SITE INFORMATION AND CORRESPONDENCE

Home Browse Search

MAY, -07' 99 (FRI) 08:55 UNOCAL r-'T NORTH RE TEL:510 0`4 3781 K003 j UNOCAL Page 2 Monitoring wells should be installed off site and downgradient with these deeper sareened' Intervals to show the lateral and vertical extent of the groundwater contamination. Conbaminant levels in groundwator from 25 to 50 feet b9g on site appear to have ierfuced since 1988 when MW 1. MVV2, and MW3 were first installed. This probably is due to dilution and dispersion from the increased amount of water in this interval resulting frnm 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, aliliuuyli quarterly sampling reports show total depth 55 feet) evidenced benzene and MTBE fior the first time. This may be evidence that the contamination is continuing to migrate. Comparing the levels of contaminants in the soil samples from WW2 (drilled in February 1992)with the soil samples from NP1 (drilled in January 1999)which are about 20 fleet apart, it appears that vapor extraction was a viable remedial altemative. 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 woo oporetirtg wea about 35 feet below grade. Vapor rxirdction well 2 is screened from 24 to 51 feet bsg, so it appears that vapor extraction was successful in remediating the sol below water. As previously discussed in Pf S/EHD correspondence, feasibility tests must be conducted to deiermine the most cost-effective remedial altemative. 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 effectivc in areas of Skx ton. On page 22 of the February 26, 1999 report, there is discussion about the Basin Plan not dlNerentiating 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 MCtz, the bows and regulations gvvei Hing public drinking water cen2inly differentiate between MCL classifications. Primary MCLS are based on health concerns and secondary MCLS are based on constituents that may adversely aft nt the taste, odor or appearance of the drinking water. In addition, best available treatment technologies differ for the constituent of coneem. On page 7 of the February 26, 1999 report, there is discussion about the saline front in the Skx*tnn area and references a map From the 1997 Son 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 MW1 D NPI is smaller in diameter than the other monitoring wells which made its' development difficutt. The pH of the water from NP1 reduced as the well was developed an February 1, 1999. Also, the pH of the water from NP1 during its' purginy on February 4, 1999 was similar to the pH of the water from Mm, MW5, MUV7, MW8, or MUIl9.