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ARCAUIS GERAGHTY&MILLER 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 Gettler Ryan's(GR) Site Safety Plan#24004202 ssp,dated May 8, 1998 (GR, 1998) Focused Subsurface Investigation i SJCPHS requested, in meetings with UNOCAL, that the vertical extent of ' impacts at the site be delineated by advancing a soil bonng 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 formulate a cost-effective method of cleanup at the site A groundwater cleanup strategy should be formulated based on an evaluation of sampling data from the medium that is the target of cleanup,groundwater Lastly,uncertainty is associated ' with the actual vertical flow path that the potential deeper hydrocarbon release may have followed This uncertainty is related to differences in soil type and heterogeneity that occurs in the subsurface There is no way to know that any given spot selected for ' dnlling will be the actual location where the potential vertical migration of the hydrocarbons occurred One soil boring is being proposed in this investigation to be advanced using ' hollow-stem augers at the approximate location provided on Figure 1 Based on available groundwater data, this location is downgradient of the known potential source area and within the dissolved-phase hydrocarbon plume This approach will provide samples taken from a location designed to intercept dissolved constituents migrating and dispersing immediately downgradient of the location of the area of release while incurring a much reduced risk of cross contaminating lower strata of the geologic profile Further, because of the broadening of plume width that typically