Hydrochemical Evolution Mechanism And Optimization Of Monitoring Network For Groundwater In Pearl River Delta

The geomorphic types of the Pearl River Delta are complex,and the Quaternary sedimentary thickness is relatively thin.There are many basement uplifts in the plain area of the Pearl R iver Delta,which block the groundwater flow field.At the same time,the river network is densely developed,and the aquifer is very cut by the river network.The above two reasons make the groundwater flow extremely complex.In the Pearl River Delta,there are high intensity domestic sewage and thousands of potential industrial pollution sources.Therefore,the groundwater environment in the Pearl River Delta is very complex?This paper takes the Pearl River Delta region as the research area,synthetically applies the multidisciplinary theory of hydrogeology,hydrogeochemistry and so on,The groundwater quality data were collected between 2016-2018 and the previous water quality data and results were also used.The groundwater chemical components,hydrochemical types and the spatial and temporal evolution characteristics of groundwater quality were analyzed by using mathematical statistics and graphic methods.Gibbs diagram,ion correlation analysis and multivariate statistics were used to explain the evolution mechanism of groundwater chemistry.The single factor evaluation method was used to evaluate the organic pollutants in groundwater and their sources.The driving factors of groundwater chemical composition evolution were qualitatively studied by correlation analysis.The following results were obtained:1.A hydrochemical profile from hilly area to coastal plain was established.The spatial evolution law of groundwater environment along the section was analyzed.In the section,the groundwater environment was generally acid,and it evolved from acidic to neutral along the section.And the p H value of near surface water increased.The salinity was generally low.HCO₃-Cl type water was most widely distributed.The mineralization degree decreased obviously with the change of recharge and drainage conditions in the uplift part of the basement,and gradually tended to the reduction environment.From hilly area to coastal plain area,groundwater environment tended to be reduced gradually.Groundwater pollution was increasing.In the vadose zone where the particles were coarse,the pollution sources were concentrated and the river channels were excavated,the detection rate of pollutants was high and the pollution concentration was also high.2.426 sites in the first round of groundwater pollution survey from 2005 to 2008 were selected.These points represent the different intensity of human activities and hydrogeological conditions in the study area.The changes of water quality before and after ten years was compared.The results showed that the groundwater quality was generally optimized,and the regional water quality standard rate increased from 30%to 49%.The proportion of HCO₃^-type water increased,the average concentrations of NO₂^-,F^-,PO₄^3-and Cr increased slightly,and other water quality indicators showed a better trend.In the delta plain area where the intensity of human activities was the most intense,the proportion of groundwater quality improvement was the highest.In hilly areas with poor anti pollution performance,the groundwater quality changes most acutely.3.Five principal components were extracted from the study area.Principal component 1(water rock interaction and seawater intrusion)explained the largest original variable(36.81%),principal component 2(dissolution of iron bearing minerals),principal component3(impact of agricultural activities),principal component 4(domestic and industrial sewage)and principal component 5(dissolution of fluorine minerals)accounted for 14.068%,9.16%,6.767%and 6.282%of the original variables,respectively.The results of R-type cluster analysis show that 10 hydrochemical indexes can be summarized as water rock interaction,seawater intrusion and agricultural activities;3 hydrochemical indexes(NH₄⁺,COD and NO^-₂)can be summarized as domestic and industrial sewage;3 hydrochemical indexes(p H,F^-,Fe and ORP),acid rain recharge and mineral dissolution(fluorine-bearing minerals and iron bearing minerals)dissolution.4.Acid rain,vadose zone characteristics and hydrogeological conditions were the main driving factors of groundwater acidification in the study area.Wastewater discharge,chemical fertilizer application and atmospheric deposition were the main driving factors of groundwater nitrogen pollution.Seawater,reducing environment,cation exchange and solution filtration were important sources of high ammonium groundwater.Redox conditions,acid-base conditions,groundwater runoff conditions,aquifer properties,salinity effect and human activities were the main factors driving the excessive Fe and Mn content in groundwater in the Pearl River Delta region.5.The main controlling factors of groundwater quality evolution in the study area in ten years are as follows:(1)the increase of rainfall is a main controlling factor of groundwater quality optimization.One is the rapid recharge and discharge of groundwater due to the increase of rainfall.The second reason is that the increase of runoff of main rivers caused by the increase of rainfall in the basin can weaken the influence of saltwater upwelling on plain river network.(2)The atmospheric environment and water environment treatment can promote the improvement of local groundwater environment.The intensity of acid rain decreased with the decrease of acid gas emission.The improvement of sewage treatment capacity has improved the water quality of major urban rivers.(3)Close to the polluted surface water,the accumulation of pollutants leads to the increase of pollutant concentration.The main reason for the increase of nitrate and other pollution indicators is the continuous increase of domestic pollution sources and intensity,and the gradual expansion of the scope.6.Based on the groundwater flow numerical model and groundwater pollutant transport model,the multi-objective simulation optimization model of groundwater monitoring was established.Different regions were divided according to the degree of groundwater pollution.The NSGA-? algorithm was used to determine the number of monitoring wells in the high groundwater pollution area;Finally,109 monitoring wells were determined in the high groundwater pollution area;On the basis of finding out the pollution source,the monitoring wells should be arranged in triangle along the direction of pollutant migration to ensure the maximum monitoring range.7.According to the hydrogeological conditions and the distribution of pollution sources in the Pearl River Delta,the anti pollution performance of groundwater system was evaluated.Combined with the analysis of the main pollution ways,the groundwater pollution model in the Pearl River Delta was generalized for the first time.In the transition zone between hills and plains,the main pollution paths were vertical infiltration,and in plain areas,the main pollution sources were diffuse pollution.The pollution pattern of the river downcutting area was lateral recharge.This paper put forward suggestions on pollution prevention and control in Pearl River Delta,which provided scientific basis for regional groundwater pollution prevention and control.