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PUBLIC HEALTH SERVICES �oP...... <br /> SAN JOAQUIN COUNTY <br /> JOGI KHANNA M.D.,M.P.H. '3 <br /> Health Officer P <br /> P.O. Box 2009 • (1601 East Hazelton Avenue) • Stockton, California 95201 iF o a <br /> (209) 468-3400 <br /> DAN KIRK <br /> SHELL OIL CORPORATION � 2 ��9� <br /> P O BOX 4023 >^ <br /> CONCORD CA 94524 rf <br /> RE: 620 West Charter Way SITE CODE: 1058124 <br /> Stockton, Ca <br /> San Joaquin County Public Health Services, Environmental Health Division (PHS/EHD) has received and <br /> reviewed the letters from Aegis Environmental dated July 8, 1992 and July 29, 1992 and has the following <br /> comments. <br /> Both of the above mentioned letters from Aegis Environmental were sent to PHS/EHD in response to our <br /> request for an additional sampling and monitoring point to the east of this site. The well is needed to <br /> fully define the existing groundwater contaminant plume in that direction and also to help in monitoring <br /> the efficiency and range of influence of the proposed remediation system. <br /> Aegis' letters makes reference to "off-site contributors" to this existing groundwater contaminant plume <br /> and that "Aegis will evaluate the possibility of additional site assessment as necessary", "upon the <br /> completion of the off-site contributor's investigations". Based on the existing data, this assumption is <br /> unjustified. Shell is responsible for defining a groundwater contaminant plume documented as resulting <br /> from the unauthorized release from the former tanks at the site. <br /> If a co-mingling of two separate groundwater contamination plumes is suspected, physical data can be <br /> submitted to PHS/EHD for review and concurrence. <br /> Therefore, any further delays of future assessment work at this site is unwarranted. Please submit a <br /> workplan to investigate the extent of the contamination to the east of the site by September 21, 1992. <br /> In addition, telephone conversations between PHS/EHD and Aegis Environmental have revealed the <br /> decision to use vapor extraction technology exclusively to remediate the soil and groundwater <br /> contamination at the site. PHS/EHD has the following concerns about this approach. <br /> 1.) The groundwater contamination plume identified to date is extensive, has migrated off-site <br /> and may be out of the range of influence of the existing vapor extraction system. <br /> 2.) There may be significant undissolved hydrocarbon contamination adsorbed to the soils <br /> underneath the water table in addition to the soils at the water table level in the area of <br /> the free product plume. The magnitude of the free product plume and the volume of free <br /> product recovered to date suggests that this may have occurred at this site. Vapor <br /> extraction alone may not be sufficient to mitigate this area of the contamination plume. <br /> Additionally, it is doubtful that the groundwater extraction technology previously <br /> proposed would mitigate this problem area effectively. <br /> A Division of San Joaquin County Health(are Services j <br />