Field monitoring and numerical investigation on the regulation of a large dam structure on the patterns of lateral hyporheic exchange and residence time distributions - The Xinglong Water Conservancy Dam, China

Large in-stream structures such as dams can regulate the dynamic of the river stage, which potentially alters the patterns of hyporheic exchange flow (HEF) and mass transfer between the river and adjacent aquifer. However, current studies haven’t focused on how a large dam affects the evolution of HEF and residence time distribution (RTD), especially in the upstream area. In this study, we conducted the geophysical survey and groundwater stage monitoring of three monitoring transects around the XingLong Water Conservancy Dam (XLD) in China, which covers both upstream and downstream riparian areas. Based on these field data, a two-dimensional, horizontal numerical model was built to assess the spatiotemporal evolutions of lateral HEF, hyporheic zone (HZ) and groundwater RTD under the regulation of the XLD. The monitoring and simulation outcomes highlighted the different impact patterns of the XLD in upstream and downstream regions. For instance, the groundwater in the regions upstream of the dam was found to be recharged for the most of time, while the groundwater immediately downstream of the dam was significantly discharged. As the river stage fluctuates, the XLD significantly enhanced the HZ along the upstream river boundary whereas substantial HZ downstream was mainly observed in response to rising or high river stage. Furthermore, the XLD resulted in flow paths around the XLD with short lengths and high flow velocity, which consequently resulted in a significant HZ in the perimeter surrounding the XLD. Results of RTD show that water in the HZ downstream of the XLD was rejuvenated, whereas the HZ upstream of the XLD comprises a mix of aged and rejuvenated waters. These findings emphasized the need for full consideration of river stage dynamics surrounding the dam in analytical or numerical analysis when aiming to assess the bank storage and the aquatic environment in a dam-regulated river corridor aquifer, and have potential implications for the decision of the construction or removal of the dam, river restoration and purification of pollutants in aquifer.