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f • • <br /> Unocal Corporation <br /> Diversified Businesses <br /> 2000 Crow Canyon Place,Suite 400 <br /> "ll, San Ramon,California 94583 <br /> a° 1 ;�V "i A l T81 0 510)867-0760 <br /> 10)277-2309 <br /> q! )1!t <br /> 1� <br /> UNOC 4 1 2a <br /> July 25, 1996 <br /> Mary Meays <br /> Public Health Services <br /> West Region 304 E. Weber Avenue, 3rd Floor <br /> Environmental Remediation Services <br /> P.O. Box 388 <br /> Stockton, CA 95201-0388 <br /> Unocal SS#6981 <br /> Site Code 1217-123 <br /> Pacific and March Lane <br /> Dear Ms. Meays: <br /> Per our conversation the other day and as a follow up to your June 24, 1996 letter, I have instructed <br /> MPDS to sample the groundwater from the shallow vadose monitoring wells. After speaking with Alan <br /> Kemp at Sequoia Laboratory, it was determined that a 1 ppb detection limit for MTBE cannot always <br /> be obtained, but a 5 ppb detection limit is usually possible. Therefore, MTBE will be included in the <br /> analyses from the wells using a detection limit of 5 ppb. Alan also reviewed the data from the April 25, <br /> 1996 sampling event and determined that MTBE was non-detectable for all sampled wells to a <br /> detection limit of 5 ppb. A revised report will be issued to reflect this information. <br /> Construction activities in the intersection are scheduled to start on August 1. Upon completion of the <br /> construction activities and after reviewing additional groundwater data,Unocal will submit a work plan <br /> to install additional wells necessary to characterize the groundwater contamination. Attached are <br /> several figures showing the groundwater elevation and TPH concentration versus time for each well. <br /> Each figure also indicates the well's screen interval plus the time frame where the vapor extraction <br /> system was operating. As you can see from the figures, the depth to water from 1990 through most of <br /> 1995 exceeded 50 feet below grade. Since then, the groundwater has risen around 10-15 feet. A close <br /> look at the data around January 1995 suggests that hydrocarbons in groundwater have reduced in <br /> concentration in part because of the vapor extraction system. <br /> If you have any questions, please call me at(510)277-2335. <br /> Sincerely, <br /> David J. dfLille <br /> Senior Environmental Engineer <br /> attachments <br /> cc: Greg Gurss, GSI- Sacramento <br /> File SS #6981 : 3 <br />