Abstract: Hydrochemical variations caused by the 15April 2015 MS5.8Alxazuoq earthquake were discussied based on the ratios of hydrogen and oxygen isotopes and ion concentrations of the spring waters from the fault zones around the Hetao basin in western Inner Mongolia.17water samples were collected from the springs and wells in the Wulashan fault zone,Serteng fault zone,Langshan fault zone and Zhuozishan fault zone.The hydrochemical data attained indicated that the TDSvalues of the waters ranged from 143.8to 42553.0mg/L,which can be classified into 12chemical types using the Schukalev hydrochemical classification method.The water samples in the Wulashan fault zone were characterized by low TDS and Ca-HCO₃ type,showing an origin of atmospheric precipitation. The waters in the Seerteng and Langshan fault zones were enriched in HCO^-₃ and SO^2-₄with higher TDS,resulting from water-rock reaction in Cretuceous aquifer.The waters collected from the Zhuozishan fault zone were characterized by higher TDSand enrichment of SO^2-₄ and Cl^-,caused by mixing water with coal bed.Values ofδD andδ¹⁸O ranged from -83.6‰to 66.56‰ and-11.16‰to 8.2‰,respectively,indicating the waters were mainly meteoric origin.Ratios of ion concentrations between the samples of 2015and the samples of 2015as well as milliequivalent ratio,such asγ(Na+K)/γCl,γ(SO⁴+Cl)/ γHCO₃,γHCO₃/γCl,can better reflect the earthquake activity.After the April 15,2015 MS5.8Alxazuoqi earthquake,the TDS values of the waters in the Hulusitai and Diyanaguimiao areas shown a slight decrease trend while the proportion of HCO^-₃ in anion increased,which might result from mixture of low TDS water in the shallow aquifer. However,TDSvalue of the water(S12)from the deep well increased obviously and the ratio ofγ(Na+K)/γCl decreased,indicating mixing of higher TDS water from the deep strata.The results provide a hydrochemical background of the study area and are in favor of seismic monitoring and forecasting.

Key words: Ion concentration, H and O isotopes, Hydrochemical precusor, Hot spring