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9.3 Impact of Residual Hydrocarbons on Beneficial Uses <br /> Residual petroleum concentrations In soil at this site appear to be limited to the vicinity of the <br /> former tank location Horizontal migration from this site has been minimal as observed in <br /> surrounding vapor extraction well soils Vertical migration beneath the farmer tank location <br /> was originally noted to a maximum depth of 40 feet below grade Following a series soil <br /> remediation activities, residual diesel is found in an isolated zone beneath the former tank <br /> excavation only, from 10-20 feet below grade <br /> Diesel concentrations have significantly decreased from a maximum of 7,100 ppm in 1988 to <br /> 900 ppm in 1994 No detectable volatile petroleum hydrocarbon concentrations are <br /> encountered No impact to groundwater has been noted based on analytical results from the <br /> onsite (apparently downgradient) water well Based on the current soil analytical data it <br /> appears that minimal residual diesel fuel remains in onsite soils and it is unlikely that this <br /> residual will migrate and effect the beneficial uses of groundwater It is likely that the <br /> residual diesel fuel concentrations will continue to decrease through natural biodegradation <br /> 10.0 Conclusions and Recommendations <br /> The Pacific Bell site contained a 1,000 gallon underground storage tank (diesel) which was <br /> removed in 1988 Upon removal of the tank a release of diesel to the subsurface was noted <br /> A series of soil remediation activities with subsequent soil verification sample collection has <br /> been conducted at the site The results of the soil remediation activities were primary <br /> source removal, disposal of approximately 100 tons of impacted soil (original excavation and <br /> subsequent overexcavation), reduction of volatile petroleum hydrocarbon concentrations to <br /> below detection levels and a decrease of in place residual diesel fuel concentrations from a <br /> maximum of 7,100 ppm in 1988 to 900 ppm in 1994 The areal extent of diesel impacted <br /> soils has also been significantly decreased. <br /> No evidence of to impact r <br /> p groundwater exists The distance between the remaining diesel <br /> impacted soils and groundwater is greater than 100 feet Results of a leaching potential <br /> analysis indicate that a maximum in place diesel concentration of 1,000 ppm should not pose <br /> a significant threat to site groundwater The volatile petroleum constituents of fuel (benzene, <br /> toluene, ethyl benzene, and xylenes) are the components generally of concern in fuel <br /> remediation projects, as they pose the greatest health threats (carcinogenic), their vapors can <br /> be flammable if they are concentrated and they have the greatest leaching potential Based on <br /> MZ/01-27-95/UST/95-0011 nbr 13 <br />