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2057 South El Dorado
<br /> Stockton, CA
<br /> Page 2
<br /> benzene 45,000 ug/1 toluene, 3,400 ug/1 ethylbenzene, 19,200 ug/l total xylenes, and
<br /> 350,000 ug/1 MTBE. Analtyical results of the soil samples collected at 20 feet bsg during
<br /> installation of VW-1 were 3,600 milligrams per kilograms (mg/kg). AGE also reported
<br /> in the latest Quarterly Report-First Quarter 2006 dated April 11, 2006, that the TPH-g
<br /> and aromatic hydrocarbons concentrations rose in the downgradient well MW-1
<br /> groundwater samples, to 290,000 ug/1 TPH-g and 120,000 ug/1 benzene, higher than all
<br /> previous quarters. In contrast, MTBE concentrations decreased in all three wells
<br /> compared to previous quarters, with 52,000 ug/I MTBE in MW-1 compared to 170,000
<br /> ug/l the previous quarter. Contaminant concentrations in MW-3, an upgradient well
<br /> (located near the previous underground storage tank) also rose compared to November
<br /> 2005, to 110,000 ug/l TPH-g, 11,000 ug/l benzene, and 29,000 total xylenes, but these
<br /> concentrations were much lower compared to May 2005 when the results were 3,800,000
<br /> ug/l TPH-g, 38,000 ug/l benzene, and 2,410,000 ug/1 total xylenes.
<br /> The increase of TPH-g and benzene concentration in groundwater in all three wells may
<br /> be indicative of continued leaching of soil contaminants to groundwater. A leaching
<br /> source in the vadose zone still exists based on soil and vapor extraction data. The relative
<br /> drop of contaminant concentrations in MW-3, the upgradient well, versus the continued
<br /> increase in MW-1, the downgradient well, may be indicative of a migrating contaminated
<br /> groundwater plume. Given the high concentrations of TPH-g, aromatic hydrocarbons,
<br /> and MTBE in soil and in groundwater, EHD concurs with AGE that interim remediation
<br /> should begin immediately in order to begin recovering the contaminant mass in the
<br /> vadose zone before more contaminants enter the dissolved phase, and also to reduce and
<br /> hydraulically control the contaminated groundwater plume. EHD concurs with AGE's
<br /> conclusion that based on the findings of the two pilot tests, SVE/GWE are viable
<br /> alternatives for interim remediation at the site. EHD directs you to submit an interim
<br /> remediation work plan to this office by June 20, 2006, and implement the proposed work
<br /> within 30 days after work plan approval.
<br /> The lateral and vertical extent of the contamination at this site has not been defined. A
<br /> work plan for further investigation of the extent of the contamination is to be submitted to
<br /> EHD by July 15, 2006.
<br /> Please feel free to contact Jeffrey Wong at (209) 468-0335 if you have any questions.
<br /> Donna Heran, REHS, Director
<br /> Environmental Health Department
<br /> Jeffrey Wong, Senior REHS Margaret Lagorio, REHS
<br /> LOP/Site Mitigation Unit IV Supervisor
<br /> Cc: CRWQCB, Central Valley Region—James L. L. Barton,PG
<br /> Cc: Advanced GeoEnvironmental, Inc. — Anna W. Behrens
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