Characteristics Of Arsenic Accumulation And The Causes Of Abnormal Enrichment In Cultivated Soil In Typical Karst Areas In Central Guizhou

Karst area is the main distribution area with high levels of heavy metal geological background in cultivated soil in China.Many studies have shown that the background value of arsenic（As）in the soil formed by carbonate rocks（limestone and dolomite）in this area is high.Excessive As accumulation in soil may pose risks to ecology and human health.In this study,198 topsoil（0-20cm）samples and 9 soil profiles were collected from typical karst areas in central Guizhou.By analyzing the content and speciation of As in topsoil,deep soil and rocks,the enrichment characteristics of As in soil were explored.The main factors and sources of As enrichment in soil were revealed through correlation analysis,PMF model,chemical alteration index（CIA）,etc.The conclusions reached are as follows:（1）The accumulation of As in the surface soil of the cultivated land in the study area is different,and the distribution is heterogeneous.The As concentration in the weathered soils of the Triassic carbonate rocks（12.2-113 mg/kg）are higher than that in the weathered soils of the Permian carbonate rocks（7.04-52.7 mg/kg）.The vertical distribution of As content in the cultivated soil profile is quite different.The average As concentration（124.60 mg/kg）in the weathered soils profile of the Triassic carbonate rocks is higher than that in the soil profile weathered in the Permian carbonate rocks（27.11 mg/kg）.（2）The results of As content in the rocks show that the Permian Maokou Formation limestone（6.61 mg/kg）is smaller than the Triassic Anshun Formation dolomite（9.57 mg/kg）.Soil weathering strength,p H,OM and constant oxide content all have an influence on As enrichment.The CIA results indicate that the chemical weathering of the soil profiles in this region are strong.The results of correlation analysis show that the content of As in the weathered soils profiles of the Triassic carbonate rocks are negatively correlated with p H and OM,and negatively correlated with the content of Si O₂and Ti O₂,positively correlated with Al₂O₃,TFe₂O₃,Mg O,K₂O.In particular,the TFe₂O₃content affects the adsorption of As in strongly weathered soils.（3）PMF model results show that the main sources of heavy metal pollution in the study area are daily activity sources（8.81%）,agricultural activity sources（19.65%）,transportation sources（25.56%）,natural parent material sources（26.24%）,and industrial activity sources.（20.03%）.The natural parent material source of As（60.97%）in the weathered soils of the Triassic carbonate rocks is higher than that in the weathered soils of the Permian carbonate rocks（43.90%）,indicating that the enrichment of As in the regional soil is mainly of geological origin.（4）The results of As speciation analysis indicate that As in the weathered soils profile of the Triassic carbonate rocks are mainly stable crystalline hydrous oxides（As F4）and the residual fraction（As F5）,with low bioavailability and low risk pollution level.The results of the potential ecological risk index show that the ecological risk in the weathered soils of the Triassic carbonate rocks（E_rⁱ=19.05）is greater than that in the weathered soils of the Permian carbonate rocks（E_rⁱ=9.67）.The contents of As in crop samples（tea,plum and corn）are not exceed the corresponding crop pollution limit,indicating that the accumulation of As in the regional soil had no effect on the As enrichment of crops.In conclusion,the accumulation degree of As in the weathered soil of the Triassic carbonate rocks is greater than that of the Permian,and its abnormal enrichment is mainly derived from the weathering deposition of the mother rock.The results of this study will provide scientific basis for the prevention and control of regional soil As pollution.