Abstract: The waterrock interaction(WRI) process is a stochastic one.The mathematical model of WRI is set up on the basis of principles of chemical therm-dynamics,which is solved by using the Monte-Carlo method.A case study is carriedout, in which some characteristic values,i.e.average one,standard deviation and coefficient of standard deviation,are used to reveal the WRI state and its variation within the domain.It is known from the result that within the system:(1)the related interactions may take place by themselves,(2) the relative large value of standard deviation shows the probability of the system approximate to equilibrium state,and (3) the acidification of groundwater is mainly produced by the oxidation of organic substances in shale.