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w � 437 East Miner • December 1997 Page 2 As stated in the above referenced feasibility study, "the highest concentrations of TPH-g were detected in the boring U-L at 45 feet bgs (19,000 ppmy, It was not reported that the vertical extent of this contamination remains undefined PHS/EHD has stated in correspondence dated January 3, 1994, November 4, 1994,December 23, 1994, June 3, 1997, and June 25, 1997 that the vertical extent of the hydrocarbon contamination has not been defined PHS/EHD focussed regulatory efforts,during the years of 1995, 1996,and 1997,towards Unocal performing a vapor extraction feasibility test when the highest concentrations identified to date were above the water table level,however,compliance was never achieved The point of defining the vertical and horizontal extent of the soil and groundwater contamination is to comply with the Health and Safety Code and Underground Storage Regulations and to prepare a corrective action plan which addresses the contamination which resulted from the operation of former underground storage tanks Unocal has not demonstrated willingness to comply with the California haws and Regulations i The vertical and lateral extent of soil and groundwater contamination has not been delineated Until conclude that the plume is not migrating Already,the groundwater plume has migrated off site such time as the full extent of the soil and groundwater contamination is determined,it is premature to impacting groundwater beneath neighboring properties The work plan to complete the investigation should have included soil borings on site to define the extent of on site contamination and to install air sparge and vapor extraction wells to perform the required feasibility study The above referenced feasibility study failed to include at least two alternatives for restoring or protecting the beneficial uses of the groundwater which has been impacted as a result of the operation by Unocal of former underground storage tanks PHS/EHD has observed that as groundwater levels have risen, resulting in a longer column of water within monitoring wells, that samples collected from these wells may not be representative of groundwater contamination levels due to dilution effects Wells which have been installed with proper screened intervals,in plume conditions similar to this site conditions,have evidenced significantly higher Ievels of groundwater contamination Should Unocal wish to evaluate natural attenuation as one of the remedial alternatives,additional monitoring wells will be required which are screened discretely to monitor the full vertical and lateral extent of groundwater contamination If you have any questions please contact me at (209) 468-0337 Donna Heran,REHS,Director /Enjviironmental Health ivision �If Mary Meays,Senior REHS Margaret Lagorio, REHS Site Mitigation Unit Supervisor cc David Irey,San Joaquin County Deputy District Attorney cc Pat Anderson,CVRWQCB cc Sherri Kirk,Starsky&Kirk,601 University Ave Ste 245,Sacramento CA 95825-6708 cc Peter Nienuec,Unocal Corporation, 376 S Valencia Ave Brea CA 92823 . cc Stephen Carter,GeoStrategies 3164 Gold Camp Ste 2401 Sacramento CA 95670 cc Sutter Office Center, 2001 Union Street Ste 300,San Francisco CA 94123