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The exchange of water and solutes between the stream and groundwater along a stream network affects the source composition of discharge and solute load in the stream. To date, this hydrologic turnover has only been analyzed with respect to the exchange of water. In this study, we extend the concept of hydrologic turnover to solutes and analyze the effects of different hydrologic conditions on the spatial patterns and magnitude of exchange fluxes. Based on a coupled stream‐groundwater model built in MODFLOW using the Streamflow‐routing package, we simulated stream‐groundwater exchange along a stream of 30 km in length and evaluated the evolution of stream water source composition under different precipitation and streamflow conditions. Results show that even for highly variable hydrologic conditions, the direction of stream‐groundwater exchange (loss/gain) remained unchanged in consistently gaining or losing reaches, but changed in interspersed transitional reaches. The comparison between the source composition of discharge and nitrate load in stream water revealed a decoupling of discharge and solute load contribution from groundwater to the stream, as zones of high water gains do not necessarily coincide with zones of high solute concentrations. Overall, nearly 80% of total groundwater contributions to the outflow of water from the catchment were generated over only 20% of the total stream length. Our research highlights the importance of distinguishing water and solute load contributions to streams. This in turn implies that to reduce groundwater‐borne nutrient loads to streams, measures need to focus on the specific reaches with highest load contributions. Plain Language Summary: Streams and groundwater are connected and continuously exchange water and solutes. For example, the inflow of groundwater into streams not only increases streamflow, but also influences the chemical composition of stream water if the groundwater contains different solutes than the stream. Therefore understanding the spatial patterns of water and solute contributions from groundwater to the stream is important for the management of stream water quality. Taking the lower Selke River in central Germany as an example, we investigated the influence of stream gains and losses on the source composition of discharge and solute loads along the stream network as well as the influence of different precipitation and streamflow conditions. We found that higher precipitation and the associated elevated groundwater recharge or lower stream flows can increase the total length of gaining reaches, decreasing the relative importance of the source water from the upper stream. Due to the heterogeneous distribution of groundwater and solute gains, the magnitude of solute contribution from groundwater to stream can be quite different from the associated discharge contribution. However, both water and solute contributions are typically dominated by focused groundwater gains along only a few key reaches. Key Points: Across a wide range of hydrologic scenarios, at least 37% of the stream length is characterized by losing conditionsHeterogeneous solute concentrations in groundwater induce a different solute source composition from stream water source compositionMost of the groundwater contributions to stream water are generated over only a small fraction of total stream length [ABSTRACT FROM AUTHOR]