Ecological water replenishment and groundwater response in Chaobai River in spring

The middle and upper reaches of the Chaobai River alluvial fan is the area with the most abundant groundwater resources in Beijing,and it occupies an important role in the urban water supply.Due to over exploitation for many years,the groundwater level has decreased continuously and formed a large groundwater level drop funnel.At the same time,it has caused a series of environmental problems such as ground subsidence and groundwater pollution.At the end of 2014,after the middle route of the South-to-North Water Transfer Project was opened officially,it not only eased the shortage of water supply in Beijing,but also provided an opportunity for groundwater conservation.Since April 30,2021,the Chaobai River basin implemented the first large-scale experimental water replenishment in 22 years.The replenishment of water was an exploratory practice to promote the comprehensive management and ecological restoration of the river basin.It is also an important part of improving the ecological environment of the river basin and promoting the comprehensive management of the overexploitation of groundwater in the river basin. The research scope was determined by analyzing the hydrogeological conditions in the Chaobai River area borehole data,replenishment items,drainage items and long-term observation hole data.By the Unstructured Grid (USG) module embedded in GMS 10.0 to refine the replenished water channels and water sources,a long-term groundwater flow model from January 2000 to December 2020 was established.To evaluate model accuracy,the determination coefficient (R2) and root mean square error (RMSE) of evaluation factors was taken as the evaluation criteria for parameter optimization of the evaluation model.After identification and verification of the model by the observation data,the identified model was applied to the spring ecological water replenishment of Chaobai River to evaluate the effect of ecological water replenishment and groundwater response. The results show that the water level around the replenishment path has risen at the end of the replenishment period.The water level in the case of replenishment minus the water level in the case of no replenishment.The maximum increase is close to 18 m.The area that the water level has a rising value greater than 0 is the impact area in this replenishment.It is calculated that the affected area is about 842.8 km 2,accounting for 58.6% of the total area of the study area.The groundwater level near the mainstream of Chaobai River rises more than 10 m,and the affected area is about 44.2 km2,accounting for about 3%.During the water replenishment period (April 30-May 27,2021),the total water replenishment volume is 206 million m3,and the groundwater replenishment by water leakage is 157 million m3 (approximately 76% of the total water volume).The storage capacity of the system has increased by 149 million m3 in the first aquifer of the study area.Obvious delay in the groundwater levels in observing wells with responses after water replenishment,and the lag response time of observation wells at different locations of the river channel is different.The water level of observation wells that are 3 km away from the river channel rises obviously,and the lag response time is short (3-6 d and the lag response time of observation wells that are 6 km away from the river channel is longer (18-30 d). The implementation of ecological water replenishment projects in the Chaobai River basin was conducive to the recovery of water levels on both sides of the river and the restoration of groundwater resources in the region.After replenishing water,the water level rose significantly and the groundwater storage was increased within the water replenishment period.Overall,the replenishment effect was good.There was a time lag in the response of groundwater to replenishment,and the lag response time varies with distance.

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