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' Mass Balance for Remaining TPHg • Colberg Boat Works,401 North Stockton Street, Stockton, CA ��e ' Page 2 For this mass balance calculation, it is assumed that the soil plume extends15 fee n radius from MW-3 The thickness of impacted soil will be assumed to be 15 feet since no ydrocarbon impacts were detected at 20 feet bg in boring MW-3 The soil analytical results were taken from the report prepared by Apex on August 21, 1997 titled Results of Subsurface Investigation A table of the data from this report is included as Appendix B In order to estimate the amount of ' impacted groundwater, it is assumed that TPHg impacts will remain within the top 10 feet of the aquifer Analytical results from soil sampling performed in 1997 indicate weathered TPHg at concentrations ranging from 5 2 parts per million (ppm) to nondetectable, and analytical results from groundwater sampling conducted on January 20, 1999 indicate concentrations ranging from 1,100 parts per billion (ppb) to nondetectable concentrations Using analytical data from the groundwater sampling event, iso-concentration contours were created for the distribution of TPHg in groundwater(Figure 2) As mentioned earlier, the iso-concentration contour for TPHg impacts to soil is assumed to be 15 feet in radius from soil boring MW-3 (Figure 3) The mass of TPHg present at the site will be estimated by finding the area of each iso-contour, calculating the volume of soil and water in each contour, and by using an average concentration of TPHg inside each iso-contour '. Calculating Mass The first step to calculating the mass of TPHg remaining in water and soil is to identify the water ' and soil volume inside of each contour Assuming the TPHg in the aquifer is contained within the top ten feet, the thickness of the water table is multiplied by the area inside the contour to determine a volume of water present in this contour interval Also, assuming that impacts to the soil are limited to 15 feet bg, the thickness of the soil is multiplied by the area of soil inside the contour For a sample calculation, the volume inside the 100 µg/L contour for water and the area inside the nondetectable contour for soil will be determined 1 Volume"""'= 191 1 ft,* 10 ft= 1910 f? Volume,a = 706 9 ft * 15 ft= 10604 ft3 An average concentration inside the iso-concentration contour is determined by averaging the highest and lowest (boundary) concentrations of each contour interval It is assumed that the average between the higher and lower contour concentration for a particular interval will be a good approximation for the average concentration of a particular contour interval volume The ' average concentration inside the 100 pg/L iso-concentration contour for groundwater and inside f the nondetectable iso-contour for soil will be calculated by averaging the maximum concentration found in groundwater and soil samples and the established lower boundary for those iso-concnetrations c (1f . Concentrationwpter= (1,100 + 100) µg/L/2 = 600 lcg/L SB C 1_PROJECT1COLo 1 001-Colberg Boat Workslmassbalance doc