SITE INFORMATION AND CORRESPONDENCE FILE 2

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Potential Risk of Indoor Vapor Intrusion There may be need for further discussion regarding what constitutes an "active""service station". Our opinion is described below: • Active Service Station ➢ We classified this property as having an active service station operating on it. ➢ We classified this property as active since an auto service&repair facility is currently operating on the site.The business currently stores, handles,uses and disposes of petroleum based products. ➢ Internal combustion engines are currently used in the service bays. ➢ The service bay doors are open during business hours,so operational fumes and outdoor traffic fumes are constantly exchanged inside the services bays. ➢ The office door is open to both the outdoors and to the services bays during business , hours. • Given the facility operational parameters described above,the fact that this facility does not dispense gasoline for auto fueling purposes seems to be irrelevant or incidental at best. • If by definition -to qualify as an"active service station requires dispensing of petroleum fuels,than this facility would not qualify. • However, since they use petroleum products,cleaners,solvents, and operate internal combustion engines inside the building, these sources far exceed the minor concern of possible indoor air intrusion from gasoline soil vapors. Weconclude that the current use of the facility qualifies this site as an"active service station". Time Needed for the Groundwater to Reach Maximum Containment Levels(MCL's) In order to determine the time needed for groundwater to reach MCL's, GZA plotted the benzene concentrations reported in various wells as a function of time, indicating that the contaminant distribution can be represented by a first order decay function with attenuation rates ranging from - 0.0004 to -0.001 (Figures I thru 6). Applying the decay function forward in time results in an estimate to reach the clean up goal of I Ng/I for benzene by October 2071, or if implementing a safety factor of 30% by May 2094(Tables 1 thru 6). It is important to note that this is an estimate, based on information known at the time of preparing this report and conservative assumptions, and actual time to reach groundwater quality goals may differ. First order decay rates and the time to reach MCLs were calculated for the following wells: MW-5,MW-3,MW-8, MW-108, MW-6 and MW-I. MW-5 Historically, MW-5 has been the most contaminated shallow depth well at the site. If we plot the natural log of benzene concentrations reported in MW-5 (from September 1999 forward) as a function of time, we see that the contaminant distribution can be represented by a first order decay function with an attenuation rate of k = -0.001 per day and an R2 value of 0.6084 (Figure I &Table 1). Applying the decay function forward in time results :in an estimate to reach the clean up goal of l pg/I for benzene by May 2026. If a factor of safety of 30% is applied to this estimate to accommodate for the low R2 value, then it is anticipated that the groundwater cleanup goal of 1 pg/I for benzene will be reached by May 2034. MW-3 Historically, MW-3 has been the most contaminated intermediate depth well at the site. If we plot the natural log of benzene concentrations reported in MW-3 (from July 1996 forward) as a function of t