Spatial Distribution And Migration Enrichment Of As In Soil Of Karst Region In The Southwest Of Xingyi City

The karst region in southwest China is a typical geochemical anomaly region of As element.Besides human activities,the carbonate rocks deposits could be another significant factor for the geochemical high background of As in soil.Due to the special topography and the soil texture of the karst area,there may be spatial variability of As content in the small scale areas.In this paper,we conduct soil sampling on a small scale.The As anomaly polluted regions?typical karst region,semi-karst region?in southwestern Xingyi City of Guizhou Province was selected as the research objective,with the non-karst region as the control group.Geostatistical analysis was combined with GIS to study the spatial variability and pollution conditions of As in soil.The leaching loss ratio,enrichment coefficients and mass balance coefficients was used to study the migration transformation and the leaching degree of As in the soil profile and the weathering profile,and the following conclusions:?1?The arithmetic mean values of As content in the typical karst region soil and the semi-karst region soil is 52.1mg·kg^-11 and 33.5mg·kg^-1,respectively.In the meanwhile,the As content in the typical karst region and the semi-karst region is range from 14.5mg·kg^-11 to 140.0mg·kg^-11 and 10.1mg·kg^-11 to 105.5mg·kg^-1,respectively.The content of As in soil from these two regions is significantly higher than the background values of As in non-karst region.If the background value of As in the soil of Guizhou Province is used as reference,the over-standard rates of As in those two regions are 98.8%and 93.9%,respectively.Among these three afore-mentioned landform types,the results of independent sample T test shows that there is no significant difference of As content in soil?P>0.05?.?2?The Moran's I coefficient of As content in soil is 0.55,the Z value is 13.51,and the significance is greater than 1.96.Compared with the other three directions,the As content has significant positive spatial autocorrelation in the soil of the northeast-southwest direction?P<0.05?,which is both high value or low value spatial aggregation,and the structural variation was dominant.The As content in soil of typical karst region is mostly greater than 40 mg·kg^-1,while it is in the non-karst region is mainly less than 15mg·kg^-1.The spatial heterogeneity of As is positively correlated with lithology type,soil type and altitude,positively correlated with topographic relief.?3?The mean and variation range of As in the soil profile of typical karst region was 67.4 mg·kg^-11 and 60.6⁷2.0 mg·kg^-1,respectively.In this area,the water washing coefficient of As ranged from 0.48 to 0.50,and the migration pattern was positively exponential.The mean and variation range of As in the soil profile of semi-karst region was 38.9 mg·kg^-11 and 20.1³0.9 mg·kg^-1,respectively.In this area,the water washing coefficient of As ranged from 0.39 to 0.55,and the migration pattern was positively exponential.The enrichment coefficients of As in those two regions are less than the critical value of 2.0,which is no obvious enrichment.The content of As in different soil depths had a strong correlation?P<0.01?,and the correlation coefficient reached 0.685⁰.996.?4?The average content of As is 43.6 mg·kg^-11 in soil formed by different carbonate rocks,and the content of As in pure limestone and pure dolomite is higher than that of sandy limestone and sandy dolomite.There are two reasons for the high As content in the soil:one is the high background value of carbonate rock,and the other is the relative enrichment of As in the weathering of carbonate rock.The leaching degree of As is related to the rich and poor of mineral elements in the carbonate weathering profile,and the mass balance coefficient of As between different soil depths is 15.27%⁸9.59%.The As element has a strong positive correlation with Ca,Mn,P and S elements?P<0.01?,and it has a moderate positive correlation with Al element and pH?P<0.01?.In the meanwhile,the As element has a weak negative correlation with the Mg element?P<0.05?.?5?The potential ecological risk index indicated that the As samples with moderate ecological hazard and strong ecological hazard level in typical karst region accounted for 40.24%and 1.22%,respectively,while the moderate ecological hazard level in the semi-karst region accounted for 17.65%.The geoaccumulation index indicates that the As samples with mild pollution and light pollution grade in typical karst region accounted for 34.15%and 54.88%,respectively,and the two grades in the semi-karst region were 30.88%and 20.59%,respectively.Thus the typical karst region is stronger than the semi-karst region in the ecological hazard and accumulation of As.