Immiscible Contaminant Transport in Soils and Groundwater with an Emphasis on Petroleum Hydrocarbons: System of Differential Equations vs Single Cell Model

Abstract

Soil and groundwater contamination resulting from leaks of petroleum products, from faulty storage tanks and pipelines has become a recent environmental concern. A generalized mathematical model, incorporating the physical, chemical and biological processes which collectively describe the transport of a reactive and immiscible contaminant in soils and groundwater is presented. The problem is one of multiphase transport, that is, the contaminant can be transported as solutes in water, vapors in air and as unreacted constituents in an immiscible phase. Additionally, it may be adsorbed onto the soil surface. Conservation principles lead to a system of nonlinear partial differential equations governing the phenomenon. As an alternative, a single cell model as a simplified version of this generalized system is also presented. The whole system is represented by a single element. Such an approach yields conservative estimates of a single constituent contaminant as only advective solute transport is allowed. The model has been applied to study the fate and transport of individual hydrocarbon constituents in an unsaturated zone. The results predict the concentrations of each hydrocarbon in all phases in space and time allowing the user to estimate the amounts of hydrocarbons which enter the underlying aquifer or which leave the soil via volatilization into the atmosphere. Such predictions are presented for selected cases. Comparisons of the single cell model with the generalized one are also given.