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rn n FILE COPY PUBLIC HEALTH SERVICES SAN JOAQUIN COUNTY '^k ENVIRONMENTAL HEALTH I)WHE ION Karen Furst, M.D. M.P.H. Health O (icer ' aL+i=oR� 304 East Weber Avenue, Third'Floor • Stockton, CA 95202 2091468-3420 j PAUL SUPPLE OCT 0 6 2000 ARCO PRODUCTS COMPANY P O BOX 6549 MORAGA CA 94570 RE: ARCO Station #4493 SITE CODE: 1042 205 N. Center Street Stockton CA 95202 San Joaquin County Public Health Services, Environmental Health Division (PHSIEHD) has reviewed the "Additional Site Assessment Report" dated September 5, 2000 and the"Quarterly Groundwater Monitoring Report, Second Quarter 2000" dated Septembei 18, 2000 that were prepared by SECOR International Inc. (SECOR)on behalf of the above referenced site and has the following comments. On May 8, 2000 one continuous core soil boring (B-1)was dvanced on site to obtain vertical definition of the contaminant plume in both soil and groundwater. PHS/ HD directed that this boring utilize a technology that would allow for the collection of depth discr to soil and groundwater samples. SECOR chose to use Precisions"Enviro-Core"sampling system. T ie boring was advanced to a depth of 60 feet below surface grade (bsg). Selected soil samples from 16-E 1 feet bsg were submitted for analysis. Groundwater samples for analysis were collected at depths of 30, 45 and 60 feet bsg. Analytical results indicate that the vertical contaminant plume extends to 25 f et bsg in soil, and that it remains undefined in groundwater. Concentrations of total petroleum hydrocarbons as gasoline, benzene and methyl tertiary butyl ether(MtBE) in the water sample collected at 60 feetbsg; were 663 parts per billion (ppb), 56.4 ppb and 28.8 ppb respectively. On June 2, 2000 three off site monitoring wells were install d. These wells were installed to a depth of 25 feet bsg to monitor the horizontal extent of the shallow groundwater contaminant plume in the down- gradient direction. The new wells were developed and a routine quarterly monitoring of all seven wells belonging to this site occurred on June 16, 2000. Results of the analyses of the collected water samples indicate that the lateral extent of the shallow plume has not been defined. Two of the new off site wells, MW-5 and MW-6, had benzene at 2.75 ppb and 1.87 ppb, is-1,2-dichloroethene at 5.46 ppb and 17.6 ppb and trichloroethene at 20.2 ppb and 3.28 ppb respectively. MW-6 had MtBE at 144 ppb. The"Additional Site Assessment Report"recommends that a multi-completion well be installed near boring B-1, and that following the review of data from this well and another quarterly groundwater event, the need for more wells can be evaluated. PHSIEHD agre s that deeper, discretely screened wells should be installed on site. Prepare a work plan to further define the vertical and lateral extent of the documented contaminant plume in groundwater at this site Deep, discretely screened wells will be required both on and off site and must be included in thenext phase of work. Submit the work plan to PHSIEHD by November 17, 2000. Quarterly groundwater monitoring must continue at this site until directed otherwise by PHSIEHD. Analysis for all oxygenate , dichloroethane (1,2-DCA)and ethylene dibromide (EDB) must be completed by EPA Method 8260 A Division of San Joaquin County Health Care Services