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Mass benzene=(cone )(area)(thickness)(porosity)(pounds/eu ft) 68 x 10"9 x 750 ftz x 10 ft x 30 x 7 48 gal/ftp x 6 5 lbs /gal = 0 025 pounds, or 0 011 kilograms I Therefore, the computer model simulated a 24-hour release of 0 01 kilograms of benzene Hydraulic Conductivity and Gradient The hydraulic gradient at the site has been calculated numerous times over the course of the investigation The gradient varies over the site, but a value of 0 002 ft/ft is characteristic of the contaminated area To insure that the modeling result is not overly optimistic, a steeper hydraulic gradient of 0 004 ft/ft was input to the model No aquifer tests or hydraulic conductivity tests have been made at this site, so quantitative data on the hydraulic conductivity of the water-bearing sediment are lacking A typical value for medium-grained sand is 102 cm/sec, but qualitative information at this site (slow recharge rate in the sparging wells, excessive infection pressures required during downhole infection of the Oxygen Release Compound during site remediation) suggest that the hydraulic conductivity is less than the typical value (10-4 cm/sec is probably a more realistic number at this site) However,to insure that the results are conservative and compatible with the modeling program, the typical value was used The result of this substitution is to increase the groundwater flow rate above its true velocity Benzene Degradation Rate and Partition Coefficient A relatively low(slow) benzene aerobic half life biodegredation rate of 6,931 days was selected The benzene partition coefficient in water is a known constant I ' I , Other Parameters Other,characteristics, such as soil density and total organic carbon, are needed,by the I model to compute such parameters as retardation rates These characteristics were not I measured during any phase of site assessment, and were therefore estimated from similar I I sites I OUTPUT DATA EVALUATION ` 'Output data for each time step over the`9 5 year simulation period are presented in I , Appendix A and summarized in Table 2 Output data indicate a maximum predicted I benzene concentration of 138 ppb at the release location within the first six months after' 1 release Four the first four years,benzene remains localized within 10 meters I' (approximately 30 feet) of the source, and concentrations decrease steadily to less than 50 I I I ppb within three years The simulation predicts that benzene will increase to a, E concentration of 4 ppb at a distance of 20 meters Eby the tenth year, but will not be, , detectable at a distance of 20,meters(60 feet) from the;sourcelat any time Factor`s that ' �f�l lRi ' ` Illlr€I'{ rl!'I,f rlrl 0. ,[E1 ,I r'IM I``II'I 11 i ,� ` IF Ilii 411 1l'il I I II I`l 11 lil Ilii ' I II Oil`I' '� ' 7 , �1 I r �I +Iip l �Ilo h I 11 I a 'I'll II i " i t Ill Ill 11 1 II 11 l F'I illlrll II I III ill I�, I I I rE I I I I`rfll II�'Il'E'illt'tl'l it I'r I fIl II II V �l I II"I,tI< E�lll ll' r*`' k ` �f lir( 1 li s pip I 411 I I I III til II' 1111 ll J I I�l 11 id I�'I I I I I Il`I I I I S nl Ill ul I t I'