Vertical Hyporheic Exchange and Nitrogen Transport and Transformation in Prairie Meandering Rivers
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摘要: 潜流带是地表水与地下水之间发生水分和物质交换的关键区域，研究河流潜流带氮素转化对于改善河流水质，维持水生态系统稳定具有重要意义.为探究半干旱区草原曲流河潜流带内的氮素迁移转化过程，以锡林河弯曲河段为研究对象，结合水动力学和水化学法，分析了潜流带内的垂向潜流交换模式及氮素转化特征.结果表明：非降雨时期垂向潜流交换以微弱的上升流为主，平均水力梯度为-0.023.降雨和上游来水可能导致交换方向逆转，出现下降流，平均水力梯度为0.086.垂向潜流交换速率随深度增加而递减，河床表面以下20、50和100 cm处的平均交换速率分别为0.102、0.041和0.017 m·d-1，100 cm是垂向交换的下边界.潜流带中可能存在氨化、硝化、反硝化及异化还原反应，50 cm是热点反应深度，出现生物地球化学梯度的逆转.草原曲流河的垂向潜流带是NO3-的汇，且对NO3-的去除作用存在空间差异，深层沉积物的去除作用强于浅层.上升流和下降流条件下的NO3-平均去除率分别为34%和28%.曲流河段顶点处相对于入流及出流处，其垂向交换较弱，而氮素浓度较低，可能是曲流驱动下生物地球化学反应发生的热点位置.Abstract: The hyporheic zone is a critical interface between surface water and groundwater where water and materials exchange actively. The study of nitrogen transport and transformation in the hyporheic zone of rivers is of great significance for improving river water quality and maintaining the stability of water ecosystems. To investigate the nitrogen transport and transformation processes in the hyporheic zone of prairie meandering rivers in the semi-arid region, a meandering reach of the Xilin River was taken as the research object, and the vertical hyporheic exchange patterns and nitrogen transformation characteristics in the hyporheic zone were analyzed by combining the hydrodynamic and hydrochemical methods. The results show that the vertical hyporheic exchange is dominated by a weak upwelling flow with an average hydraulic gradient of -0.023 during the non-rainfall period. Rainfall and upland water can lead to a reversal of the exchange direction and an appearance of downwelling flow with an average hydraulic gradient of 0.086. The vertical hyporheic exchange rate decreases with depth, with the average exchange rate at 20, 50 and 100 cm below the surface of river bed is 0.102, 0.041 and 0.017 m·d-1, respectively, the depth of 100 cm is the lower boundary of vertical exchange. Ammonification, nitrification, denitrification and dissimilatory reduction reactions are likely to occur in the hyporheic zone, with the depth of 50 cm being the hot spot where the reversal of biogeochemical gradients occurs. The vertical hyporheiczone of prairie meandering rivers is a sink for nitrate. The removal effect on nitrate indicates spatial differences with stronger removal rate in deep sediments than in shallow ones. The average removal rate of nitrate were 34% and 28% under upwelling and downwelling conditions, respectively.Compared with the inflow and outflow points, the vertical exchange at the apex of the meandering reach is relatively weak, and the nitrogen concentration is low. The apex may be the hot spot of biogeochemical reactions in the hyporheic zones driven by meanders.
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Key words:
- meandering river /
- hyporheic exchange /
- nitrogen transformation /
- nitrogen sink /
- riverbed /
- groundwater /
- hydrogeology
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图 1 研究区概况（a）及监测井布设（b）
Fig. 1. Overview of the study area (a) and the layout of monitoring wells (b)
图 2 监测期内河水流量（a）及垂向水力梯度随时间的变化(b~f)
Fig. 2. Changes of surface water discharge (a) and vertical hydraulic gradient during monitoring periods (b-f)
图 3 水化学指标浓度在河水及潜流带内的垂向分布
Fig. 3. Vertical distribution of concentrations of hydrochemistry indicators within the river and hyporheic zone
图 4 上升流和下降流条件下硝酸盐的产生和消耗
Fig. 4. Nitrate production and consumption under upwelling and downwelling conditions
图 5 曲流河段不同点位的垂向水力梯度及氮素浓度的垂向分布
Fig. 5. Vertical distributions of hydraulic gradients and nitrogen concentrations at different positions in the meandering reach
图 6 锡林河垂向潜流交换及其氮素迁移转化的概念示意图
Fig. 6. Conceptual schematic diagram of vertical hyporheic exchange and nitrogen transport and transformation in the Xilin River
表 1 水化学参数在河水及潜流带中不同深度孔隙水中的平均值及标准偏差
Table 1. Mean concentrations and standard deviations of hydrochemical parameters in river water and in pore water at different depths in the hyporheic zone
水化学指标 T
(℃)EC
(µs·cm-1)SAL Cl-
(mg·L-1)DON
(mg·L-1)NH4+
(mg·L-1)NO3-
(mg·L-1)NO2-
(mg·L-1)DO
(mg·L-1)DOC
(mg·L-1)pH 河水 21.8±2.71 341.71±35.68 0.10±0.00 7.61±0.74 3.29±2.22 1.95±1.87 3.57±2.05 0.42±0.43 7.74±0.36 19.43±7.47 8.149±0.24 孔隙水 20 cm 19.37±2.70 396.00±32.15 0.10±0.00 13.24±3.95 2.77±1.18 1.52±1.14 3.85±2.32 0.50±0.45 3.24±1.59 18.06±4.83 7.629±0.42 50 cm 18.08±4.00 408.29±36.44 0.10±0.02 12.35±3.36 3.03±2.29 1.25±1.11 3.97±2.49 0.35±0.30 0.96±0.79 18.75±7.87 7.701±0.10 100 cm 19.13±3.90 367.00±14.40 0.10±0.00 9.42±0.96 2.79±1.75 1.32±0.88 3.06±2.34 0.31±0.22 0.94±0.62 13.64±7.72 7.697±0.15 注：表中所列数值为5组监测管，7次分批取样所获得的共112个样品的实测浓度的平均值及标准偏差. 
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