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' ARCADIS GERAGH Y&MILLER groundwater analytical data between 1989 and 1993. Based on the results of an analysis presented in PEG's Work Plan (August 6, 1993), PEG concluded that the data indicated that the configuration of the hydrocarbon plume has remained rela- tively stable over the 4-year period Based on recent groundwater monitoring data, the distribution of hydrocarbons in shallow groundwater beneath the site appears to remain stable PEG has attributed this finding, and ARCADIS Geraghty and Miller concurs, to the combination of the fine-grained sediments underlying the site, the ' relatively shallow gradient, and the effects of natural attenuation and degradation processes ' PROPOSED SCOPE OF WORK ARCADIS Geraghty & Miller developed this scope of work to vertically ' delineate dissolved-phase petroleum hydrocarbon impact in groundwater. The scope of work includes installation of one soil boring, collection of depth-discrete groundwater samples from the saturated zone, completion of a nested piezometer ' beneath the current level of groundwater, and preparation of a Corrective Action Plan All work will be performed under the supervision of a geologist registered in the State of California. In addition, all field work will be performed in accordance with a site- specific health and safety plan previously developed for the site by Pacific Environmental ' Focused Subsurface investigation ' SJCPHS requested, in meetings with UNOCAL, that the vertical extent of impacts at the site be delineated by advancing a soil boring at the center of the plume and taking depth discrete soil samples until a depth is reached at which sample chemical analysis indicates concentrations that are nondetect for the constituents of ' concern It is the opinion of ARCADIS Geraghty and Miller that drilling through the ' center of the area of known soil contamination is not advisable and would unnecessarily present the risk of cross-contaminating lower geologic strata with liquid petroleum hydrocarbons which, although not previously detected, could potentially ' exist in strata higher up in the geologic profile Such cross-contamination, if it occurred, could increase the degree of impact to groundwater quality and significantly increase the cost and time to achieve site cleanup Saturated zone soil sample results ' also do not provide a superior basis for determining potential impacts to groundwater quality beneath the site. The most accurate and representative method of assessing impacts to groundwater quality is to sample the groundwater Soil sampling in a ' borehole placed in,the center of the plume will also not provide a superior data set with which to formulatte a cost-effective method of cleanup at the site A groundwater flur rer_ Page ' ucM sarrrA,.eaKsnsnAMArA%APRo_ssowiuocw.+c YnLtenwas arv+aaCWWZkooc 5111