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04 January 2000 <br /> AGE-NC Project No. 95-0106 <br /> Page 2 of 5 <br /> • 16 February 1998: advancement of three soil probe borings to collect soil and ground water <br /> samples and installation of a ground water monitoring well (MW7) on the adjacent property <br /> east of the subject property, conducted during May 1998; and <br /> • 02 December 1998: proposed advancement of up to 10 soil probe borings to collect soil and <br /> ground water samples for laboratory analysis of the contaminants of interest and <br /> physicochemical and microbiological state of selected samples;in addition,one probe boring <br /> was to have been continuously cored. The work plan addendum was approved, but <br /> reconsidered by the PHS-EHD,with comments and suggestions conveyed telephonically to <br /> AGE personnel on 27 August 1999. The scope of work of the 02 December 1998 work plan <br /> addendum has not been implemented. This revised work plan addendum incorporates the <br /> comments and suggestions of the PHS-EHD and replaces the 02 December 1998 work plan <br /> addendum. <br /> A total of 15 soil borings have been advanced on and off the site, seven of which have been <br /> completed as ground water monitoring wells,providing data concerning hydrocarbon impact on soil <br /> and ground water, soil composition, and ground water flow direction. Laboratory analysis of soil <br /> samples collected from monitoring wells and soil probe borings constrain the plume of impacted soil <br /> to the immediate vicinity of the former UST excavation. The lateral extent of impacted ground water <br /> appears to be delineated northeast, east, south, west and northwest of the former UST excavation, <br /> but laboratory analysis of ground water samples collected north of the former UST excavation, from <br /> well MW4,have consistently reported total petroleum hydrocarbons quantified as gasoline(TPH-g), <br /> benzene, toluene, ethylbenzene and total xylenes (BTEX). Over the last nine monitoring events, <br /> benzene and TPH-g concentrations in ground water samples collected from MW4 have averaged <br /> 107.3 micrograms per liter(,ug/l) and 2,466.6 gg/l, respectively. Benzene concentrations exceeding <br /> the California Maximum Contaminant Level (MCL) for drinking water of 1 µg/1 is a concern to <br /> human health and ground water in a residential area. Lower concentrations of petroleum <br /> hydrocarbons have been detected periodically in samples collected from MW1, located <br /> approximately 25 feet north-northeast of the former UST excavation. <br /> The occurrence of TPH-g and BTEX in ground water encountered in an off-site, down-gradient well <br /> (MW4) indicates that dissolved hydrocarbons have migrated north of the site. The lateral extent of <br /> the impacted ground water has not been delineated in all directions. The plume is generally <br /> constrained on the west by monitoring well MW-5, on the northeast and east by MW-6 and MW-3, <br /> respectively, and on the east and south by MW-7 and MW-2, respectively. However, the plume is <br /> not constrained north and west of MW-4. <br /> Two potential migration routes for the hydrocarbon-impacted ground water are suggested by site <br /> geology and the impacted ground water encountered in well MW4: <br /> Advanced GeoEnvironmental.Inc. <br />