| River is an important part of riparian river ecosystem,and the interaction between river water and groundwater is the core content of water circulation in basin.Frequent fluctuation of river level will promote the exchange of river water and groundwater and change the hydrological condition of riparian zone.In this paper,the river water and groundwater in the upper and middle reaches of the Xilin River were selected as the research objects.Based on field experiments,hydrodynamic analysis and hydrochemical analysis,the spatiotemporal variation characteristics of the river water-groundwater interaction were studied.The main results are as follows:(1)Due to the seasonal influence of river hydrology,the exchange process of river water and groundwater in the upper reaches of the Xilin River varies significantly during the year.The exchange is stable in the horizontal period.The summer flood season is a sensitive period for the exchange of river water and groundwater because of the frequent fluctuation of river level and obvious change of hydraulic gradient.There is no exchange between the river water and groundwater during the frozen period.Due to the thawing of the permafrost in riparian zone lag behind the river opening in spring flood season,there is basically no exchange between river water and groundwater.Further analysis of the flow exchange in the upper reaches of the Xilin River during the summer flood season of the three years shows that the water level fluctuates less in 2018 and 2019,groundwater is mainly recharged from river.The vertical recharge of river water to groundwater and the lateral recharge to the left and r ight banks are 2.1×10-2?9.7×10-2 m/d,0.75×10-3?5.13×10-3 m/d and 0.7×10-2?3.0×10-2 m/d,respectively.In 2020,frequent rainfall and water level fluctuation changed the exchange direction between river water and groundwater.The vertical and lateral exchange capacity between river water and groundwater was-1.6×10-2?4.6×10-2 m/d,-8.00×10-3?1.72×10-3 m/d(left)and-3.5×10-2?10.8×10-2 m/d(right).(2)The hydrochemical types in the upper and middle reaches of the Xilin River are HCO3--Ca2+·Mg2+.The ions in the upper reaches are derived from the weathering and dissolution of carbonate rocks and calcium-containing minerals.In addition to the above natural factors,human activities have also led to an increase in the ion concentration in the water.Under the influence of evaporation and concentration and river level,TDS concentration of river water and groundwater in normal period is higher than that in flood season.The TDS concentration of groundwater at riverbank 7 m away from the river is higher than that of the river water and the groundwater at 12 m away from the river.It is speculated that the mixing of river water and groundwater at nearby point leads to a strong biogeochemical reaction.During the monitoring period,the range of TDS concentration of the upper reaches of the Xilin River was 238?395 mg/L,higher than the average concentration of rivers in the world.The average TDS concentration of the middle reaches of the river in 2019-2020 is 1196 mg/L,and the degree of salinization is high,which needs to be paid attention to.(3)From 2018 to 2019,there was significant exchange between river water and groundwater at 7 m from the river bank and 1.3 m below the river bed,while there was no significant exchange at the riverbank 12 m from the river.In 2020,there was significant exchange between river water and groundwater even at 12 m on both banks.Frequent rainfall and fluctuation of water level increase the intensity of exchange between river water and groundwater,making the boundary of river water and groundwater exchange move far away.The hydraulic connection between river water and right bank is stronger than that between river water and left bank.In the summer flood season,the mixing ratio of river water and groundwater within the range of7 m from the river varies from 48.8%?70.5%,while in the normal flood season,the mixing ratio decre ases to 45.8%?49.1%. |
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