MODELING FREE PRODUCT MIGRATION AND RECOVERY AT HYDROCARBON SPILL SITES

The numerical model ARMOS, which simulates areal flow of water and lig ht hydrocarbon in an unconfined aquifer, is described. Based on the as sumption of local vertical equilibrium, areal flow equations for water and hydrocarbon are derived which exhibit reduced dimensionality and nonlinearity. A finite-element method is used to solve the water and o il equations using an efficient semidecoupled approach. Input required by the model includes areal boundaries, elevations of the aquifer low er boundary, and initial water and hydrocarbon levels in monitoring we lls. Soil and fluid properties include hydrocarbon density, viscosity and surface tension, saturated hydraulic conductivity, van Genuchten a ir-water capillary pressure curve parameters, and the maximum residual hydrocarbon saturations in the saturated and unsaturated zones. Fluid heads or fluxes may be specified on the domain perimeter and pumping rates are prescribed at recovery wells. The water pumping rate is auto matically limited when drawdown reaches a pump-off set point (or the s creen bottom), and hydrocarbon recovery is limited when well hydrocarb on thickness becomes zero. Model output includes water and hydrocarbon levels in monitoring wells, cumulative product recovery, and free and residual hydrocarbon volumes in the soil. A hypothetical problem invo lving optimization of free product recovery and a field application of the model to a large pipeline leak are described.