AN EFFICIENT SCHEME FOR STOCHASTIC SIMULATION OF THREE-DIMENSIONAL VARIABLY SATURATED FLOW AND TRANSPORT WITH VOLATILIZATION

Many hazardous waste sites, including the Superfund sites and others, are contaminated with a number of chemicals which pose potential threats to human health and the environment. The potential exposure and associated health risks due to the contaminants are dependent upon the environmental transport mechanisms, as well as the pathways by which receptors may be exposed to the transported chemicals. For the groundwater pathway, particularly in shallow unconfined aquifers, the transport of volatile organic chemicals (VOCs) is of special interest because active migration of the contaminants could occur in the vapor phase, either volatilizing directly from the source or from the water table. The simulation of transport of contaminants in unconfined aquifers requires a solution of the variably saturated flow and transport equations which are highly non-linear, and subsequently computationally intensive. As modern risk assessment relies more and more on stochastic simulations to assist in quantifying uncertainty associated with the evaluation of contamination levels at receptor locations, it is imperative that an appropriate simulation methodology for flow and transport of VOCs, taking into account of the vapor- phase transport stochastically, be developed. This paper presents an approximate solution technique designed to minimize the computational efforts, while maintaining the accuracy of the solution. It is based on a composite technique which allows the flow and transport processes to be divided into three stages: (i) flow and transport in the vadose zone in the vicinity of the source or waste management unit, (ii) in the saturated zone, and (iii) in the vapor phase due to back volatilization at a distance away from the source or the waste management unit. The developed solution technique assumes that the waste management unit is rectangular, and that the vadose zone thickness, the saturated thickness, and material properties are uniform. Semi-analytical and numerical techniques are used to provide solutions to the three-dimensional flow and transport equations. Details of the solution procedures, verification and application examples, and substantial savings in computational efforts are discussed.