SITE INFORMATION AND CORRESPONDENCE FILE 2

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AECOM Ms.Victoria L.McCartney May 29,2013 1 Page 2 SUMMARY OF HYDROCARBON CONCENTRATIONS AT DEPTH(-50 fbg) Screen/Sample TPHg Benzene Analytical Well ID Depth(fbg)' Direction** (Ng/I)*** (Ng/I)*** Trend MW-15D 46-51 -- 19,000 650 variable MW-7 32-52 Southeast/downgradient 170 <0.5 decreasing MW-8 22-42 Southeast/downgradient <50 <0.5 -- MW-5 35-60 Southeast/downgradient <50 <0.5 -- CPT-8 47-50 East/crossgradient 1,100 <0.5 -- MW-2 35-60 East/crossgradient <50 <0.5 -- MW-6 35-60 West/crossgradient <50 <0.5 - *Approximate depths **Direction relative to monitoring well MW-15D ***Most recent data from HFA's site assessment report dated January 24,2012; and low-threat closure request report dated March 26, 2013 SJCEHD COMMENT.Provide the technical justification for the use of the geometric mean, rather than the arithmetic mean, when calculating the mass estimates of total petroleum hydrocarbons as gasoline(TPHg) in soil, and total petroleum hydrocarbons as diesel(TPHd)in groundwater. RESPONSE: The usage of geometric mean for calculating mass estimates of TPHg in soil and TPHd in groundwater provides a more conservative value for the concentrations which are log normally distributed. Usage of the arithmetic mean would oversimplify the distribution of the concentrations throughout the plume resulting in a lower residual mass calculation. SJCEHD COMMENT:Also clarify why the height(h) was multiplied by 0.8 to get a volume of 7,700,000 cubic feet when calculating the TPHd mass in groundwater. RESPONSE: For calculating the TPHd mass in groundwater, the formula used is "volume= r1 x r2 x 0.8 x h", where"h" is defined as vertical depth and is not changed. Because the plume is modeled as an ellipse, it is assumed that the plume is 80% of the volume of the water within the rectangular prism r1 x r2 x h. Therefore, the volume of the plume within the rectangle is(r1 x r2 x h) x 0.8 or 80% of r1 x r2 x h. Although it is likely that the volume is still an overestimate, the resulting volume is more representative of a dissolved-phase hydrocarbon plume. 60289045