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Memorandum v Environmental Resources Management To: Jim Adams, Catellus Development Corporation 7777 Botelho Drive Suite 260 From: Arun Chemburkar, P.E. Project Engineer Walnut Creek,CA 94596 (925)946-0455 Date: 9 February, 2000 (925)946-9968(fax) Subject: Chemical Mass Calculations - Former Tank Sites #1 and #2 1515 and 13255 West Weber Avenue, Stockton, California ERM. In response to a Regional Water Quality Control Board request, Environmental Resources Management (ERM) has performed calculations to estimate the mass of chemicals of concern (COCs) in ground water at 1515 and 1325 West Weber Avenue. This memorandum summarizes the results and methodology of these calculations (Tables 1 and 2). As a first step in determining the mass of COCl, ERM estimated the volume of water in the ground water system. Figures 1 and 2 present the estimated dimensions of impacted ground water at both sites. The plumes at these sites are roughly elliptical. ERM applied the mathematical equation for the surface area of an ellipse to determine the surface area of each of the plumes. Estimates for the major and minor axes as well as the calculated surface areas are presented in Tables 1 and 2 for Tank Sites #1 and #2, respectively. ERM assumed an average thickness of 15 feet for the aquifer at Sites #1 and #2. Total volumes for the plumes were calculated by multiplying the surface area of each plume by the depth of the aquifer. Assuming a porosity of 0.3 (EPA Technical Protocol for Evaluating Natural Attenuation of Chlorinated Solvents in Ground Water, Appendix C- Data Interpretation and Calculations, EPA/600/R-98/128,September 1998), ERM calculated the volume of water contained within the void space (total volume multiplied by the porosity; Tables 1 and 2). For each site, analytical results reported for each sample collected during the most recent ground water monitoring event (22 July 1999) were averaged to estimate the mass of COCs remaining in ground water. For samples in which COCs were not detected, concentrations were assumed to be equal to half the detection limit. The average concentration for the plume was multiplied by the volume of water in the void space of the ground water system. The result was an estimate of the mass of COCs remaining in the ground water. Results of these mass calculations for Site #1 and Site #2 are presented in Tables 1 and 2, respectively.