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CONESTO 'A-ROVERS <br /> &ASSOCIATES <br /> ,. K. <br /> June 19,2009 "Reference No. 241660 �., , <br /> .- <br /> a. <br /> _. •. g W M1r'su4 .alt T <br /> - of subsequent groundwater"monitoring data compared to the VLEACH model predictions "Y4 <br /> Concentration versus time graphs Fi1 res 3 through 5) depict TPH and benzene concentration <br /> trends in wells MW-1, MW-3,'and MW 4.,. - <br /> The groundwater data aver':the first 3 years agree reasonably weIf ith the VLEACH r - <br /> predictions. The TPHg concentration in:well MSN-1 during jure 2006 was'1,SQ0 µg/L, _ <br /> ^ 1 <br /> } - compared with the 3;326"�g/L VLEACH prediction.yHowever;since ithen the MW-I;TPHg <br /> 41 concentrations have been-decreasing <br /> The graphs clearly show that TPHg and benzene concentrations in:wells MW-1, MW-3,and: <br /> 2 <br /> ` MW--4 increased between 2004 and-2006,-peaked some time-near 2006,and have been- eneralfy <br /> - .. -. .,.. ��i l <br /> decreasing since,then."�Thefirst quarter 2009-TPHg,groundwater,concentrations in weIls MW-L J <br /> MW-3, and MW-4 ranged from 330"µg/L.in NIW-11 500.µg/L-`in MW-3. These fin' ngs:do not f <br /> -support the VLEACH predictions."The apparent inaccuracy,of the VLEACH model may duel <br /> to the following: the model assumes a one=layer homogeneous profile:and a constant:recharge' <br /> rate at the surface; iii the subsurface;the.model assumes constant flow and transport o_f TPHg a _ <br /> --_. <br /> through soil by,sorption, volatilization, diffusion to the atmosphere; and leaching to= c;m <br /> groundwater; and the model does not take into account TPHg degradation and dispersion, - =� a <br /> which.are.fundamental fate:and transport mecliarusms <br /> r.,. _ <br /> x. : . . While modeling may sometimes be a useful,tQo1; decisions should be made oii'actual' <br /> groundwater concentrations, which are the primary line of}evidence. CRA notes that benzene - <br /> was only detected in one groundwater sample (MW-1 at 2 0 �tg./L);at concentrations that exceed <br /> the California.Drinking Water Standards Maximum Contaminant Level (MCL)'of_1'.0 µg/L 'The <br /> graph clearly"shows that^natural attenuation of=l�enzerie rn MW_1"will-reach the MCL"wxthin a ,.. <br /> reasonable period of.time - CRA calculated the projected target date for benzene to reach the <br /> " MCL in well MW-1 using the slope equation:of,the;trend line,on.Figure 3 and presents ft-in the <br /> table below. <br /> Well ID.-,. Constituent ` Equation f '° MCL" Target', <br /> x w h ._;,(a )� Date <br /> MW-1 "= "" .Benzene e-2, -03x : '41:0 : 6/9/2010 <br /> Note:1.Trendline begins from historically highest concentration <br /> .Groundwater elevations are also shown on the concentration versus time graphs for'wells ' W <br /> u MW-1, MW-3;and MW-4. The graph for.MW'1}(Figure.3)%shows a possible correlation between. . <br /> groundwater elevations and petroleum hydrocarbon concentrations in groundwater since 2006 <br /> - t. <br /> � 4i .�., <br /> Prior to that"there is no correlation between groundwater elevation and petroleum hydrocarbon•' <br /> - concentrations in groundwater. "Trend graplis;for MW-3 and MW-4 do not show.a strong <br /> _ n <br /> nMa ldwide.Engineering,"Environmental,Construction,andAT Services <br /> Jt: <br /> K <br />