. The results show a site-specific negative correlation of HT with discharge, while hydraulic gradients and reach scale absolute discharge changes correlating with HT only at the upstream site which is characterized by steeper hillslopes compared to the downstream section. Analyzing the variation of silicate concentrations between stream and wells shows that in-reach silicate variation increases significantly with the decrease of HT under groundwater dominated flow conditions.. In Summary, our results show that discharge shapes the influence of HT on solute transport as visualized by silicate variations. Yet, reach specific drainage behavior shapes seasonal states of groundwater storages and thus, can be an additional control of HT magnitudes, influencing physical stream water composition throughout the year.