SITE INFORMATION AND CORRESPONDENCE FILE 2

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r time, we see that the contaminant distribution can be represented by a first order decay function with an attenuation rate of k = -0.0004 per day and an R2 value of 0.8514 (Figure 2&Table 2). Applying the decay function forward in time results in an estimate to reach the clean up goal of 1 pg/I for benzene by October 207 1. 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/l for benzene will be reached by May 2094. Extensive remediation has occurred on-site within the core of the plume (MW-3 & MW-5) including: • June 2007 thru July 2009—Dual-Phase Extraction • August 2011 thru July 2012—Air sparging and soil vapor extraction GZA believes that following the extensive mass removal, groundwater within the core of the plume (MW-3 & MW-5) will show a more rapid attenuation factor than the -0.0004 calculated for MW-3. The attenuation factor of -0.001 calculated for less impacted wells MW-I, MW-5 and MW-6 is believed to be more representative of the natural attenuation expected to occur in MW-3 following extensive remediation. The recent benzene concentrations in MW-3 are at levels first observed in MW-1, MW-5 and MW-6 prior to remediation, therefore we expect MW-3 to attenuate at a similar rate. Based on an attenuation rate of 0.001, MW-3 would reach MCL's by May 2036, as shown in Table 2 &Figure 2. MW-8 MW-8 is the shallow depth down-gradient well at the site. If we plot the natural log of benzene concentrations reported in MW-8 (from March 2004 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 = -0.0006 per day and an R2 value of 0.7749 (Figure 3 & Table 3). Applying the decay function forward in time results in an estimate to reach the clean up goal of I pg/1 for benzene by January 2019. 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 June 2023. MW-108 MW-108 is the intermediate depth down-gradient well at the site. If we plot the natural log of benzene concentrations reported in MW-108 (from August 2004 forward) as a function of time, we see that the contaminant distribution can be represented by a positive first order decay function with an attenuation rate of k = 0.0004 per day and a low R2 value of 0.2732 (Figure 4 & Table 4). The concentrations of benzene in well MW-108 appear to have been on a slow increase with minor fluctuations from August 2004 thru the present. GZA agrees that the on-site concentration trends and years of remediation are expected to influence the concentrations of hydrocarbon constituents in down-gradient well MW-108. GZA believes that the declining on-site mass is unlikely to maintain neither an expanding groundwater plume nor an increasing trend in MW-108, deeming the positive decay rate of 0.0004 not representative. The decreasing trend in MW-108 is likely to mimic the decay rate of on-site wells MW-1, MW-3 and MW-6. When the first order decay function from on-site wells MW-I and MW-6 (-0.001) is applied from the last monitoring event forward in time results in an estimate to reach the clean up goal of I Vg/1 for benzene by November 2025. 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 December 2029.