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PUBLIC HEALTH SERVICES <br /> SAN JOAQUIN COUNTY 1 ;:,� <br /> RECEIVED ENVIRONMENTAL HEALTH DIVISION s <br /> MAR ., 7 1997Ernest M„ Fuji mmoto, M. D., M.F-H., Acting Health Officer •. <br /> 304 Fmt Weber Avenue,Third Floor a Stockton, CA 995202 y�a.. <br /> 209/"8-3420 <br /> UNOCAL CORPORATION FILER Z�_SS8 <br /> BOB ROUST RPT QM TRANSIVIITT� 97 <br /> P O Box 5155 2-3-1/-4— 5— Rr,_,_ <br /> SAN RAMON CA 94583 <br /> RE: FORMER UNOCAL#2859 SITE CODE: 1213 <br /> 1665 PACIFIC AVE <br /> STOCKTON CA <br /> San Joaquin County Public Health Services, Environmental Health Division <br /> (PHS/EHD) has, reviewed your February 7, 1997 request for closure letter. <br /> Your letter states that eight years of monitoring data strongly suggests the <br /> groundwater plume is naturally attenuating. The monitoring wells with 8 years of <br /> data are wells 1, 2 and 3, wells 4 and 5 have 7 years of data, wells 6 and 7 have <br /> 4 years of data and wells 8 and 9 have 2 years of data. <br /> Well 1 has rarely exhibited evidence of contamination. Well 2 is the most highly <br /> contaminated well exhibiting 4200 ppb as the current benzene level. Well 3 <br /> exhibited a flat trend from 1988 to 1990, was dry from 1990 to 1993 and after <br /> 1993 exhibited decreasing levels of benzene. Well 6 has shown a definite <br /> decrease from 19 ppb benzene in 1992 to ND in 1996. Wells 7, 8 and 9 have <br /> never shown any benzene. Well 5 has shown very low benzene levels <br /> sporadically. Well 4 has shown levels between .5 and 2 ppb at least annually <br /> from 1989 to 1996. <br /> The data indicate a stable and declining plume with MW-2 at the core. The overall <br /> area of the plume is oriented downgradient approximately 130 feet long and 60 <br /> feet wide with an average benzene concentration of 32 ppb. <br /> Remedial activities which have occurred to date include overexcavation of 1100 <br /> cubic yards of Impacted soil from an excavation under the tank pit area, and <br /> removal of 2745 lbs of gasoline and 12 lbs of benzene using a pilot vapor <br /> extraction system (VES). <br /> A ni,.kion of son Jooquln county Raft am Se j=s <br /> EXHIBIT 4 <br /> Z00 'd 60£Z LLZ 01s:131 311 HISON 1330 'IYOON11 0:01 ((13A)L6 ,90-- '5aV <br />