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Answers to Mass Balance Questions of June 30, 2003. 1 have some questions concerning the Mass Balance calculations found in the Work Plan for Additional Groundwater Assessment for the site as 3250 WesCHammer Lane in Stockton. This site is referred to as Parkinson's Hammer 1-5 Arco. I believe that the benzene concentration for MW-5 is incorrect. Instead of 3.4 ug/L,the benzene result should be less than 0.5 ug/L. Please correct the Zone 2 calculation based on this revision. If the benzene concentration in Zone 2 is changed to <0.5 ug/L, effectively eliminating Zone 2 from the sum of Total Dissolved Benzene, the mass of benzene remains 0.074 lbs when rounded off. Page 2 includes the Mass Balance Calculations with Zone 2 removed. Also, please explain the benzene concentrations of 760 ug/L for Zone 3 and 382 ug/L for Zone 4; and the MTBE concentrations of 10,335 ug/L for Zone 3,20,500 ug/L for Zones 4 and 5, and 12,050 ug/L for Zone 7 that are used in the Mass Balance calculation Regarding the benzene concentrations used in Zones 3 and 4,because Zone 3 is between the USTs and Zone 1, it was assumed to have a similar concentration as Zone 1, or 760 ug/l that was used for that zone. For Zone 4, an average of Zones 3 and 5, or 382 ug/L, was used. i Regarding the MtBE concentrations used, Zone 4 is an average concentration from wells MW-4 and MW-5 in that zone. Because Zone 5 is located between Zone 4 and the USTs, the concentration in Zone 4 was assumed to be present in Zone 5, For Zone 3, and average of Zones 2 and 4 was used, which should have been 10,635 instead of the 10,335 used. Page 3 includes revised MtBE calculations. Zone 7 is an average of Zones 5 and 6. I have corrected the first paragraph under the section titled Site Conceptual Model. The soil results should be reported in mg/kg, not ug/L. There were higher contaminant concentrations in the shallow soil found at three feet below the dispensers of 4450 mg/kg gasoline, 0.134 mg/kg benzene, and 179 mg/kg MTBE but this was not mentioned in your report. The Site Conceptual Model will be updated in the Final CPT Investigation Report, and will include revisions to this section that more accurately represent soil conditions beneath the site. I i A f V k i E