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U 5.0 NATURAL ATTENUATION AND FATE/TRANSPORT OF MTBE 5.1 Natural Attenuation of MTBE As previously discussed in:this document (section 3.3.2), it has been clearly established that the dissolved-phase MTBE plumes in groundwater beneath the site are laterally and vertically characterized. To estimate the natural degradation rate of MTBE in groundwater beneath the site, Stratus prepared lognormal graphs of attenuating MTBE concentrations over time for nine monitoring wells (Sand A wells MW-2, MW-3, MWO-2, MW-7, MW-9, and MWO-3, Sand B wells MW-8 and MWO-lx, and Sand A/B well MW-13A/B). These graphs, included as Figures 9 through 17, also present first order rate equation analysis for MTBE in groundwater in each well. The first order rate attenuation evaluation conducted was in accordance with the USEPA's Calculation and Use of First-Order Rate Constants for Monitored Natural Attenuation Studies, November 2002. On each graph, a linear trendline was fit to the data and the trendline was then extrapolated forward in time until it intersects with a concentration of 5.0 µg/L (secondary MCL for MTBE in water), thereby estimating the time required for MTBE concentrations to reach the MCLs. From these graphs, a site-specific half-life for MTBE as it degrades in the groundwater beneath the site (specific to the geochemistry of the water in the A and B zones) was estimated at about 1.91 and 2.20 years, respectively. This estimation of half-life was performed by measuring along the trendline on the graph the period of time required for the MTBE concentration to decrease by half(then averaging estimated half-lives, separately for Sands A and B). LJ At wells MW-2, MW-3, MWO-2, MW-7, MW-9, MWO-3, MW-8 and MWO-]x, the rate constants in the first order decay equations are negative, indicating that MTBE in groundwater beneath the site is naturally attenuating and resulting in a prediction of the date when concentrations in these wells will approach the MCL. For these eight wells, the negative MTBE concentration trends predicted reaching the secondary MTBE MCL in 2009 (MW-3), 2010 (MW-9, MWO-3, MW-8), 2011 (MWO-2), 2012 (MW-7), 2013 (MW-2) and 2019 (MWO-1x). At cross-screened well MW-13A/B, the rate constant in the first order decay equation is slightly positive, indicating that MTBE in groundwater as measured in this well is not naturally i attenuating. Well MW-13A/B is relatively new and, due to access issues, has been sampled a total of only ten times between late 2006 and present; therefore, insufficient data has been collected at the well to establish clear trends and alternate methods of predicting the behavior, extent (fate and transport) of MTBE in this well are employed in the next section. 5.2 Fate and Transport of MTBE Not only has the MTBE in groundwater beneath the site been demonstrated to be naturally attenuating and predicted to reach MCLs within a reasonable time frame, but the MTBE plumes in Sands A and B also appears adequately assessed to the secondary MCL in all directions in both zones, with one exception. The lateral extent of MTBE in Sands A and B to the northwest of well MW-13A/B is precluded from additional assessment by the presence of Interstate 5. To further understand the extent of MTBE in groundwater to the northwest of the site, SJCEHD ( requested the use of a fate and transport modeling to accomplish two goals: 1) to evaluate how �J Page 18 5TKA TU5