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• Corporation ember of The IT Croup Ms. Devra Lewis Project 812717 April 9, 2001 Page 3 IT proposes to defer the installation of additional downgradient wells and proceed with installing the proposed wells that are situated downgradient of the main plume boundary and upgradient of CPT/1-100-16. The groundwater analytical results from these wells will be used to evaluate the validity of the analytical results at CPT/H00-16. The STTC would like to defer the installation of additional upgradient groundwater monitoring wells. The wells that are requested north of the site and northwest of monitoring well TO/WC-1M and PS/P-29 are both upgradient of the source areas. Historical records show that benzene was detected in groundwater samples at two upgradient CPT locations at concentrations of 0.6 and 2.7 gg/L during the Interim Pollution Assessment conducted by EMCON in 1993. Petroleum hydrocarbons were not detected in groundwater samples collected from a CPT located north of the ARCO terminal during the Phase II Pollution Investigation Assessment, (EMCON, 1994). Benzene was periodically detected in groundwater samples from upgradient well PS/MW-IM at concentration near the lower detection limit. The RWQCB consented to eliminating this well from the quarterly groundwater monitoring program in December 1998. The historical groundwater analytical data shows that the petroleum hydrocarbons have not migrated upgradient from the source areas. 5. The isoconcentration maps need to show all contour lines. Several isoconcentration maps contour to 1,000 parts per billion (ppb), and the next contour line is less than 50 ppb or less than 0.5 ppb. The concentration gradients for many of the constituents are low at several of the terminal boundaries and decrease significantly toward the interiors of the site. We chose to use these contour intervals because we can best account for irregularities in the concentration gradients and best portray the lateral distribution of the various constituents. We considered using a logarithmic scale in areas with steep concentration gradients but found that we would loose much of the detail. We also hand contour each of these maps instead of using a contouring program(i.e., Surfer"11) so that we can better account for some of the irregular concentration gradients. Additional contour-intervals will be difficult to fit on the map and will likely obscure important site features. 6. Isoconcentration maps must reflect reported concentrations. Examples of discrepancies area • Figure 7 has a value of 6,420 ppb on the 10,000 ppb contour line; Benzene was reported at a concentration of 6,420 gg/L in the A-Zone groundwater sample at CPT/H00-05. This result was plotted near the 10,000 gg/L concentration contour because we were inferring a steep concentration gradient in this area due to separate phase hydrocarbons detected in the adjacent well TO/MW-1. • Figure 8 has a value of II ppb contoured inside the 100 ppb contour line; tT CORPORAT/ON SAC:\C^My DocumentsTroimts\Pon of StocktomCPT HP Study\Response to Comments.doc-Mit:1 812717