Distribution,migration And Transformation Of Cr(Ⅵ) In Groundwater Of Xianyang Region,china

During the past decades,groundwater pollution caused during economic development has often been neglected,leading to groundwater nitrate,organic matter and heavy metal contamination problems that exacerbate the conflict between groundwater supply and sustainable socio-economic development.In particular,the problem of heavy metal pollution is becoming more and more prominent in the process of urban development.Groundwater in the Xianyang District is an important water supply source for the region.In recent years,high concentrations of Cr（VI）have been found in the groundwater of the Guanzhong Basin,which is undoubtedly a great threat to the health of local residents.Therefore,in this paper,we analyzed the chemical characteristics of groundwater in which Cr（VI）is present in the Xianyang District by combining field sampling,indoor column experiments and simulations,distinguished the factors affecting the chemical composition of groundwater,and analyzed and discussed the migration and transformation processes of Cr（VI）by different influencing factors.（1）Groundwater in the Xianyang District has a more serious problem of Cr（VI）pollution.The exceedance rate of Cr（VI）in shallow groundwater is 44.23%,while that in deep groundwater is 27.59%.The high concentration of Cr（VI）water samples were often accompanied by higher TDS,Na⁺,SO₄^2-,and NO₃-N,while Ca²⁺and Mn contents were at low levels.The water chemistry type of shallow groundwater is complex,and the water samples with Cr（VI）concentration>0.05 mg/L are mainly of SO₄-Cl-Na type and HCO₃-Na type.The water chemistry type of deep groundwater is divided into HCO₃-Na type and HCO₃-Ca-Mg type,and the high concentration of Cr（VI）（>0.05 mg/L）water samples are mostly of HCO₃-Na type.（2）Groundwater chemical types and Cr（VI）concentrations have obvious spatial variation patterns.Groundwater Cr（VI）concentrations are characterised by a low west-east trend,with high concentrations mainly distributed in Qindu District,Weicheng District,Jingyang County,Sanyuan County,northern Xingping City and southern Liquan County in the south-eastern part of the study area.The trend of water chemistry type from west to east is HCO₃-Ca-Mg type→HCO₃-Na type→SO₄-Cl-Ca-Mg type→SO₄-Cl-Na type.Groundwater Cr（VI）concentrations in Qian County,Wugong County,Yongshou County,Chunhua County and southern Xingping City were low（≤0.05 mg/L）and the water chemistry type was mainly HCO₃-Ca-Mg type;high concentrations of Cr（VI）water samples in Sanyuan County and Jingyang County（>0.05 mg/L）were mainly SO₄-Cl-Na type water;high concentrations of Cr（VI）water samples in Liquan County,Qindu District and Weicheng District（>0.05 mg/L）were mainly of the HCO₃-Na type.（3）Natural processes such as rock weathering dissolution,Na-Ca exchange,evaporation,and human activities act together to influence the chemical composition content of groundwater,and the content of Cr（VI）and manganese in groundwater is also influenced by the redox environment.The results of cluster analysis showed that the water chemical constituents of different clustering groups were clearly distinguishable,and the spatial distribution pattern in the study area was obvious.The chemical composition of groundwater in loess hilly areas was not affected or less affected by human activities;the chemical composition of groundwater in loess plateau areas was affected by Na-Ca exchange,mineral dissolution and evaporation,and the ion concentration increased compared with hilly areas.The groundwater in alluvial plain areas of Jingyang County and Sanyuan County was significantly affected by human activities.The higher p H in groundwater,the exchange of Na⁺with Ca²⁺in the aquifer and the longer water flow path are favorable to the enrichment of Cr（VI）,and the industrial pollution discharge containing high concentration of Cr（VI）is also one of the main factors for the enrichment of Cr（VI）.（4）The experimental results of chromium migration transformation in soil column under different influencing factors show that the larger the p H,the more favorable the migration of Cr（VI）,which is mainly influenced by redox and adsorption.When the infiltration solution was acidic,most of the Cr（VI）was converted to trivalent chromium by reduction and adsorbed in the soil,resulting in a low concentration of Cr（VI）in the leachate.On the contrary,the alkaline infiltration solution could promote the oxidation of trivalent chromium to Cr（VI）,which reduced the adsorption of Cr（VI）by the soil.The presence of inorganic salt（sodium nitrate）promoted the migration of Cr（VI）,mainly influenced by ion-competitive adsorption,while the presence of organic matter was able to inhibit the migration of Cr（VI）through the reduction of Cr（VI）;the concentrations of Cr（III）in the leachate were all smaller due to the strong adsorption of the soil.（5）The main forms of Cr（VI）in groundwater are Cr O₄^2-,HCr O₄^-and Na Cr O₄^-.Among the Cr（VI）-containing minerals,the dissolution of Na₂Cr₂O₇ makes the greatest contribution to Cr（VI）in groundwater.p H increase can promote the dissolution of Na₂Cr₂O₇,K₂Cr₂O₇ and Cr O₃,thus increasing the concentration of Na Cr O₄^-and Cr O₄^2-in groundwater.An increase in nitrate concentration promotes an increase in Cr O₄^2-concentration and a decrease in HCr O₄^-and Na Cr O₄^-concentration.The dissolution of carbonate along the flow path affects the migration of Cr（VI）by changing the p H of the groundwater,and the higher the dissolution of carbonate,the more favourable the migration of Cr（VI）,and the increased concentration of NO₃^-also promotes the migration of Cr（VI）.