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Post-it®Fax Note 7671 Date paogea arm To 0 From MEMORANDUM Co./Dept Co. Phone# Phone# To: Brett Hunter,Chevron Fax# Fax# 1 CC: Margaret Lagorio Supervising REHS,Unit IV, San Joaquin County Public Health Services Environmental Health Division(PHS/EHD) From: Ron Rowe, Sr.RENS,Unit IV, LOP/Site Mitigation San Joaquin County Public Health Services Environmental Health Division(PHS/EHD) Date: December 3, 1998 Subject: Chevron Site#9-6258, 2409 Main St., Stockton Brett, A review of the LOP files for the above referenced site has been completed in response to the November 20, 1998 letter from Chevron. Please refer to the memorandum attached from my Supervisor, Margaret Lagorio dated December 2, 1998. Chevron's request is lacking sufficient justification as it relates to omitting oxygenates analyses required by the State of California and this Local agency. Testing for MtBE by EPA Method 8020 alone or the highest MtBE 8020 hit for 8260 confirmation will not suffice, presence or absence by antiquated/conventional methods in preliminary assessments does not allow this agency to conclude that other fuel oxygenates/components do not exist at this site as method 8020 cannot reliably detect or report ETBE, TAME, DIPE, or TBA (nor Pb, DCA, EDB etc). EPA Method 8260, 8260B, or 8240 analyses is required to maintain consistency at all UST sites currently under investigation. As can be observed, preliminary assessment work at this site confirms the presence of significant vertical distribution of fuel hydrocarbons. Water is currently at 58' bgs and is impacted by fuel hydrocarbons (TPH-g 32,000 ug/1). This agency would expect the vertical distribution of fuel hydrocarbons as likely to exist at depths greater than 58' bgs and smear zones may be large with the knowledge of increasing rising groundwater phenomena in this area. Chevron must perform a historical record search to determine the all time low groundwater level during the operational period of the facility. Boreholes and discrete sampling depth must conform to the findings of the all time historic low groundwater elevation during the operational period of the facility at a minimum. This requirement is issued recognizing the potential for dissolved, entrained, and adsorbed contaminants to exist at this interval or deeper. In numerous instances this agency has documented fuel hydrocarbon distribution at these depths and deeper particularly where pumping influences (i.e.-municipal/industrial wells)and or recharge areas create net vertical groundwater flow regimes. In the Chevron November 20, 1998 letter to this agency Mr. Hunter states "The fate and transport characteristics of a weathered gasoline plume without oxygenates are well understood and a limited migration distance can be assumed." Generalization such as this fails to consider the potential complexity of the undefined site specific hydrogeologic setting, nearby pumping influences, depth to water fluctuations,the mass of the release and the like. This agency has observed 1970's tank closure sites which have reported detection's of TBA and MtBE directly beneath the release, Benzene 700 feet down gradient and confirmed by several Quarterly Monitoring Events at 153' bgs in short screened wells although water is currently 50' bgs. Cross-sections at groundwater impact sites will eventually be required to visualize contaminant fate and transport and to provide contaminant mass volume estimates regardless of the contaminants of concern (i.e.-benzene,MtBE). In order to create reliable cross sections continuous core borehole logs are necessary. Non-continuous core boreholes at best represent only 30%of the subsurface(three 6"sleeves at the bottom