STOCHASTIC MODEL FOR MOBILE-IMMOBILE FLOW AND TRANSPORT

Mobile-immobile flow and transport can be modeled by two coupled partial differential equations, one for the mobile concentration and one for the immobile. Convolution in time with a memory function represents anomalous retention in the immobile zone. In this talk, we describe a new stochastic model that coincides exactly with the mobile-immobile equations with a memory function. The memory function is proportional to the probability tail of the random waiting time in the immobile zone. Hence, particle tracking can be used to solve the mobile-immobile equations with space-variable coefficients and a general memory function. Both classical and fractional advection-dispersion terms can be accomodated. An application to the MADE site illustrates the utility of this approach. The exact relationship between the mobile-immobile equations and CTRW (Continuous Time Random Walks) will also be discussed.