Global random walk stochastic simulations for the assessment of the contamination risk in groundwater systems

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The predictability of contamination in groundwater systems will be investigated via stochastic simulations using the Global Random Walk method (GRW). To this aim, statistical ensembles of transport realizations, computed by GRW simulations on high performance parallel computers, will be used to estimate ergodicity ranges. The latter are defined by the root mean square deviation of actual observables from predictions provided by stochastic models and quantify the ergodicity in a large sense. The investigations will focus on pre-asymptotic regime and on memory effects induced by the nonlinear dependence of transport on the shape and dimensions of the initial solute plumes. Stochastic analyses based on Fokker-Plank and Itô equations will be developed and used for the interpretation of the stochastic simulations results. To increase the computational efficiency, the algorithm GRW also will be used as part of an ergodic simulations procedure. This approach exploits the ergodicity of the geo-statistical model of the groundwater velocity field with finite correlation range by inferring transport statistics from space averages over simulations done for a single velocity realization.