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CAP Addendum:Former Fue lg Station, 7500 West Eleventh Street, Tracy, L Page 17 rate of attenuation of petroleum hydrocarbon contaminants in groundwater. Obviously, if 100% of the LNAPL can be extracted there will be a very rapid onset of aerobic conditions in the groundwater and that will be accompanied by greatly increased rate of groundwater remediation. From the foregoing we see that optimal performance of the proposed groundwater remediation system is controlled by the system's ability to remove LNAPL from the subsurface. If all LNAPL can be removed, remediation of groundwater by aerobic processes of natural attenuation will reduce the concentration of dissolved contaminants at a favorable rate. If less than 100% of the LNAPL can be removed, the remediation system would still generate a significant decrease in the total time required for natural attenuation to remediate groundwater because the duration of the initial anaerobic stage of the process would be reduced in direct proportion to the fraction of the total volume of LNAPL that can be extracted from the subsurface. With that understanding, it is now evident that the ability to predict the likely performance of the proposed groundwater remediation system depends on our ability to predict the quantity of LNAPL that can be extracted from the subsurface. As was discussed in Section 2.1.5, the state of the art methodologies for making such predictions a priori yield widely varying estimates This problem occurs because, although the general nature of the physical and geochemical processes that control the disposition and extraction of LNAPL are reasonably well understood, 'it is essentially impossible in practice to measure the majority the very large number of parameters that control such processes under specific in situ conditions. Unfortunately there is no more than a handful of empirical laboratory studies that have addressed the subject and those have all modeled simplified and idealized soil deposits. There.are even fewer case histories presented in the technical literature and each of those, of course, relate only to the site- specific conditions encountered by the authors. 2.4 Importance of Application of the Observational Method Experienced geotechnical engineers, when confronted with the requirement to generate solutions to problems associated with the very complex interactions of small or even large-sale phenomena that influence the behavior of earth systems or human infrastructure that interact with earth systems have long relied on application of the Observational Method when appropriate data from field observations is available. Simply stated, the Observational Method (Peck 1969) relies on observed and measured conditions as they actually occur at a specific site to predict future behavior of geotechnical systems or to assess compliance of a constructed system with its design objectives. It is essential, of course, that interpretation and use of the field observations and measurements be informed by a good understanding of all of the natural physical and geochemical processes and parameters that give rise to the observed conditions. The practitioner must also be keenly aware that apparently minor variations in geotechnical conditions from one location on a site to another can have important effects on the