SITE INFORMATION AND CORRESPONDENCE_FILE 2

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19g2 4 0 : zoo 1 419 • • 19%o < 1vv9 sewfi una 9/17/ 11 by 34eve & 'k)L4 This letter is written in response to a telephone conversation between the San Joaquin County Department of Environmental Health(SJCDEH)and Steve Strait of Stantec Consulting Corporation (Stantec)on August 12,2011. The SJCDEH requested additional information on the initial TPH-g concentrations used in setting up the FLOWPATH II model for the site at 4707 Pacific Avenue,Stockton. This letter responds to that comment. The beginning of the leak was estimated to occur in 1990 as that is the earliest data for the site. The initial concentration contours were used with the following rationale: 6ej9 d s?444ty 7G• 1972 1) Just upgradient of VW-2 an initial TPH-g concentration of 80,000µg/L was used. This location was assumed to leak for approximately 19 years. A TPH-g concentration of 50,000 µg/L surrounding wells VW-2 and VW-7 was also used and the initial concentration input and assumed to leak for 1 year. These values were chosen to calibrate the model based on observed concentrations. This is also a conservative worst case scenario approach since the maximum concentration in VW-2 was 78,000µg/L in 1996, had some lower detections in 1999 and 2000 than in 2011 had detections of TPH-g of 23,000 and 18,000µg/L in January and June, respectively. The amount of time the leak occurred is an integral part of the calibration process and the values were chosen to match the historical concentrations observed. 2) The initial TPH-g concentration spanning from MW-5 toward VW-2 and VW-7 had an initial concentration in the model of 70,000µg/L. This was assumed to leak for 35 days. This concentration,configuration and leak time were chosen from calibration of the model with historical observed concentrations in well MW-5. This initial concentration input parameter does not match with the concentration of TPH-g observed in well MW-5 in February 1990 (3,600µg/L). However, it does match with other observed concentrations in this well. One limitation of FLOWPATH II is the inability to have leaks occur at different times. There is a certain degree of uncertainty in the concentrations of the plume because numerous wells were not sampled for extended periods of time. If you have any further questions with the model or the results please contact Steve Strait at 559-271- 2650.