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Page 9 <br /> January 30, 1992 <br /> Job No 400511-SA <br /> 1 <br /> VW-33 The horizontal extent of TPHg is defined by the non-detect (<0 5 ppm) levels in MW-28 and <br /> MW-30 On Figure 7, the horizontal extent of TPHg is defined by the non-detect (<0 5 ppm) levels <br /> 1n MW-29 and VW-36 The hydrocarbons>n the soil have also been vertically defined by the non-detect <br /> levels which occur below the soil plume <br />' 5.4 Occurrence and Extent of Hydrocarbons in the Groundwater <br />' Groundwater samples were collected from monitoring wells MW-27 through MW-30 on October 3 <br /> and 4, 1991 The groundwater samples were analyzed for TPHg by the DHS method and for BTXE <br /> by EPA Method 8020 The laboratory reports are summarized in Table D, and are presented 1n <br /> Appendix E Dissolved concentrations of TPHg and benzene were below the laboratory detection limits <br /> 1n MW-27 and MW-28 Monitoring well MW-30 had the highest concentration of TPHg at 340 ug/l <br /> and benzene at 38 ug/l Figure 8 illustrates the extent of the dissolved benzene plume 1n groundwater <br /> beneath the southwest corner of the site <br /> 6.0 CONCLUSIONS <br /> Petroleum hydrocarbons have accumulated in sandy soil at the level of a former groundwater table <br /> located approximately 51 feet below the ground surface Both the horizontal and vertical extent of <br />' hydrocarbons in the soil have been defined by this study The existing vapor extraction system has been <br /> effective m reducing the concentration of petroleum hydrocarbons in the soil, as reported in 1985 and <br /> 1 1989,by removing the volatile components and by aiding the m situ biodegradation of the hydrocarbons <br /> Concentrations of petroleum hydrocarbons dissolved in the groundwater at the site are generally low <br /> twith the highest concentration of TPHg being 340 ug/1 and benzene at 38 ug/l in MW-30 The <br /> groundwater gradient is very slight and directed towards the southwest The plume of benzene M <br />' groundwater is also oriented toward the southwest <br /> I <br /> 4005111 Rev 1 Du Pont Envtronâ–ºnental Remedtahon Services <br /> Printed on Recycled Paper <br />