Investigating the hydrodynamic and biogeochemical evolutions of the hyporheic zone due to large-scale reservoir impoundment

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The impact of large reservoir construction on the basin water cycle, ecological environment, and engineering safety needs to be studied urgently, especially, the evolution of the hyporheic zone in the dam site area connecting the basin SW-GW. Few hyporheic zone research investigates the evolution of SW-GW interaction in a certain period of large reservoirs. The Xiluodu Reservoir, located in the Yangtze River's upper reaches, has a dam height of 285.5 m and covers a basin area of 454,400 km2. The isotope balance calculation of water samples demonstrates that following impoundment, the upper boundary, lower boundary, and extension width of the hyporheic zone expands from 370–395 m, 320–330 m, and 125 m to 560–600 m, 370–390 m, and 200 m, respectively. Meanwhile, the upstream increment is bigger than downstream increment. Both temperature tracing and hydrodynamic methods show that the hyporheic flow velocity increases about three times during the initial impoundment cycle and then gradually fallbacks. The hydrochemistry of hyporheic zone water, which changed significantly, was strongly related to the season, depth of burial, and distance perpendicular to river but not to distance along river. The biological activities in the hyporheic zone strengthened after impoundment and gradually changed from the initial impoundment nitrification-dominated to denitrification-dominated. The SW-GW interaction shifts from alternating upwelling and downwelling to continuous downwelling according to the computation of increasing NO3- flux. The study helps better understand scientific and technical concerns such as the evolution of the hyporheic zone, river valley contraction, and dam foundation leakage in the Xiluodu reservoir. Meantime, the thoughts and approaches would also be useful to other large reservoirs.