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Ms Diane Hinson Project 0805-118 03 <br /> February 13, 1995 <br /> Page 3 ' <br /> volume of groundwater beneath each site and multiplied it by the concentration of TPHG <br /> in groundwater TPHG concentrations in groundwater recorded during quarterly <br /> groundwater monitoring events was used in the calculations <br /> EMCON developed isoconcentration lines for TPHG in groundwater based on the results <br /> of the third quarter, 1994 quarterly sampling event Although fourth quarter, 1994 results <br /> were available, EMCON chose to use the third quarter results as the TPHG concentrations <br /> in the third quarter were consistently higher at both the sites The depth to the bottom of <br /> the first water bearing zone was not available at either site therefore, EMCON assumed <br /> that the depth of the deepest monitoring well to be the depth to the bottom of the water <br /> bearing zone The porosity of the water bearing zone was established based on the known <br /> lithology of the sites <br /> The following assumptions were made to simplify the calculations <br /> 1 TPHG concentration remains constant between two isoconcentration lines / <br /> 2 Soil type and porosity remain constant throughout the aquifer i <br />. The following formulas was used to calculate the volume of groundwater and then mass of <br /> TPHG beneath the site <br /> Volume of = area between x porosity of x depth of <br /> groundwater isoconcentration lines aquifer aquifer <br /> Mass of TPHG = volume of groundwater x concentration <br /> in groundwater of TPHG <br /> Figures 1 and 2 illustrate the isoconcentration lines for TPHG for the two ARCO stations <br /> 760 and 548 respectively Attachment A presents the detailed calculation sheets for both <br /> the sites Table 3 summarizes the total mass of petroleum hydrocarbons at each site <br /> EMCON employed identical procedures and assumptions to both the sites EMCON also <br /> incorporated the PHS/EHD suggestion to use the inner line to calculate the area of <br /> impacted groundwater <br /> The total mass of dissolved TPHG in groundwater at ARCO station No 760 is <br /> 1 02 pounds The total mass of dissolved TPHG in groundwater at ARCO station No 548 <br /> is 6 20 pounds <br /> WATER WELL SURVEY <br />• In addition, EMCON conducted a well survey for the ARCO Facility No 548 <br /> Information on nearby water-supply wells was obtained from the California Department of <br /> Water Resources According to its records, there is one domestic well within 1/4 mile of <br />