资源环境科技发展态势分析平台

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 (STR) 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.