Study On The Hydrogeochemistry And The Circulation Of Shallow Groundwater In The Jianjiang Alluvial Fan

The Jianjiang Alluvial Fan is located in the northwest of the Sichuan Basin,where is the transition zone between the Chengdu Plain and the Longmen Mountains,is one of eight medium-large alluvial fans on the west side of the Chengdu Plain.Due to its unique water-bearing structure,abundant groundwater reserves,and good water quality,it has become an ideal area for human beings to live,engage in agricultural productions and industrial activities.In recent years,as the dramatic development of urban and rural economies,the impact of anthropogenic activities on its groundwater has become more and more intense.And it may cause such a series of environmental problems as the groundwater resource depletion,the water levels decline,and the water quality deterioration.However,we cannot find anyone research or report specifically for the groundwater in this area.Thus,understanding and studying the current status of groundwater environment quality,chemical characteristics,and circulation pattern not only has an important practical value for the reasonable utilization,protection and sustainable development of groundwater resources in this region,also has a reference significance for the hydrogeochemistry and the groundwater circulation researches in other alluvial fans.This article relies on the"Evaluation of Groundwater Environmental Impact of West Sichuan Gas Field Productivity Construction Project",based on analyzing the geological and hydrogeological conditions of the Jianjiang Alluvial Fan thoroughly,we have used descriptive statistics,multivariate statistics,GIS spatial analysis,ion ratio method,forward calculation method,and 3D-simulation to study the chemical characteristics and circulation patterns of the quaternary shallow groundwater in this region.Finally,we have got some results and understandings as below.?1?The chemical type of groundwater in the study area show the law of the transition from upstream HCO₃ to downstream HCO₃·SO₄.The fan top area is HCO₃-Ca type water,and the downstream is mainly HCO₃·SO₄-Ca type water.The spatial variability coefficients of main anions and cations in groundwater are 28.6%to 74.6%and 22%to 49.5%,respectively,all showed moderate variability.Except for HCO₃^-,which is relatively high in Danjingshan Town where is located on the alluvial fan top,all ions showed a certain degree of enrichment near the Gexianshan Town in the north of the study area.Trace ions show higher variability than main ions,with coefficients of variation from 54.5%to 270.7%,and NO₂^-,NH₄⁺,and Mn are relatively enriched near in Gexianshan Town,and Fe is more enriched between Longfeng Town and Lichun Town.Besides,the TDS concentration is generally low,and its distribution has a certain relationship with the distribution of cultivated land and urban construction land.?2?The single index evaluation results show that 84.81%of the groundwater in the study area meets Class III drinking water quality standards,and Class IV and V waters,which are not suitable for human drinking directly,account for 13.57%and1.62%,respectively.As the pollution factors?NO₃^-,Mn,and Fe?all are non-absolutely controlled impurities and the evaluation results are difficult to truly reflect the overall status of groundwater quality.Therefore,the Bayesian groundwater quality evaluation method was used to comprehensively evaluate the water quality.The results show that the groundwater in the study area does not exceed the type III quality standard,but it still shows the characteristics of the groundwater quality of the south area is better than the north.?3?The qualitative analysis results of water chemistry show that the groundwater in the study area has been affected by human activities.Other factors controlling the chemical characteristics of groundwater include the carbonate dissolution,the silicate dissolution,and the atmospheric rainfall input.The average contribution rates of these factors to groundwater cations are 75.74%?carbonate dissolved?,21.97%?silicate dissolved?,1.49%?input from human activities?,and 0.8%?input from atmospheric rainfall?,respectively.?4?By establishing a 3D flow model,the groundwater circulation of the study area is under the control of the surface water,the topography,and the lithology.Therefore,it can be divided into three different patterns:the Yazihe-Xiaoshihe pattern,the Xiaoshihe-Shihezi pattern,and the Shihezi-Tuqihe pattern.These patterns can be summarized as"three sources and four sinks,good runoff","two sources and three sinks,strong runoff",and"four sources and three sinks,good runoff",respectively.