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r ARCADIS GERAGHTY&MILLER 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 drilling will be the actual location where the potential vertical migration of the hydrocarbons occurred ' One soil boring is 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 potential source areas 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 occurs downgradient of a ' release point, the chances of intercepting and sampling a released constituent are increased downgradient of the release area Groundwater samples will be collected from the saturated zone at a minimum of 10-foot intervals using a Hydropunch— or similar sampling device The fust water sample will be collected at the water table based on the water elevation measured in the nearest groundwater monitoring well Groundwater samples will be collected to a minimum depth of 70 feet bgs unless a competent(greater than 3 feet thick)confining unit is encountered at which point dnIlmg and sampling will cease Water samples will be analyzed by a State-certified on-site mobile laboratory for TPHg, benzene, ' toluene, ethylbenzene, and xylenes (BTEX), and methyl tertiary butyl ether(MTBE) The vertical extent of dissolved-phase hydrocarbon impact will be determined to be adequately defined when two groundwater samples in succession, collected below the 1 last sample interval reporting dissolved-phase hydrocarbons, contain no detectable concentrations of TPHg,BTEX or MTBE ' Following the collection of the final Hydropunch— sample, two 1-inch diameter piezometers will be installed in the borehole One piezometer will be screened in the upper impacted zone and the second piezometer will be screened in the ' lower nonimpacted zone as identified by the results of the Hydropunch— sampling The exact depth of the screen intervals will be determined by the field geologist based on site specific lithology and constituent distribution information. Water level data ' provided by the piezometers will assist in determining whether a vertical groundwater gradient is present. In the absence of a vertical gradient, groundwater will only flow laterally. Thus, in the absence of a vertical gradient, there is no substantial reason to believe that groundwater would be impacted beneath the depth at which nondetect 191 our rd Page ' 116141 SANTiJ�_BARBARATATAWPRO EOWNOCAL%C VALLEAVVPS_REVWACWKP2A.DOC 6111