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Mr. Gabriel Gonzalez, SJCEHD April 23,2015 Supplemental Information to Support Environmental Case Closure Project No.2129-1140-01 7835 Thornton Road (formerly 1 140 W. Hammer Lane), Stockton,CA Page 3 Year GRO Benzene 1,2-DCA 2008 230,000 38,000 880 S 2aaj , , y o °►bc I'68a 2009 160,000 34,500✓ 780 2010 160,000 40,500 2011 125,000 28,000 330 3 c 2012 140,000 33,000 V <450 1" ti n 4 ItA Kj' SkOr.�t ece Percent Reduction 39.1 13.2✓ 62.5 ✓ Between 2008 and 2012 (2011 for -Lao>oc-a-I`16 G'D i�� " 33�poa'�° � 3a-", sc 1,2-DCA) caters ;_a° Note: All concentrations depicted are annual averages and in units of micrograms per liter(pg/L). Using the data shown above for well MW-11, Stratus is assuming that GRO and benzene concentrations at well MW-12 will attenuate at rates of 39.1 percent and 13.2 percent, respectively, over a five year period, and that 1,2-DCA concentrations at well MW-12 will attenuate at a rate of 62.5 percent over a four year period at MW-12. (A Mcg 1.Y The attached Table 1 presents�computations that estimate the time to reach WQGs for GRO ,( gg/L), benzene (1.0 pg/L), and 1,2-DCA (0:5 µg/L) using data from the first quarter 2015 as_a baseline, and historical attenuation rates observed at nearby well MW-11 between 2008, and either 2011 (for 1,2-DCA) or 2012 (for GRO and benzene). Based on these computations, Stratus estimates that approximately 80_years will be needed for GRO concentrations to decline to a level of 5.0 gg/L, approximately 245 years will be needed for benzene concentrations to decline to a level of 1.0 pg/L, and approximately 16 years will be needed for 1,2-DCA concentrations to decline to a level of 0.5 gg/L. Since the time needed for benzene to reach WQGs is anticipated to require the longest time, Stratus thus estimates that approximately 245 years will be needed for groundwater at the site to attenuate to WQGs as a whole. LIMITATIONS This document was prepared in general accordance with accepted standards of care that existed at the time this work was performed. No other warranty, expressed or implied, is made. Conclusions and recommendations are based on field observations and data obtained from this work and previous investigations. It should be recognized that definition and evaluation of geologic conditions is a difficult and somewhat inexact science. Judgments leading to conclusions and recommendations are generally made K:\Knowles-Hammer Lane\Reports\knowles-hammer-supplemental-closure-info.doe