The arsenic pollutants generated by the arsenic industries and mining enterprises in karst area flow into subterranean streams and contaminate groundwater easily because of the unique hydrogeological charac-teristics of karst area,which is a serious threat to the water ecologic security and local residents’health.In order to elucidate the geochemistry reaction mechanisms of arsenic in karst subterranean streams,the Lihu subterranean stream in Nandan county,Guangxi Province,Southwest China,is selected for the study.The main outcropped lithology in the Lihu subterranean stream catchment is Carboniferous-Triassic limestone and the karstification rate is 31.67 %.Arsenic species,sediment physical-chemical properties and hydro-chemical characteristics of the water are analyzed by inductively coupled plasma mass spectrometer (ICP-MS),X-ray fluorescence spectrometer (XRF)and plasma spectrometer (ICP)respectively.The results show that arsenic concentration in the Lihu subterranean stream is high and it is closely related to human activities.The con-tent of As (III),higher than As (V),occupies 53 % of the total inorganic arsenic in the Lihu subterranean stream due to the low redox environment,which increase the arsenic ecological risk in this region.The con-tents of total arsenic and As (III)as well as As (V)are attenuated by 5 1 %,36 %,59 % respectively in the underground river.Correlation coefficient analysis calculated with SPSS indicates that sediment Fe,sediment Ca,sediment organic matter,sediment Mn and water Ca2 + are the positive factors that promoting the deposi-tion of arsenic from water.While water Cl- ,SO 2 -4 and HCO -3 ,negatively correlated with arsenic in the wa-ter,do not facilitate to absorb arsenic.Compared with other water bodies,such as Quaternary groundwater, surface rivers and lakes waters,Ca and HCO -3 turn out to be the important factors for mobilization and transformation of arsenic due to the high calcium and bicarbonate content in karst water.Therefore,karst hydrogeological characteristics should be considered during arsenic contamination treatment in karst ground-water.