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APR. -28' 99 (DYED) 15: 10 UNOCAL C NORTH RE TEL:510 oQ 3781 _ P. 004 <br /> APR-2a-1999 89:05 AW 4 C RM M.EA N1.iP Tp�F5. 91fi 227 4349 P.24i@4 <br /> . PfttIt'Fast Mate 7671 oam Zmap <br /> ra <br /> ,• <br /> Qo. <br /> James pro** <br /> Ed Ed f :; M jFft# <br /> RE; UNOCAL- STOCKTON <br /> This is a point by point rexponse to,_xlln;�ataat fzv�iz;l -an S. As YOU know, this is the <br /> second around on. „t _dttozL ould think by now <br /> mmmer�tdevelo " ' fir: '� ;w the <br /> Ped bX N.-D- d;bn e�' a bo 1evel'af supervisor and te,ehnirW <br /> QA/QC before sending sheat`oa fi t.irit�ui c i� t eiFO's rt pottse <br /> 1. MW-7, Ioaated x90$�down-gtadnleAt:fzes' ca<liyr detected hydrocarbon <br /> constituents since it was fwd in talled-in March cf 1992. 0.9 98 ppb TPHg in 1992 <br /> 1.3 to 4 ppb toluene in 1992, 1995, and 1999, 2.6 and 0.63 ppb ethylber=ne in 1992 <br /> and 1995, and 4.5 to 23 ppb xylene in 1992, 1993, and 1999. 71he Webnu ry 1999 <br /> analysis detected benzene(0.V ppb), tolueb" '{4 ppb), xylene(4.8 ppb), and MTBE <br /> (8,5 ppb). Site wells were aarnpled•i n Marcb of 1999 (RB had throe data before <br /> Canton's response)acrd the sample from this-well tested non-detect for all <br /> constitnzenrs. My impression Is thatiRB stadate using thi F6njmy became"hit" to <br /> argue that the plume-is not "I&'' <br /> ti I& Tbiiatgument however ignoresyears of data from <br /> this well and other wells closer"to the site- "i .daady demonstrate a stable plume. <br /> With this type of logic, I wotildn't be surpria6d;if the" has UNOCAL,looldng <br /> further oTsite for the 0.85 ppb benzene•detcctesd in Febna'Lry that was not there in <br /> Mardi. <br /> 2. There are thane Sets Of water'lovetO tg'for.th"was(M W-2 and NON 1)R8 staff'used <br /> to calculate a 0.055 dowiYvgerdta'taii'g i& •T.o_attain'such.e gradient, the head <br /> differential in the two wells fid-Iiav+e'1'0,bk'•oh i6e ondeaur of 2.8£bet (dl-about 50 <br /> feet) which hes not been the case. Based on-those� the Vmfiant betwa= the two <br /> web has been 0.002 up (2JIMI 0.007 down(2/4199) , and 0,00W up (3/1199). <br /> Va:tical gradients Cal U.slso'be,e titztstad 8Om'tiie head difi'�ntWs in we�s NP-1 and <br /> MW-10 (d1 =about 32 fW): 'The data.for this pair of wells produce upward <br /> gradients of 0.003, 0.02, and 0.0006. I suspect that RB staff incomnraly calculated the <br /> "vertical' gradient by ging the horizontal distance between the two well location& <br /> (about 14 flet) rather dW the vortical distance between the mid-points of the <br /> respective screen intavali, <br /> 3. rm not sure what pow this camnuM.1i unci&W to mala. The water sample in <br /> question(coliected whsle dritlirgrthe'bornng);W Method 8020 concentrations of <br /> TP 48 of 1 I0,000 ppb aid riM. ppb.;TThisanalytical method is known to over, <br /> estimate or produce,"false positive" MTBE•roncentmions in samples kith high TPHg <br /> concentnstions hence the confirmation procedure of the Method 82do analysis. when <br /> the subject waxer sample is viewed in the context ofothcr soil and water Samples <br /> 1' <br /> TOTAL. P.A4 <br />