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AliS G s 10 ` 2.4 Extent of Hydrocarbon Impacted Groundwater lved in The distribution of total petroleum hydrocarbons-as-gasoline (rPH G f the and plumes anzene lreooughly groundwater are illustrated in Figures 7 and 8,respev.ively. The lateral extentP conceni;ic around the location of the former underground gasoline tank,as expected from the low(0.001) hydraulic gradient observed at the site. The highest dissolved petroleum hydrocarbon entr tions were detected in on-site wells. The TPH-G concentration decreases about 730 pp per foot awaIf e former i inc;arground tank. The entire TPH-G plume,to its zero line,has a radius of approximately 100 - - feet. The vertical concentration gradient is also steep. MW-21 screened in the upper sand zone, b TPH-G and 28,000 b benzene on June 12, 1990. MW-7, contained 73,000 parts ��r billion (pp ) pp adjacent to MW-2 and screened in the lower sand zone,had 11,000 ppb TPH-G and 2,600 ppb benzene on Ja, :12,1990. }-fence,hydrocarbon concentrations are approximately an order of magnitude greater in the upper sand zone than that' are in the lower sand zone. The only groundwater samples in which organic lead and EDB were detected came from MW-2Y MW-5,and MW-7. The maximum concentrations to Maximum Contaminant were detected in Ml and were 200 ppb lead and 53 ppb EDB compared Levels(MCL)of 5o ppb and 0.02 ppb respectively. The extent of hydrocarbon impacted groundwater has been defined in the PAR investigation and through subsequent sampling. Separate-phase petroleum has been measured in monitoring wells MW 1,MW-2, MW 3, hiW 4, MW-5, and MW-6, with a maximum of 2.97 feel measured in MW-2 a e ha�petroleum has beeneremoved 250-gallon product storage tank was installed on site,and sep p with a hand bailer since that time. , 3.0 REMEDIATION SYSTEM DESIGN 3.1 Overview The PAR defined the extent of hydrocarbon impacted Solt and groundwater,some of the hydraulic properties of the subsurface materials, and evaluated remedial technologies. To develop a final design for the remediation system, Groundwater Technology conducted s fail vapor matixandlohe ilot to su st o determine the pneumatic characteristics of the unsaturated zone ro hydrocarbon concentration in the extracted soil vapor. Collection of these data allows complete design of the vapor extraction and treatment system, and provides necessary information to obtain an air discharge permit. r.__ Review of the.aquifer.characteristics and groundwater quality data collected for the PAR are used to design a groundwater extraction and treatment system. Final design and per.mittfng will-require performing an aquifer test to verify the design parameters. Initially,one 12-inch diameter groundwater GROUNDWATER TECHNOLOGY,INC.