SITE INFORMATION AND CORRESPONDENCE_1988-1993

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SHELL OIL COMPANY -2- 3 June 1992 STOCKTON FACILITY • the State. As specified in 40 CFR 112.7(e), the above provisions of the state law should be discussed in the plan. Item 33: I agree that Section 25270.5(d)(3) of the APSA pertains to farms, nurseries, log ing sites, and construction sites. The proper citing is 40 CFR 112.7(e)(2�(ii) . This section specifies the same secondary containment requirements that Section 25270.5(d)(3) of the APSA. Thus, the comments in the checklist still apply. Furthermore, the U.S. EPA is proposing to require containment of the largest tank volume plus precipitation from a 25-year storm event. Shell has stated its containment area will hold 120% of the largest tank volume but has not' shown what the additional 20% containment volume represents in terms of precipitation. Item 34: See Item 26. Item 35: See Item 5. Item 36: See Item 5. Shell 's responses on the other items are adequate. • Site Status Comment 1: Shell ' s response is adequate. Comment 2: Shell 's response is adequate. Comment 3: Shell states that additional assessment work should not hold up installation of the additional recovery pumps. I am concerned that the addition of two extraction wells prior to fully defining the horizontal and vertical extent of contamination could accelerate the spread of contamination. (See Comment 4 below) . Comment 4: The Cooper and Theis methods used to determine aquifer properties assume, among other things, that the well used in the pumping test fully penetrates the aquifer. Thus, estimation of hydraulic properties would be more accurate if the conditions used during testing approach the assumptions used in calculating the hydraulic properties. Furthermore, if the wells are partially penetrating the aquifer, the cleanup is effective only at elevations corresponding to the screened zone. Contaminants in the nonscreened zone may not be captured by the extraction wells. Also, those previously uncontaminated portions of the aquifer that form peripheral bounds of the contaminant plume may become contaminated if the entire plume is not captured by the extraction wells. Another result of not capturing the entire plume is • that the spread of contamination could be accelerated since extraction would increase the velocity of ground water in the downgradient direction.