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437 E. Miner Street, Stockton <br /> Page 2 <br /> San Joaquin County has a history of substantial fluctuations in groundwater depth <br /> caused by years of drought and overdraft, then years of heavy precipitation and reduced <br /> groundwater usage allowing groundwater recharge. San Joaquin County Flood Control <br /> Maps show that the depth to groundwater in the area of this site in 1978 was about 70 <br /> feet bsg and in 1986 the depth to groundwater was about 30 feet bsg. Since it is <br /> unknown when the release of petroleum hydrocarbons occurred, it is possible that soil <br /> contamination exists to 70 feet bsg or deeper. <br /> On December 15, 1997, a water sample was collected from the vapor extraction well, <br /> located in the area of the UST's removed in 1967, and it was noted that there was <br /> sheen on top of the water as well as a strong hydrocarbon odor. Monitoring wells <br /> installed to a depth of 55 feet bsg, across the street from the site, have indicated that <br /> the plume of groundwater contamination extends laterally to at least that distance. <br /> Therefore, the vertical and lateral extent of groundwater contamination also remains <br /> undefined. <br /> PHS/EHD has directed UNOCAL to define the vertical and lateral extent of petroleum <br /> contamination in the soil and groundwater both on site and off site. UNOCAL has also <br /> been directed to install air sparge and vapor extraction wells to evaluate the feasibility of <br /> air sparge and vapor extraction as remedial alternatives. PHS/EHD has explained that <br /> the alternatives for remediation cannot be properly evaluated until the mass of <br /> contamination is determined and remedial feasibility studies conducted. <br /> UNOCAL has proposed installation of one additional off site monitoring well to define the <br /> groundwater contaminant plume with subsequent quarterly groundwater monitoring for a <br /> minimum of one year. It is the opinion of PHS/EHD that the installation of one <br /> monitoring well off site will not define the vertical extent of contamination on site and <br /> may not be adequate to define the contamination off site. Until the vertical extent of <br /> contamination is defined both on site and off site, the lateral extent cannot be defined. <br /> Because the release of contamination occurred over 30 years ago and there is still a <br /> sheen on the groundwater, evaluation of natural attenuation by quarterly groundwater <br /> monitoring is not appropriate. Monitoring wells screened discretely to monitor the full <br /> vertical and lateral extent of groundwater contamination are necessary. <br /> California laws, regulations, and policies are based on the protection of and restoration <br /> of groundwater quality so that it can be used. PHS/EHD does not have regulatory <br /> authority over the placement of municipal water wells in the City of Stockton. Although <br /> municipal wells are screened well below first water, it has been shown that their <br /> pumping causes contamination to migrate laterally and vertically. Water purveyors are <br /> not and should not be limited in their placement of municipal wells because dischargers <br /> are not required to thoroughly investigate and evaluate remedial alternatives for their <br /> discharges. <br />