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r " r 0 laniary 8, 1998 } 1)age 3 - . most recent, when a thin layer of SPH was detected in a well UV-1 on November 3, 1997 Quarterly groundwater monitoring data are included Attachment C The nearest downgradient water supply well is located over 1,500 feet from the site In a work plan and feasibility study document dated October 9, 1997, UNOCAL's consultant GeoStrategies, Inc (GSI), proposed completion of site assessment by the installation of a dawngradlent groundwater monitoring well The GSI work plan is included as Attachment D Attachment D also includes a site assessment report and Nvork plan which were submitted 1994 and 1996, respectively Groundwater quality beneath and downgradient of the site is currently monitored by nine wells (Attachment C, Figure 1) Historic lmonitoring data indicate that hydrocarbon-impacted groundwater is concentrated immediately downgradient of the former first generation USTs (see Attachment C, Figure 2 for most recent quarterly groundwater monitoring data) Historic groundwater quality data indicate that time proposed well would be beyond the anticipated downgradient edge of the plume A partial feasibility study was included in the October 9, 1997 GSI report Based on the feasibility study, GSI recommended monitoring and sampling the current and proposed groundwater monitoring wells on a quarterly basis for a mimimum period of one year, at which time natural attenuation and stability of the plume would be assessed No active remediation was proposed at the time This recommendation was based on stability of • the hydrocarbon plume, decreasing hydrocarbon concentrations, and the low risk to groundwater resources (nearest downgradient well located approximately 1,575 feet from the site) In their June 25, 1997 letter to UNOCAL (Attachment A, Document A-1), PHS/EHD concluded that it was unlikely that the site posed an immediate sianifcant human health risk by stating "Until such time as the f7111 exlelll of the still aildg7oiiildii)aler C.o111al77111atio77 is deleliiiilied if is pleliial7i7e to assi,�,/7 a ilSk slatins ,Holi ei ei, sllice ll7e'1 e 1 v iiot a dl liiAiiig mi'ater silppl}I 11'el1 O77 the Site, 1107 CTI e thel e airy I7ioii n vapol 1711gr7alron 1)a117ivays, it is wilikely that the site pose% ali iminediale tilgwf said hilmai1 heallh 71 sk ' After a review of the October 9, 1997 GSI report, PHS/EHD in their letter of December 10, 1997 stated that Unocal's proposed work plan for final site characterization and the feasibility study were both deficient PHS/EHD concluded by stating "The ,i1vlkplan to eonyVele the investigation should ham'e uichided soil hot lilt;% on site 10 define the exte171 of on slle eo111a1711ilalion and to nislall air tpaige and vapol extl aclimi wells 10 pet fo7771 lite 7egrined feasibility clod}f" The PHS/EHD letter continued ",Should Ihwcal ii %h to ei�alliale iialii7al atleiiiicilioll as olie the 1 emedlal alleMallve's, addif7wia11710171107177g ii ell,c ii,lll he 7 egin!ed li hlc,h ale SG7 eelled dis(-retel)� to nioi7ltor the fllll i,ei tical and lalelal exlei it ofg7ouncAvaiei eo111an,mall o17 " It is PEG'S opinion that any additional site assessment activities should be performed only if necessary to further evaluate the site's risk to the environment and/or public 1101 s 07 am 187RE\1ENV