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Robert Boust/UNOCAL 95098 page 2 5606 Pacific Ave , Stockton In PHS-EHD correspondence dated September 30, 1996 and February 26, 1997, UNOCAL was advised that due to the high levels of groundwater contamination consistently recorded in the down-gradient webs, additional soil investigation north of MW-11 and MW 12 was required PHS-EHD concluded the "source" of this contamination as undefined and would require the assessment of the site to continue until the source was delineated ERI's work pian included a copy of an August 25, 1989 Central Valley Regional Water Quality Control Board (CVRWQCB) letter as support for not including the required sod borings in the work plan In 1989 PHS- EHD was not the lead agency for this underground storage tank investigation and was not included in the evaluation of the effectiveness of the soil remediation The CVRWQCB letter does not relieve you of any liability under the California Health and Safety Code or Water Code for past, present or future operations at the site Nor does it relieve you of the responsibility to clean up existing, additional, or previously unidentified conottrons at the site, which cause or threaten to cause pollution or nuisance or othen,vtse pose a threat to water quality or public health The August 25, 1989 CVRWQCB letter expressed the most current conditions at the time Unfortunately the long periods of minimal monitoring and lack of remedial activity since have supplied data that indicates a source still remains at the site and it continues to impact the groundwater The consistent elevated levels of petroleum (50 000 ug/I TPH-G and 1 500 ug/I Benzene in MW-11 , Sept-1997) support this conclusion With this consistent impact and degradation to the groundwater below this site, PHS- EHD has concluded that ssgnrficant contamination, not included, investigated, or remediated in Unocal's 1989 investigation, still remains at this site and must be investigated further PHS-EHD will allow this work plan to proceed with the following conditions The missing Mems noted above are to be included in an addendum submitted to PHS- EHD immediately Additionally, soil borings are to be placed north of MW-11 and MW-12 Soil contamination at the nistorical drought depth is to have the lateral, as viell as the vertical extent defined Monitoring v,ells may be required if the soil and/or groundwater samples appear contaminated In a telephone conversation with the City of Stockton Public WoWEncroachment Permitting Department, Mr Jason Ender has indicated that monitoring wells cannot be placed in traffic lanes Proposed morntonng wells MW-20S and MW-20D cannot be constructed in the proposed locations Mr Ender has indicated that although monitoring wells cannot be installed, borings or push-points to groundwater may be possible in the traffic lane at special times and days if your modified plan meets PHS-EHD and City of Stockton requirements You may contact Mr Enders at (209) 937-8349 to obtain the push-point and other monitoring well encroachment and/or revocable permit requirements