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The mass transfer between oxygen in the formation water and hydrocarbon in the plume will limit the rate of biodegradation in the"heart"of the plume At the leading edge of the plume, a zone of reduced ' oxygen and hydrocarbon concentrations will develop As the plume continues to move,the zone of reduced oxygen and hydrocarbon concentrations will increase in size and limit mass transfer In the third region,downstream of the contaminant plume, oxygen is present in excess and hydrocarbon will be absent ' or present at trace concentrations Sensitivity analysis with the one-dimensional model indicated that the various microbial parameters (Km, K, k,Y, F) had little or no effect on the hydrocarbon distribution in the body of the plume and on the t time to hydrocarbon breakthrough This suggested that the consumption of hydrocarbon and oxygen by microorganisms in the body of the plume(Region 2,Figure 1 1)can be approximated as an instantaneous reaction between oxygen and hydrocarbon In explicit finite difference form,this approximation is written as H(t+1)= H(t)-O(t)IF (7) ' 0(t+1)= 0 where H(t) > O(t)IF ' 0(t+1) =O(t)-H(t)•F (8) H(t+1) = 0 where O(t) > H(t) - F where H(t), H(t+1), 0(t), 0(t+1)are the hydrocarbon and oxygen concentrations at time t and t+1 Results obtained using the instantaneous reaction approximation were compared with the complete one-dimensional solution in Figure I 1 The instantaneous reaction approximation closely matched the I complete solution except in Region 1 The width of this region will depend on the mixing properties of the aquifer near the contaminant source,the ground water velocity and the nature of the contaminant If ground water velocities are very high or the hydrocarbon is poorly degradable,the area in which the instantaneous ' reaction assumption is not applicable may be significant 1 2 3 Two-Dimensional Simulations 1 12 31 Vertical 2-D Simulations Borden and Bedient(1986) conclude that vertical exchange of oxygen with the unsaturated zone ' could potentially result in significant fluxes of oxygen into a hydrocarbon plume and that this exchange would be most important for contaminants such as gasoline which occur at or near the water table Simulation results with the two-dimensional vertical model indicated that the effect of vertical exchange of 1 oxygen with the unsaturated zone can be approximated as a first order decay in hydrocarbon concentrations Sensitivity analysis with the two-dimensional model indicated that the vertical dispersion coefficient and the saturated thickness had the greatest impact on the first order decay rate 1.2 3 2 Horizontal 2-D Simulations Two-dimensional simulations generated plumes with similar characteristics to the one-dimensional simulations The most notable characteristic was the lack of lateral spread, i e,the plumes were long and narrow The HC plume has a somewhat lower peak concentration and is much narrower in the cross- 1-6