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APR. -2$' 99 (WED) 15: 10 UNOCAL 7 NORTH RE 'i'EL:510 3781 P. 003 <br /> gPR"p9-•1999 ea:04 _uRca/cLEAN WATER 916 22'7 4349 p.02/04 <br /> 1665 Pacific Ave., Stockton. Scut IonquNi 4,61int, <br /> Response to Rearponse to Regional 86ard;C-6m nix lgeg lr+cdlnig Draft Oradea for Closure or.UNOCAL <br /> Site. <br /> Gordon, here arc my irnme dimes ehair''hts:rarR=the-$t;it aeiE Rt.• to our co <br /> 8 �onse mmcnts on.tete Drat <br /> Order for Closure of UNOCAL UST'Site'uf I G65`!)ac)Fc-A.vc., Stockton, San Joaquin County. <br /> Scare BooA Miff say They have reviewed Me► !'t of rerenr work..and fmd that it conr&bw no new <br /> mformamm rltat would altar!iu crnictusioira acid-r..mpmefans fir clasaere... <br /> The following are new data froth diaJanuiir�i-Md.cV.r' x1999 activities and is, illy opinion, sited <br /> question on die plume stability Arid-vaixic' ;stiic_tri3tieia' <br /> 1. MnE And benzene were detected;in•monitoring well MW-'l (located appmximately 290 feet <br /> downgradient of the site), at respedbye;conceutia;iorns'of 8:5,0.85µgdh Prior to the <br /> d February 1909 sampling' t, neEtber.cnn ii�eieant.fiad ever been teported in rtiis well. if the <br /> plume is stable, wiry arecontarninaints now being detected ne wly 300 feet away. {bean <br /> pluses ase not supposed to go that far, right?) <br /> 2. Based on groundwater cievation,data'frOtYi'11+1W-2(screened from 20 to 50 feet bgs), and NP-1, <br /> (screened from 85 to 90 foet-bgs)utile•is•a downvi*d vartical gradient of 0.055 Peep Tfer foot. <br /> The two wells art separated by 10 feet. (see page 21 of tete <br /> 26 February 1999 Geraghty and.Mfller•Repor.C),.Tho gt*tindwa1&r flow direction is consiswady <br /> tnotdhwest at a gradient of approximately 0.00 1.to 0.002 ftJ& <br /> 3. MTBE was detected in the displicate water saiaplo collected from die NP-1 bosiag <br /> (at 60 feet bgs)at a concentration of 77µg/L by EPA Method 8020. The:8260 coehmadon <br /> analysis was nen using EPA Mkthiodi•Sx60wid jr -in ND(1130µg/L). Sample-NP-�] (also <br /> collected at 60 feet bgs err ` ' <br /> gs 'lvifk'ri:ciCt6tl. ui'q�f Z Obµg2., <br /> 4. State Hoard staff lifer to the"hits"df' <br /> TE E3rr sevoi'al.ort and,orf-site manitod ig wells as <br /> ronrn <br /> backgd conditions. Ac coii6g"ty Mdtgaret,Lagprio of Salt Joaquin County, trate are no <br /> known fuel staticm.5 upff"ant mf th6'f6ra=VN0CAL. Furthermom, if tdwse are in fact <br /> background concentrations,.ail wells wottld contaitt'low concc>ftdons of MTBE,not just a few. <br /> MT]aF-concentt tom are coesistcst with a-prs•1992 rcic"e(this relcam is repunad to be ]988 <br /> or before). <br /> 5. Between August 1990 and early 1993, groundwater,surfAce elevation dropped below 50 feat bgs <br /> (leaving MW-1, 2, and 3 dry). During thio tint"period.Cal water also pumped ftom many of <br /> their production wells in the.araa: •Be w'exn--1§93,t#' present groundwater surface elevation has <br /> risen and is presently about 24 feet•bgs:,-iisligigp'#,�eant cha ri p In water eurf'ace and the <br /> col respoadrng lank of puMom"g frrom•thd•acarby C 1 Water wells suggests the upper and lower <br /> aquifer zones are in commume bh. Iu iise�iff t .groundwater tabic is primarily due inemased <br /> precipitation and lack of pumping in the:amu.'the lataai extent(below 59 feet) and tete vertical <br /> e:xtenEof hydrocarbons and thel addldvc impact Is still not dcfnect. At a mini imum. one <br /> downgradiertr deep monitoring well is neo Bury to determine ttte magnitude and extent of <br /> hymen impact. <br /> I do not agree Nat site cha=tadmition ac'die .%u' a Gornpktt•ut this site. <br />