Research On Evolution And Optimal Regulation Of Shallow Groundwater Based On Water Quality And Quantity

With the advancement of urbanization,many countries and regions have experienced water shortages and water environment pollution.Especially for the northern region of our country,duo to the lack of surface water,over 34%of the water is supplied by groundwater..Because groundwater is buried deep below the surface,it has the characteristics of stable water supply and excellent water quality,but it also prone to pollution,and it is difficult to control after pollution.Especially for shallow groundwater,because it is directly recharged by atmospheric precipitation and surface runoff,it is more susceptible to the impact of surface pollutants.At the same time,due to the advancement of urbanization,industrialization and population growth,the intensity of human exploitation of groundwater has been increasing,resulting in over-exploitation of groundwater and continuous decline of groundwater table.Due to the continuous decline of the groundwater table,on the one hand,the recharge path of atmospheric precipitation is prolonged,and leaching of minerals in the vadose zone is intensified.On the other hand,due to overexploitation,the oxidation-reduction environment in the formation has changed greatly,causing the deterioration of the groundwater environment.This paper takes the shallow groundwater in Daxing District,a typical area in the southern plain of Beijing,as the research object,and conducts research in three aspects:(1)Under the condition of longterm groundwater exploitation,what is the evolution law of groundwater table,exploitation intensity,water quality and hydrochemical characteristics in the study area.(2)According to the evolution law of groundwater,how to formulate a reasonable groundwater utilization plan.(3)How to optimize and control the groundwater quality and groundwater table in wet year,normal year,dry year according to the formulated groundwater utilization plan,so as to realize the improvement of groundwater quality and the recovery of groundwater table.Focusing on the above three questions,the main research results obtained in this paper were as follows:(1)The evolution evaluation model system of groundwater table,exploitation intensity,water quality and hydrochemical characteristics was constructed,and dynamic evolution law of groundwater in the study area was studied.Taking the groundwater depth data of 32 long-term monitoring wells in the study area from 1986 to 2020 as samples,based on the seven interpolation methods in the ArcGIS geostatistics module,the ordinary kriging interpolation method was selected as the best interpolation method for groundwater depth in the study area.Based on the geostatistical variation function model,the nugget effect value of groundwater depth from 1986 to 2020 was calculated,and it was found that spatial correlation of groundwater depth showed a gradual weakening trend from 1986 to 2020.The nugget effect value was the largest from 2006 to 2016,was 27%,indicating that the human groundwater exploitation activities had the greatest impact on the spatial correlation of groundwater depth from 2006 to 2016.Based on set pair analysis and subtractive set pair potential,the main factors causing the increase of groundwater exploitation intensity in 2006～2016 were quantitatively evaluated and identified.The evolution trend of groundwater quality in the study area from 2006 to 2016 was analyzed using partial connection number.The results showed that:① NO3-index mainly involved Ⅰ-Ⅳ types,total hardness index mainly involved III-V types,and total dissolved solids index mainly involved Ⅲ-Ⅳ types;② The water quality grades of comprehensive water quality,NO3-,total hardness and total dissolved solids in the study area showed an increasing trend,indicating that the groundwater quality in the study area was in a deteriorating trend.Using hydrochemistry,principal component analysis,partial connection number and other methods,the spatiotemporal trends of the shallow groundwater chemical characteristics and influencing factors in the study area were analyzed.The results showed that:① The cations and anions in the groundwater in the study area increased with time,indicating that the groundwater quality had a trend of deterioration.② With the change of NO3-concentration,the hydrochemical types,anion and cation components of groundwater would also change accordingly,indicationg that the evolution of groundwater chemical components was closely related to NO3-concentration..(2)According to the law of groundwater dynamic evolution,reasonable groundwater exploitation schemes were formulated and the optimization of water resource allocation schemes were selected.According to the dynamic evolution law of groundwater,the groundwater environment in the study area would continue to deteriorate under the current exploitation mode.To change the existing problem of groundwater overexploitation,the water supply of surface water,reclaimed water,and transferred water should be increased under the premise of ensuring water safety and water consumption.For areas with serious groundwater pollution,measures should be taken to reduce the mining output of groundwater or prohibit the exploitation,so as to gradually reduce the pollution after selfpurification and fresh rainwater replenishment,so as to achieve the minimum cost of treatment and the best effect of groundwater pollution treatment.Therefore,we selected the water quality data of surface water and groundwater in the study area from 2012 to 2016.analyzed the applicability of their water quality,set up three groundwater exploitation modes according to the principle of quality-based water supply,and optimized the allocation of water resources,and finally selected the allocation of water resources based on the improved connection entropy.The results showed that,in accordance with the principle of quality-based water supply,under the three groundwater exploitation modes,the utilization rate of groundwater and NO3-pollution decreased significantly after optimization compared with before optimization.(3)According to the formulated groundwater utilization plan,the groundwater quality and water table in wet year,normal year and dry year were optimized and regulated,and the groundwater quality was improved and the water table was restored.By collecting groundwater table and water quality data in the study area from 2006 to 2020,under the existing exploitation model,the water demand in the study area from 2021 to 2025 was predicted.Through the combination of GIS and machine prediction model,the NO3-concentration of groundwater monitoring points in the study area in wet year,normal year and dry year was predicted.According to the pollution of NO3-,three groundwater exploitation modes were set up to optimize the allocation of existing water resources in wet year,normal year and dry year,respectively.By comparing the water shortage,groundwater utilization rate,and NO3-pollution under different exploitation modes,the optimal exploitation mode for different hydrological years was determined.According to the optimal exploitation model,the changes of groundwater pollution and water table before and after the optimal regulation were simulated in wet year,the normal year and the dry year,respectively.The results showed that the improvement of groundwater quality and restoration of water table in the study area had been achieved by optimizing and regulating groundwater in wet year,the normal year and the dry year.