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16 2 3 31 Longitudinal Dispersivity ' The longitudinal dispersivity was vaned from 10 ft to 1 ft Figure 2 6 presents the variation in contaminant and oxygen concentrations along the centerline of the contaminant plume (section A-A. Figure 2 2) for three values of longitudinal dispersivity It can be seen that the longitudinal dispersivity also has a slight effect on biodegradation The maximum contaminant concentrations vaned from 29 6 mg/l(10 ft)to 41 7 mgll (1 ft) The change in biodegraded mass with longitudinal dispersivity is illustrated in Figure 27 ' 2 3 3 2 Transverse Dispersivity The transverse dispersivity was vaned from 1 ft to 5 ft Figure 2 8 shows the variation of contaminant and oxygen concentrations with transverse dispersivity along a transverse cross section through the centerline of the plume(cross section B-B, Figure 2 2) The transverse dispersivity does not seem to have ' an appreciable effect on biodegradation The areal extent of the plume is not very sensitive to the transverse dispersivity, however,the maximum concentrations exhibit a wide range of vanation The maximum contaminant concentrations vaned from 26 5 mgll(5 ft)to 35 5 mg/1(1 ft) The change in biodegraded mass with transverse dispersivity is illustrated in Figure 2 7 2 3 4 Variation of Concentrations with Porosity The porosity was vaned from 0 25 to 0 7 Figure 2 9 shows the variation of contaminant and oxygen concentrations with porosity along the centerline of the plume(section A-A, Figure 2 2) It is evident that porosity does not have a significant effect on biodegradation The maximum contaminant concentrations vaned from 29 0 mgll (n=0 25)to 35 0 mg/1(n=0 5) The change in biodegraded mass with porosity is illustrated in Figure 2 4b 2 3 5 Variation of Concentrations with Reaeration Coefficient The reaeration coefficient, k,was vaned from 0 0 day'to 0 005 day' Figure 2 10 presents the variation of contaminant and oxygen concentrations with k along the centerline of the plume (cross section A-A, Figure 2 2) It is evident that the coefficient of reaeration has a significant effect on biodegradation The areal extent of the contaminant plume as well as the maximum concentrations exhibit a wide range of variation with k The maximum concentrations vaned from 29 6 mg11 (k= 0 0 day)to 17 6 mgll (k= 0 005 day ) The change in biodegraded mass with k is presented in Figure 2 4a 2.4 Model Output Typical output from BIOPLUME II includes an oxygen and hydrocarbon distribution matrix at selected ' points in time (see Appendix B) These matrices can be plotted as contour plots(Figure 4 1) or surface plots using the SURFER graphics package (Golden Software, 1987) The graphics option in the BIOPLUME II preprocessor will transform the oxygen and hydrocarbon matrices to the required format for direct use in SURFER It can be seen from Figure 4 1 that whenever hydrocarbon is present in relatively high concentrations, then oygen is absent The oxygen plume forms an envelope for the hydrocarbon plume with oxygen concentrations gradually increasing to initial background levels as one moves away From the contaminant plume The model output also includes a mass balance computation for oxygen and hydrocarbon at the selected points in time The dissolved mass present in the system for each is computed, as well as the biodegraded mass The hydrocarbon mass balance computation details the biodegraded mass due to the ' different processes available in the model(aerobic, anaerobic, reaeration and radioactive decay) It is noted at this point that a detailed analysis of mass balance errors computed in BIOPLUME II is being performed for a variety of conditions and geometries The results of the analysis will be included in future ' updates to the manual I 2-5