SITE INFORMATION AND CORRESPONDENCE

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UNOCAL Page 2 Monitoring wells should be installed off site and downgradient with these deeper screened intervals to show the lateral and vertical extent of the groundwater contamination. Contaminant levels in groundwater from 25 to 50 feet bsg on site appear to have reduced since 1988 when MW 1, MW2, and MW3 were first installed. This probably is due to dilution and dispersion from the increased amount of water in this interval resulting from the heavy precipitation in recent years (1996 and 1997) and recharge, as well as the vapor extraction conducted in 1996. In February 1999, downgradient and off site MW7 (screened interval 35—59 feet, although quarterly sampling reports show total depth 55 feet) evidenced benzene and MTBE for the first time. This may be evidence that the contamination is continuing to migrate. Comparing the levels of contaminants in the soil samples from VW2(drilled in February 1992)with the soil samples from NPI (drilled in January 1999)which are about 20 feet apart, it appears that vapor extraction was a viable remedial alternative. The highest levels of soil contamination remaining on site appear to be below 50 feet from surface. The depth to first water when the vapor extraction system was operating was about 35 feet below grade. Vapor extraction well 2 is screened from 24 to 51 feet bsg, so it appears that vapor extraction was successful in remediating the soil below water. As previously discussed in PHS/EHD correspondence, feasibility tests must be conducted to determine the most cost-effective remedial alternative. Site conditions have changed since the vapor extraction tests were performed in 1992 and 1996. Vapor extraction and air sparging should be evaluated since they have proven to be effective in areas of Stockton. On page 22 of the February 26. 1999 report, there is discussion about the Basin Plan not differentiating between Primary and Secondary MCLS and that wells with constituents above an MCL are not considered safe for drinking. While the Basin Plan may not differentiate between Primary and Secondary MCLS, the laws and regulations governing public drinking water certainly differentiate between MCL classifications. Primary MCLs are based on health concerns and secondary MCLs are based on constituents that may adversely affect the taste, odor or appearance of the drinking water. In addition, best available treatment technologies differ for the constituent of concern. On page 7 of the February 26, 1999 report, there is discussion about the saline front in the Stockton area and references a map from the 1997 San Joaquin County Flood Control and Water Conservation Report. It appears from that map that the saline front changes with the level of groundwater. In 1990 first groundwater was about 50 feet bsg and the site was within the saline front. In 1985 and 1997 when first groundwater was about 30 feet bsg, the site was not within the saline front. On page 24 of the February 26, 1999 report, it is noted that the pH of the water from NPI is greater than the pH of the water from MW2 and MW10. NP1 is smaller in diameter than the other monitoring wells which made its' development difficult. The pH of the water from NP1 reduced as the well was developed on February 1, 1999. Also, the pH of the water from NP1 during its' purging on February 4, 1999 was similar to the pH of the water from MW4, MWS, MW7, MW8, or MW9.