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L %we <br /> L 5.0 Concentration Trend Analyses <br /> One method typically used to determine when natural attenuation of contaminants will meet water <br /> quality objectives is to determine the first-order decay rate constants of concentrations vs. time. <br /> This method is outlined in the EPA Document No.540/S-02/500,titled"Calculation and Use of First- <br /> Order Rate Constants for Monitored Natural Attenuation Studies' (EPA, 2002). The first-order decay <br /> rate constant can then be used to approximate the projected timeframe to reach WQOs <br /> Analytical results from the last four quarters of groundwater monitoring indicate non-detectable <br /> concentrations of TPHD in all wells following silica gel cleanup on samples from select wells. <br /> Historic concentrations of TPHD from groundwater samples collected in site monitoring wells <br /> MW-1,MW-2,MW-3,MW-5,and MW-6 were used for estimating the approximate time required to <br /> reach the WQO for TPHD to compare to recent groundwater monitoring results. In order to <br /> provide a conservative estimate to the predicted time of WQO achievement,analytical results for <br /> LTPHD concentrations prior to the silica gel cleanup process were used in applicable wells. <br /> Concentration versus time plots for historic TPHD concentrations from groundwater samples <br /> collected from wells MW-1,MW-2,MW-3,MW-5,and MW-6 are presented on Figures 8 through <br /> 12. <br /> LFigure 8 <br /> TPHD Concentrations vs.Time,Well MW-1 <br /> E Western Oil And Spreading,Inc.,Acampo Plant Site <br /> LAcampo,California <br /> 10,000 <br /> 1,000 <br /> Trendline Equation <br /> Y 1070.2,-0.1135x <br /> f O <br /> (L b 100 <br /> U <br /> 10 <br /> --o—TPHD <br /> —TPHD WQO <br /> —TPHD T-rK line <br /> i 1 <br /> 0 5 10 15 20 25 30 35 <br /> Years since January 1,2000 <br /> \\Eureka\projects\2007\007152-WOS\PUBS\rpt\20100324-1stHa1f2010GWMR&RFC.doc t97 <br /> 11 <br />