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former tanks appears to be well defined. However, the lateral extent of impacted soils, and <br /> the lateral and vertical extent of PHCs in groundwater appear to extend beyond the limits of <br />' this investigation. <br /> According to Tri-Regional Board Staff Recommendations (Central Valley Regional Water <br />' Quality Control Board, 1991 - Recommendations for Preliminary Investigation and Evaluation <br /> of Underground Storage Tanks, Appendix A), when groundwater has been shown to be <br /> contaminated as a result of a tank leak, the site investigation is to proceed in phases <br /> Following Tri-Regional recommendations, the next step recommended for Site 3 would be to <br /> develop a work plan to complete the site characterization in order to prepare a Problem <br /> Assessment Report (PAR) A PAR describes the lateral and vertical extent of the problem, and <br />' proposes mitigative or remedial actions for a site Further discussion related to the cost <br /> effectiveness, technical feasibility, and environmental appropriateness of potential corrective <br /> actions will be more constructive once the PAR has been completed A work plan designed <br /> to collect the information necessary to complete the PAR at Site 3 njX include, but not be <br /> limited to, the followuig investigative procedures <br />' o - r i r groundwater 1 - additional soil and <br /> h �nsc e � <br /> groundwater sampling in areas peripheral to the former tank area will be <br />' necessary to define the lateral and vertical extent of contamination <br /> o Monitoring well insWlation - Monitoring wells appear to be indicated for two <br /> reasons (1) to establish the site-specific gradient direction or variations therein, <br /> and (2) to collect periodic groundwater samples to demonstrate either (a) <br />' stabilization or attenuation of PHCs in groundwater, or (b) the effectiveness of <br /> groundwater remediation should that be deemed necessary Although data on <br />' groundwater direction and gradient are available from monitoring wells located <br /> on other sites within the Port complex (e.g., SPGC, 1998, and EMCON, 1998), <br /> these data show variable direction and gradient Therefore, site-specific <br />' groundwater data will be required At least one monitoring well should be <br /> installed within or near the former tank area, to monitor trends in water quality <br /> in the area where PHC concentrations are likely to be highest. <br /> I <br />' 6.0 REFERENCES <br /> Bartow, J Alan and Tor H Nilsen, 1990, "Review of the Great Valley Sequence, Eastern <br />' Diablo Range and Northern San Joaquin Valley, Central California," USGS Open File <br /> Report 90-226 <br /> 11 "117\RfvaM%MUS W urr <br /> I <br /> BOYAJIAN & ROSS, INC <br />' Environmental ScientLSIS & Fngineer% <br />