| The detection rate and over-proof rate of Cr(?)concentration in groundwater of the research area have increased in recent years.Considering this phenomenon,the paper has investigated the formation mechanism and evolution of Cr(?)pollution.Based on the actual situation of the exploitation and utilization of groundwater in the research area,we analyzed the groundwater level,hydrochemical type,primary components including SO42-arnd Mg2+,as well as the spatiotemporal evolutional process of Cr6+,and explored the relationship between the groundwater environment evolution and Cr(?)Pollution,then we studied the plane and profile distribution characteristics of concentration and existence states(including water soluble state,exchange state,carbonate bound state,Fe-Mn oxide bound state,organic-bound state,residual state)of total chromium and hexavalent chromium in non-saturated zones and saturated zones of loose sediment soils,and studied theirs correlation with Cr(?)pollution in groundwater,as well as the effect factors and characteristics which resulted migration and transformation of chromium in loose sediments of unsaturated zones.Finally,we used the hydrogeochemical modeling software Phreeqc to simulate the water-rock interaction.Our data shows that the groundwater level of the research area in 2010 had an average decreasing value of 12.2m compared to that in 1997;concrete variation of hydrochemical type was that Ca type groundwater disappeared,the proportion of Na type decreased and while that of Mg type increased,the MEQ of SO42-increased significantly.Total hardness was generally enlarged and the largest increase amplitude is 789mg/kg.Contents of NH4-N?Cr6+?SO42-?and Mg2+ were all increased,the increase of SO42-?Mg2+ concentration occurred at the place where the groundwater level decreased greatly.Generally,the total chromium contents of the unsaturated zone are low,and most of them changed from 22.01 to 50.63 mg/kg,with an average value amount of 30.99mg/kg,less than the total chromium contents of Yuncheng basin dryland field and irrigated field.Feature of the plane distribution is that it is high in the southern area and low in the northern area,the relatively highest site is Fenhe River's.southern three terraces and the relatively lowest site is Fenhe River's northern two terraces.The distribution of hexavalent chromium characterized in that it is high in the western area,followed by the eastern area and then by the southern and middle area.The content ranged from 3.95 to 24.98 mg/kg,with an average value amount of 8.88 mg/kg.The area containing the highest hexavalent chromium contents was located at Yellow River floodplain and lower terraces,while Wucun and Xiaoliang villages had the lowest content of hexavalent chromium.The unsaturated zone in the research area showed no features of being polluted by chromium.The order of various existence states of chromium according to their contents was:residual state>exchangeable state>Fe-Mn oxide bound state>carbonate state>organic bound>water soluble state.The order of various existence states of hexavalent chromium according to their contents was:residual state>Fe-Mn oxide bound state>organic state>water soluble state>exchangeable/carbonate fraction.Hydrogeochemical modeling results show that the pH value,NH4+,Mg2+and SO42-concentration of groundwater after the water-rock interaction showed a positive relationship with the Cr6+ concentration.When the contents of these factors increased,the Cr6+ concentration in water would increased accordingly.Based on our research:Excessive extraction of groundwater has changed hydrogeological conditions of the study area,and eventually caused increasing Cr(?)pollution in the groundwater. |
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