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Mr George Takemori Project OF90-002 05 • August 15, 1996 Page 6 Based on the results of the investigations and using conservative TPHG concentrations, EMCON estimated approximately 3,195 pounds of TPHG were present beneath the site before the remediation system was operational. Groundwater Analytical results of groundwater samples collected during the previous eight quarters indicate that TPHG and BTEX were not present in groundwater in monitoring wells MW-9, MW-10, and MW-12 Dissolved hydrocarbons have been detected in monitoring wells MW-11, MW-13, and MW-14 TPHG concentrations ranged from below laboratory detection luruts to 19,000 ppb TPHG was detected at a concentration of 32,000 ppb during first quarter 1996, but it is EMCON's opinion that this concentration may not be representative of the aquifer due to the small volume of water within the well at the time of sampling or possibly due to laboratory error During this sampling event (February 1996), the groundwater sample from well MW-14 also had a high TPHG detection (17,000 ppb), which was nearly an order of magrutude greater than previous and subsequent sampling events In general, TPHG concentrations in groundwater have • declined Benzene concentrations have ranged from below laboratory detection limits to 580 ppb Benzene was detected at a concentration of 3,100 ppb in well iUiW-14 during fourth quarter 1994, but it is EMCON's opinion that this concentration may also be an anomaly Benzene concentrations within the last five sampling events have shown a significant decline Figure 2 depicts the extent of current dissolved hydrocarbons beneath the site Results of groundwater analyses are presented in Table 2 REMEDIATION SYSTEM In December 1994, a remediation system consisting of an Environmental Instruments 250- cubic-foot-per-minute thermal oxidizer was started up The system was designed to extract soil vapors from wells MW-13 and MW-14 The wells are screened from 55 to 70 feet bgs On May 8, 1995, EMCON observed the installation of one air sparge well (AS-1), which was installed to increase removal of dissolved hydrocarbons and increase the efficiency of the remediation system In June 1995, a 314-horsepower Conde blower was installed and connected to well AS-1 Air sparging commenced on June 19, 1995 Based on data collected from the remediation system, the cumulative mass of TPHG removed to date Is approximately 3,008 pounds (see Table 3) SAL\N\PJ"F9WP)00020 5AS-95\pas 1