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20 November 2003. - <br /> AGE-NC Project No. 96-0263 <br /> Page 5 of 11 <br /> T However, the majority of impacted soil was encountered between 5`and '10'feet b'sg: The inferred---- <br /> volume.of hydrocarbon-impacted soil is approximately 3,960 cubic.yards (Appendix-Q. Based-on- <br /> the <br /> ased on.the volume of.impacted soil and-the average concentration­of TPH-g and MTBE" n_soil'samples i <br /> collected from the site,AGE calculates that the plume of impacted soil contains approximately 1,110 <br /> pounds of TPH-g and 51 pounds of MTBE. Mass/volume calculations are included ' Appendix C. - <br /> The approximate lateral and'vertical extent of-impacted soil is illustrated in Figures"6jhrough $ ­-";!�' <br /> x <br /> _ r <br /> 6.0: ASSESSMENT OF HYDROCARBON-IMPACTED GROUND WATER = - <br /> Between December 1999 and July-2003, up-to fifteen ground.iwater monitoring events-were' <br /> conducted on Tracy Marine Sales on wells MW-1=through MW_41 to monitor shallow and deeper- <br /> dissolved fuel comp_onent concentrations:and <to characterize ground water flow direction -and ,,' _ <br /> = gradient on the site Iri addition, grab ground water=samples were collected from soll�bonngs'B 1; _ <br /> B-5 through B=9 and CPT-1 through'CPT-4: Grab ground water samples were collected from 1 <br /> various depths. Laboratory-analysis of water samples from the wells and-soil borings indicated that, j <br /> ground water at-the site locally hays_been impacted by TPH- ,'BTEX com ouinds;and MTBE-; <br /> (Table 4). ° <br /> , <br /> , <br /> The highest dissolved petroleum hydrocarbon concentrations detected,in ground,wate'r:monitoring <br /> well samples were,detected in the collected from monitoring wells-MW-7.TPH-g,'_benzene <br /> and MTBE have been detected' well_MW-7 at coricentrations as high as 4,400µgll;-5;000µg/l.and <br /> 130,000 gg/l, resp ectively,:duririg the site-:investigation. Moderate concentrations,of dissoived.� M <br /> petroleum hydrocarbons have been detected in ground water sa_mples from monitoring wells MW 2 =y <br /> MW-4 and .MW-5. In addition, .lessor concentrations..of target,.analytes: have been"'detected, m <br /> sporadically in wells MW-6 andMW-S{Table 4):Detectible concentrations of.cheniidals of concern-_.r _ r i <br /> have not been detected in ground water sainples'collected from perimeter monitoring_wells MW-'I,.. <br />= MW-3,MW-9,MW-10 and MW-11 since Decerriber 2001,dehriing the lateral extent of petroleum ; <br />= hydrocarbon-impacted ground water at the site.The inferred horizontal extent of dissolved TPH-g, `: 4 <br /> benzene and MTBE in ground water based on analytical results from the most recent-monitoring,- <br /> event (July 2003 is depicted in i res 9 10.and_11,.res ectivel .T= = <br /> - <br /> The extent'of TPH-g-,benzene-and MTBE-impacted ground water appears to be limited to an area <br /> surrounding the former UST and dispenser areas;andAoes not appear to have migrated extensively: <br /> Relative to the inferred source area-near the former•UST-piping and dispenser areas,the maximum,,--...,. <br /> - <br /> extent'of petroleum hydrocarbozi-impacted ground water appears Wextend approximately 1:5.feet <br /> northwest of MW-S,:approxitnate'ly 5 feeteast_of�MW-3, and just north of MW=1,and MW-8. The <br /> approximate lateral distribution of dissolved TPH-' ai d MTBE in ground water was approximated:7 L i <br /> by utilizing drafting software. The average thick,hess of dissolved TPH-g and MTBE was derived : <br /> from total depth-of wells and depth to water measurements takendurin the July 2003 monitorin - <br /> event.Assuming an average saturatedporosityof20%,the'ealculated.volume of TPH-g-.and MTBE <br /> Advanced GeoEnvironmental,Inc. - <br />