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Human Health Risk Assessment <br /> Farmer Mobil Oil Sulk Plant 04-343 <br /> SOU East Grant Line Road <br /> Tracy, California <br /> velocity of approximately 11.2 feet per year, it would take approximawly 9.8 to 11.2 years for <br /> ground water to migrate from the vicinity of onsite monitoring wells to the nearest offsite <br /> 1 monitoring wells. It should be noted that this estimate does not represent the amount of time <br /> required for dissolved-phase chemicals to potentially migrate to offsite, monitoring wells. The <br /> potential rate of migration of dissolved-phase chemicals is likely to be less than the average <br /> rlinear ground water velocity due to sorption, degradation, and dilution. <br /> The approximate time that petroleum hydrocarbons may have contac.ed ground water is not <br /> known. However, it is known that offsite down di <br /> gra ent ground water monitoring wells have <br /> ' not contained detectable concentrations of petroleum hydrocarbons during the period of this <br /> investigation. Furthermore, the concentration of benzene detected in onsite ground water <br /> monitoring wells has historically been low (i.e., less than 10 ppb). If any migration of <br /> ' dissolved-phase hydrocarbons (e.g., benzene) were to occur, it would ue reasonable to assume <br /> that the concentrations would decrease with distance from the site. .:n light of the distance <br />' between onsite and offsite ground water monitoring wells it is likely '0 at dilution, attenuation, <br /> and biodegradation would serve to reduce benzene concentrations to oelow detectable levels. <br /> As a result, Alton Geoscience would not anticipate the presence of dew-c.table concentrations of <br /> benzene in offsite monitoring wells. The continuation of the ground water monitoring and <br /> sampling program discussed in section 1.3.3.1 would, over time'-,'pro-We additional information <br /> useful to quantitatively evaluate the attenuation, dilution, and degradation factors for the subject <br /> site. <br /> 1 <br /> i <br /> 1 <br /> 30-0136-11 <br /> 2-4 <br /> 1 <br />