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Californi egional Water Quality Cotol Board
<br /> Central Valley Region
<br /> Karl E.Longley,SCD,P.E.,Chair 0
<br /> Linda S.Adams 11020 Sun Center Drive#200,Rancho Cordova,California 956706114 Arnold
<br /> Secretaryfor Phone(916)464-3291 •FAX(916)46474645 Sehwarzenegger
<br /> Environmental http://www.waterboards,ca.gov/centralvalley Governor
<br /> Protection
<br /> 25 November 2008 y U'ecevm
<br /> GE1
<br /> DD
<br /> Ms. Jennifer Sedlachek ENV 12048
<br /> ExxonMobil Oil Corporation IRAN
<br /> 4096 Oakland, CA 94520 #194 PERMIT SER ��TH
<br /> DOCUMENT REVIEW, FORMER MOBIL STATION 99CAS (AKA MARLER PROPERTY,
<br /> AKA JH GRIFFIN MOBIL SERVICE), 75 EAST ALPINE AVENUE, STOCKTON,
<br /> SAN JOAQUIN COUNTY(LUSTIS #390857)
<br /> 1 reviewed the Report of Groundwater Monitoring, Third Quarter 2008 (Report) and the Subsurface
<br /> Investigation Report and Well Installation Work Plan (SIR/Workplan) received 29 September 2008 and
<br /> 28 October 2008 respectively, from your consultant ETIC Engineering, Inc. (ETIC). The SIR/Workplan
<br /> was submitted in response to my 10 July 2008 letter (Letter), which directed ExxonMobil to continue
<br /> the investigation of the vertical and lateral extent of contamination at your site. The SIR documents the
<br /> use of Cone Penetrometer Testing (CPT) for subsurface lithology and the Membrane Interface Probe
<br /> (MIP) for detection of petroleum hydrocarbons in soils for screening and future monitoring well
<br /> placement, and gives the analytical results of shallow soil samples taken during the investigation. The
<br /> Workplan provides a scope of work for the installation of four deep monitoring wells, 3 onsite and
<br /> 1 offsite in the downgradient groundwater flow direction, to provide vertical and lateral definition of the
<br /> plume. One shallow downgradient well (MW-8) was previously not installed due to a nearby property
<br /> owner's refusal to grant property access to ExxonMobil.
<br /> The Report states that, during the third quarter of 2008, the depth to water was approximately 37 feet
<br /> below surface (bgs). Groundwater flow direction has been consistently reported as towards the east,
<br /> which suggests the groundwater plume is migrating under EI Dorado Street, hence the need for the
<br /> offsite wells. Maximum petroleum hydrocarbons groundwater concentrations were total petroleum
<br /> hydrocarbons as gasoline (TPHg), 89,000 micrograms per Liter (ug/L) and as diesel (TPHd), 4,000
<br /> ug/L; benzene, 19,000 ug/L; toluene, 51,000 ug/L; ethylbenzene, 3,700 ug/L; xylenes, 20,000 ug/L;
<br /> methyl tert butyl ether (MTBE), 850 ug/L; 1,2-dichloroethane (1,2-DCA); 76 ug/L; and ethylene
<br /> dibromide (EDB), 130 ug/L.
<br /> The SIR CPT portion of the investigation revealed the presence of a laterally continuous sand unit at a
<br /> depth of approximately 70' to 85' below ground surface (bgs), extending from boring CPT-1 on the
<br /> west side of the site to boring CPT-4 on the east side of the EI Dorado Street right of way. Zones of
<br /> MIP detector response to petroleum hydrocarbons were reported at CPT-1, 60'-72' bgs; CPT-2 (former
<br /> tank pit), 59'-83' bgs; CPT-3, 53'-74' bgs; CPT-5, 51'-60' bgs; and CPT-6, 50'-59' bgs. There was no
<br /> MIP response reported at CPT-4. The MIP screening data generally show groundwater pollution
<br /> occurs at depth in silts immediately above the sand unit. The soil samples, collected from the hand-
<br /> augured samples completed to 5' bgs for clearance prior to starting the CPT borings, reported no
<br /> California Environmental Protection Agency
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