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<br /> Work Plan for Additional,Subsurface'Assessrnent—Former Unocal Station#2859 a
<br /> March 6,200I
<br /> approximately 20 feet below round surface bgs at conceritrations'u to 239 and'370 arts per million'
<br /> (ppm), respectively,♦and benzene was .detected` in the:awo 'lorings'at up:Eto 11;.2 an 6 58;,ppm
<br /> respectively. E •�
<br /> In-1988,two 10,000-gallon gasoline USTs and one 550-gallon waste oil UST were removed. At that time' Y
<br /> approximately 1,350 cubic yards of impacted-,'soil was'excavated and removed'from;the.;site. _After"
<br /> ' excavation, soil -samples collected from the'%ase of.the_tank_pk:contained TPHg and}'benzene at t s
<br /> concentrations up to.13 and,0.29 ppm,respectively.
<br /> x ` Between June 1988 and Februaryr 1992; seven monitoring-wells (MW-1 through MW-7),`three:vapor.'. r ;
<br /> extraction wells (VW-1 throughNN-3), and brie soil bor-ing-were=installed by Applied Geosystems and `•+.
<br />`.; RESNA. The highest TPHg and benzene concentrations reported were.from an area around the northern
<br /> product island location at concentrations up'to:17,300 and�28 pprri,respectively: '
<br /> . '. .
<br /> A soil vapor extraction testwas conducted by�USNA inx992. The testshowed that a wellhead flow rate,
<br /> t of 43 cubic feet .per minute and an estimatedYradius of influence.of,approxirtiately'.25_feet'could be.
<br /> r achieved. a
<br /> _ In July 1994, Pacific Environmental Group, Inc. {PEG) installed monitoring-wells -MW-8'and-MW 9^
<br /> Groundwater samples from the,wells'were non-detect-for benzene,.but TPHg were detected iri,wells
<br /> ri .
<br /> MW-8 and.MW-9 at concentrations of 93 an 110 parts per.billion(ppb),respectively. ; t ;
<br /> PEG performed Ia six-month on-site vapor, extraction�Ypilot study; in. y1996. t During• that study
<br /> approximately .2,745 pounds=(450 gallons) of TPHg and 12' pounds (1 6,gallons) of .benzene 'were" l
<br /> ., removed.
<br /> ARCADIS Geraghty& Miller,Inc. installed'-we
<br /> lls•MW-l0 and NP-1 in January,1999. Soil.samples from I
<br /> well NP-1 showed impact. beneath the site :between 40 and 60 .feet below ground surface:(bgs).` The
<br /> R'
<br /> highest TPHg and benzene concentrations were from a' sample'at`Y60 feet::bgs at.290' and 5.8 ppm, `
<br /> t, respectively. Discrete grab groundwater samples from well boring NP-1 had concentrations of TPHg and '
<br /> benzene in samples collected between 50 and 70, feet ligs: ; The:.highest concentrations of•TPHg-and
<br /> benzene detected were 200,000 (50 feet bgs),arid 21,000.,(60•feet bgs)ppb, respectively ',`
<br /> Groundwater Monitoring and Samnlins~
<br /> Quarterly groundwater sampling started'in June,1988'withM
<br /> ,wells W-1 thr6" gh;MW 3.-' Wells MW 4, ,•
<br /> and MW-5 were,,added-.in: August 1989, -MW-6 and MW-7 in .March 1992, MW-8..and>MW=.9 ,in_,'
<br />_ Y November 1994, and and NP-1 in February 1999. Se_ parate-phase'hydr`ocarbcps have not been f
<br /> r observed in the wells.- Since monitoring of-the wells began •insthe,first quarter.'of 1993, groundwater.
<br /> beneath,the site has risen more than '20 feet In the,iitost recent-monitoring and ,smnpling:event'on,
<br /> ` November 13, 2000, depth to water., the wells`ranged-from 23.7-1.,to_25.30 feet below-top.-of casing. '
<br /> Groundwater flow.during this event'was to the northeast ata gradient ranging from 0.001,6 0.003,,which. .-
<br /> is consistent with historical data. .
<br /> During the November 13,2D00 event,- were.detected in wells MW=2.{58;000 ppb); MW-9(56 ppb),F
<br /> and MW-10 (72,000 ppb) Benzene was detected.in wells MW-2 (2,800 ppb); MW-:10 (5,100_ppb), and.
<br /> 240079.04
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