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Mr. Gabriel Gonzalez, SJCEHD May 12,2015 <br /> Supplemental Information to Support Environmental Case Closure Project No.2129-1140-01 <br /> 7835 Thornton Road(formerly 1 140 W. Hammer Lane), Stockton, CA <br /> Page 3 <br /> Year GRO Benzene 1,2-DCA <br /> 2008 230,000 38,000 ggp <br /> 2009 160,000 34,500 780 <br /> 2010 160,000 40,500 430 <br /> 2011 125,000 28,000 330 <br /> 2012 140,000 33,000 <450 <br /> Percent Reduction 39.1 13.2 62.5 <br /> Between 2008 and <br /> 2012 (2011 for <br /> 1,2-DCA) <br /> Note: All concentrations depicted are annual averages and in units of micrograms per liter(µgfl. <br /> Using the data shown above for well MW-11, Stratus is assuming that GRO and benzene <br /> concentrations at well MW-12 will attenuate at rates of 39.1 percent and 13.2 percent, <br /> respectively, over a five year period, and that 1,2-DCA concentrations at well MW-12 <br /> will attenuate at a rate of 62.5 percent over a four year period at MW-12. <br /> The attached Table 1 presents computations that estimate the time to reach WQGs for <br /> GRO (5 µg/L), benzene (1.0 µg/L), and 1,2-DCA (0.5 µg/L) using data from the first <br /> quarter 2015 as a baseline, and historical attenuation rates observed at nearby well <br /> MW-11 between 2008, and either 2011 (for 1,2-DCA) or 2012 (for GRO and benzene). <br /> Based on these computations, Stratus estimates that approximately 80 years will be <br /> needed for GRO concentrations to decline to a level of 5.0 µg/L, approximately 245 years <br /> will be needed for benzene concentrations to decline to a level of 1.0 µg/L, and <br /> approximately 16 years will be needed for 1,2-DCA concentrations to decline to a level <br /> of 0.5 gg/L. Since the time needed for benzene to reach WQGs is anticipated to require <br /> the longest time, Stratus thus estimates that approximately 245 years will be needed for <br /> groundwater at the site to attenuate to WQGs as a whole. <br /> LIMITATIONS <br /> This document was prepared in general accordance with accepted standards of care that <br /> existed at the time this work was performed. No other warranty, expressed or implied, is <br /> made. Conclusions and recommendations are based on field observations and data <br /> obtained from this work and previous investigations. It should be recognized that <br /> definition and evaluation of geologic conditions is a difficult and somewhat inexact <br /> science. Judgments leading to conclusions and recommendations are generally made <br /> K:\Knowles-Hammer Lane\ReportsUcnowles-hammer-supplemental-closure-info.doc <br />