Evaluation Of Groundwater Quality And Simulation Of Hydrogeochemical Evolution In The Xianyang Section Of The Weihe River Basin,China

The Xianyang section of the Wei River basin is located in the middle of the Guanzhong Plain,which,as the core region of Shaanxi’s economic and social development,plays a pivotal role in the socio-economic development of western China.The water resources of the Guanzhong Plain are seriously unevenly distributed,with the southern part of the plain,in front of the Qinling Mountains,being more abundant,while the northern part of the plain,in and around Xianyang,is more scarce.In recent years,with economic development,the urbanization rate around Xianyang has been increasing and is facing a surge in population pressure,which has further led to a reduction in available land resources and an increased shortage of water resources.At the same time,the potential threat to groundwater has been exacerbated by the increasing discharge of wastewater due to the expansion of industrial production and the rising standard of living of residents.Therefore,it is imperative to evaluate groundwater quality in the Xianyang section of the Weihe River basin,analyze the sources of pollutants above the groundwater standard,carry out hydrogeochemical evolution simulations,and develop relevant groundwater protection policies and prevention measures.In this study,a comprehensive hydrogeological survey,field sampling,and indoor testing were carried out on the Xianyang section of the Wei River basin based on a thorough investigation of relevant literature and existing research progress in the study area at home and abroad.Based on a brief description of the physical geography,topography and hydrogeological conditions of the study area,the hydrochemical characteristics of groundwater were analyzed,and the entropy-weighted water quality index（EWQI）classification criteria were improved and innovated by selecting specific water quality indicators.The PCA-APCS-MLR model was then used to analyze the sources of the major groundwater constituents,and finally,a typical flow path was selected and the PHREEQC software was used to simulate the groundwater hydrogeochemical evolution process.The following conclusions were mainly obtained:（1）The groundwater in the study area is generally weakly alkaline and in an oxidising environment,with 10 of the analysed water chemistry indicators exceeding the standard limits.The groundwater chemistry in the study area is mainly of the HCO₃-Ca·Mg and HCO₃-Na types;the groundwater chemistry is dominated by rock weathering in the upstream areas of the north and south sides of the study area,while in the downstream sections it is influenced by the combined effects of evaporation concentration and rock weathering.Na⁺is mainly derived from the dissolution of rock salt and aragonite,Ca²⁺and Mg²⁺are mainly derived from the dissolution of calcite and dolomite,F^-is derived from the dissolution of fluoride-containing minerals,HCO₃^-and SO₄^2-are derived from the dissolution of sulphate minerals such as carbonate and gypsum,the amount of Cr⁶⁺in groundwater is related to the ion exchange between Ca²⁺and Na⁺and human activities,and NO₃-N is mainly influenced by by human activities.Groundwater in the study area is mainly recharged by atmospheric precipitation and surface water,and water rock action and evaporation concentration occur in the process of receiving recharge.（2）By selecting key water quality indicators to improve the EWQI water quality classification criteria,the results show that the improved classification criteria are better than the previous classification criteria in water quality evaluation.The improved EWQI was used to evaluate the groundwater quality in the Xianyang section of the Weihe River basin,and the results showed that the percentage of Grade I water samples was 0.92%,36.70%of Grade II water samples,52.29%of Grade III water samples,10.09%of Grade IV water samples and no Grade V water.Groundwater quality in the study area shows a clear spatial distribution,with groundwater quality in the plains on the south side of the Weihe River being better than that on the north side.This is mainly due to the fact that groundwater quality in the area north of the Wei River is influenced by the amount of groundwater extraction,agricultural activities,domestic wastewater discharge and the primary geological environment.（3）The PCA-APCS-MLR model was used to analyse the sources of groundwater exceedances in the study area and to quantify the contribution of different sources.The results show that there are five main sources of groundwater exceedances in the study area:firstly,hydrogeochemical evolution,secondly,agricultural activities,thirdly,primary geological environment,fourthly,urbanisation and industrial production,and fifthly,land cover change.The results of the spatial distribution of the principal components show that the five pollution sources are distributed in different areas,with hydrogeochemical evolution playing a more significant role near the banks of the Weihe River and in the area around Jingyang,the influence of the primary geological environment mainly on the northern bank of the Weihe River,the compound influence of agricultural activities and rock weathering distributed throughout the study area,and the influence of urbanisation and industry and land cover change mainly around the urban development circle.（4）In this study,reverse,mixed and forward simulations of hydrogeochemical evolution were carried out for a number of typical pathways.The results of the inverse simulation show that the dissolution of dolomite,gypsum,calcite,fluorite and rock salt mainly occurs in the study area,with calcite and dolomite reaching saturation in the lower reaches of the Weihe River,while gypsum and fluorite are basically in a dissolved state,and cation exchange of Ca²⁺,Mg²⁺,and Na⁺occurs from the starting point to the end point.The mixing simulation shows that when different proportions of surface water and groundwater are mixed,the lower concentration of the water chemical components dilutes the higher concentration of the water chemical components,and the change in the concentration of the chemical components affects the hydrogeochemistry between the ions;the forward simulation shows that under the consideration of continuous infiltration of surface water,the proportion of different water chemical components in the mixed water affects the magnitude of the simulated values,and the simulated values of different water chemical components in groundwater and surface water at different mixing proportions and different evaporation ratios are close to the measured values.The simulated values for different water chemistry components are close to the measured values for groundwater and surface water at different mixing ratios and evaporation ratios.