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ARCADIS GERAGHTY&MILLER convenience of the automated calculation, unless a formal evaluation of the uncertainty is also included as part of the assessment. Some regulatory agencies have in the past used more simplified calculations than those described above to set soil concentration guidance levels which are designed to be protective of groundwater. These approaches have declined in usage and application as the knowledge of contaminant fate and transport has increased. Ultimately, these approaches utilize simplified models which are no less uncertain than the calculation methodology described above. Currently, there is no approach that can be considered more useful and viable for monitoring a resource medium than actually taking a sample of that medium itself. The approach presented in the work plan goes beyond predicting fixture impacts by providing the means (through the installation of a sampling point) of acquiring real-time, tangible data which demonstrates the water quality status of the site groundwater. It is the opinion of ARCADIS Geraghty and Miller that it is not necessary or appropriate to acquire saturated zone soil samples at this site in order to assess the current or future potential impacts to groundwater at the site. Groundwater samples, as previously collected at the site and proposed in this work plan, are a superior approach. However, in order to demonstrate UNOCAL's desire to cooperate and collaborate with SJCPHS, ARCADIS Geraghty and Miller will collect and analyze unsaturated zone and saturated zone soil samples at the sampling point proposed in the May 26, 1998 work plan according to the protocols described in Attachment A (also included in the May 26, 1998 work plan). Separate soil and groundwater samples will be simultaneously taken at 10-foot intervals, using the simulprobe technology, starting at ground surface down to total boring depth. Sampling will continue, as indicated in the May 26, 1998 work plan, to a minimum depth of 70 feet bgs and until two successive samples in a row contain no detectable concentrations of TPHg, BTEX, or MTBE. Below the first encountered water, samples will be taken at maximum of 10- foot intervals or at changes in lithology. Samples will be submitted to a California State Certified Laboratory under chain of custody and analyzed for the petroleum hydrocarbon constituents of interest SJCPHS as described in section A.6 of the Attachement to the work plan. Comment: In paragraph 6, your letter states, "The proposed soil boring should be located further south and closer to MW2." 4