SITE INFORMATION AND CORRESPONDENCE FILE 2

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When the first order decay function from on-site well MW-3 (-0.0004) is applied from the last monitoring event forward in time results in an estimate to reach the clean tip goal of I pg/I for benzene by January 2045. If a factor of safety of 30% is applied to this estimate to accommodate for the low R2 value, then itis anticipated that the groundwater cleanup goal of 1 pg/I for benzene will be reached by November 2054. In addition, based on soil and groundwater analytical data collected from down-gradient of well MW- 108 (CPT-2 and CPT-3), the contaminant plume appears to attenuate to near non-detect levels approximately 170 feet down gradient of MW-108 and does not continue past the location of CPT-3. As contaminants continue to decrease by natural attenuation near the source of the contamination, GZA believes that this trend will occur in down-gradient well MW-108 as well, despite a recent increasing contaminant trend in this well. MW-6 Historically, MW-6 has been the up-gradient shallow depth well at the site. If we plot the natural log of benzene concentrations reported in MW-6 (from July 1996 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 0.9194 (Figure 5 &Table 5). Applying the decay function forward in time results in an estimate to reach the clean up goal of I pg/I for benzene by January 2020. If a factor of safety of 30% is applied to this estimate to accommodate for the R2 value, then it is anticipated that the groundwater cleanup goal of I pg/I for benzene will be reached by February 2027 MW-1 Historically, MW-I has been the up-gradient intermediate depth well at the site. If we plot the natural 'log of benzene concentrations reported in MW-1 (from October 1994 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 0.8184 (Figure 6 &Table 6). Applying the decay function forward in time results in an estimate to reach the clean up goal of I pg/1 for benzene by September 2017. 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 I pg/I for benzene will be reached by August 2024. Conclusion A review of groundwater monitoring field parameters indicates -that the dissolved oxygen concentration for wells mentioned above ranged between 0.4 and 2.0 mg/I during the last year of groundwater monitoring, indicating a slightly anaerobic environment with a reasonable expectation for biodegradation. In conclusion, the core of the groundwater plume in the vicinity of MW-3 will reach MCL's by the following dates dependent on which attenuation factor is used: • September 2012: based on the Low-Threat UST Case Closure Policy,Groundwater Specific Media Class 2, the highest benzene concentration was 2,100 ug/L, which is below the threshold of 3,000 ug/L benzene. • October 2019: based on the Low-Threat UST Case Closure Policy, Groundwater Specific Media Class 3, natural attenuation should decrease concentrations in MW-3 to below the threshold of 1,000 ug/L. • May 2036: based on the calculated attenuation factor calculated for on-site wells MW-1, MW-5 and MW-6(4001). � I li I