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5 May 1995 <br /> • AGE-NC-95-1036 <br /> Page 3 of 4 <br /> 3.2. SITE SPECIFIC CONDITIONS <br /> Site specific parameters were derived from data of previous investigations performed at the <br /> site. A simulation time of 40 years was chosen to include a full range of program output. <br /> The contamina[lt source was chosen to be a release lasting 15 years and then stopping. This <br /> \would correspond to a tank leaking for 15 }'ears and then removed and remediation <br /> Performed. Year 15 would correspond to 1991 for this site. Data analysis points \sere I)laced <br /> at distances equivalent to the locations of M1V-1 and MSV-=1. <br /> An average hydraulic gradient \vas determined front existing ground water monitoring clata <br /> at the site. The hydrat[IIc condLICtivlty, p03•oslt}' and mass load of benzene to ground\valer <br /> Nyel'e varied until the predicted contaminant concentrations at MWA and 1` W- 1 matched <br /> field data from the site. The distance to the production well on the Cutter Lumber property <br /> (the closest d10mi-gradient \veil) ' \vas then inserted, and the predicted concentrations <br /> determined. <br /> 3.3. <br /> RL-SUM'S <br /> • An effective )o 'osity of 30%=. a hydraulic cortclucti�sty of 1000 n.r/}'r and �a mass load of 0.12 <br /> 1 <br /> k"�yr produced predicted concentrations of benzene sit M\V-1 and \1\V-4 similar to actual <br /> concentrations observed in field samples collected at the site. Graphs of predicted. <br /> concentrations are shown oil Fiairres 2, 3 and 4. When the distance to the production \vell <br /> n <br /> othe Cutter Lumber property was inserted, a [IlaNIHIL1ttl valise of 3.5 1 10' mglL-(0.000035 <br /> ppb) for benzene was obtained. The reporting limit for benzene In water is 0.5 ppb. No <br /> deal"adattolt of.benzene was ntclttdecl ',I this, predictiolt. <br /> =1.0.- CONCLUSIONS - <br /> The closest \yell down-gradient of the Impacted around water plume is located on the Cutter <br /> Lumber Property,-approximate.ly 450 feet north of the former UST location. The AT123D . <br /> modeling progratn predicted no detectable concentrations of benzene to reach thiswell, Due <br /> to the low concentrations of hydrocarbons detected III wells oil-site, and prediction of no <br /> detectable hydl'ocarbons reaching the closest down-gradient well, we recommend that any <br /> remaining hydrocarbons be left.to naturally degrade. A Closure Summary Report for the <br /> site should be prepared and submitted to the PHS/EHD and the Regional Water Quality <br /> Control Board. <br />