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1 <br /> -L � <br /> b <br /> I <br /> 10 March 2000 <br /> AGE-NC Project No 97-0312 <br />' Page 3 of 6 _ <br /> 1 <br /> 2 4 1 OVERVIEW OF SESOIL 4 r <br /> The American Petroleum Institute's (API) Risk/Exposure Assessment Decision Support System <br /> (DSS) is a modeling program developed to estimate human exposure and risk from conta:mmated <br /> sites Several modeling options are contained in DSS, including SESOIL and AT123D SESOIL uses <br /> information obtained from the site assessment of contaminants,hydrogeological, and meteorological <br /> information to estimate the mass of contaminants that will reach ground water over a given time <br /> 1, frame AT123D can be used to determine contaminant concentrations in a receptor well located <br /> down-gradient from a contaminant source AT123D can use the information generated by SESOIL <br />' as the input data <br /> k <br /> Chemical properties and health risk data for 26 chemicals of possible concern are supplied in the <br />' program including benzene,toluene,ethylbenzene and xylenes Total petroleum hydrocarbons(TPI <br /> is not an individual chemical with definite properties, and therefore is not included on the list of <br /> chemical choices Output can be displayed in several ways, including annual reports, tables and <br />' graphs <br /> 2 4 2 SITE SPECIFIC CONDITIONS <br />' The two areas of greatest concern, based on data collected from the site and cross sections, were <br /> identified and examined, the former UST cluster location and the area near monitoring well UST-5 <br /> Cross sections of the impacted soil plume were depicted in Quarterly Report-September 1999 Both <br />' areas were divided into four layers,based on hydrocarbon concentrations The soil profile beneath <br /> the site is reasonably vertically homogeneous, consisting primarily of silt <br /> t <br />' Mass calculations for the first area were based on data from samples collected in the UST cluster <br /> excavation, soil stockpile samples and boring B14 The top layer consists of the stockpiled soil <br /> returned to the excavation and non-impacted fill dirt from surface grade to 4 6 meters (15 feet), the <br />' second layer is impacted soil from 4 6 to 6 7 meters (15 to 22 feet), the third layer is slightly- <br /> impacted soil from 6 7 to 10 4 meters (22 to 34 feet), and the fourth layer is the non-impacted soil <br /> from 10 4 to 15 0 meters (34 to 49 feet) The area of impacted soil was arbitrarily defined as 5 meters <br />' by 5 meters with all mass calculations based on this area <br /> The mass of contaminants in the upper-mast section was based upon the average concentrations of <br />' soil stockpile samples for the soil placed back into the excavation Mass calculations for the second <br /> layer were based on the highest concentrations detected in the UST excavation and boring B14 <br /> samples BTEX compounds were not detected in boring B14 below a depth of 20 feet Therefore, <br />' the detection limit of 0 005 ppm was used to calculate a maximum mass of contaminants in the third <br /> and fourth layers <br /> Advanced GeoEnvironmentsl,Inc <br /> I r <br />