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t r <br /> Pombo Ranch Property <br /> Second Quarter Groundwater Sampling Report April 2006 <br /> Page: 3 <br /> A comparison of MTBE concentrations detected in monitoring wells MW-4 and MW4b <br /> located in the vicinity of the former underground storage tanks indicates that MTBE <br /> concentrations show a decrease from greater than 2000 ug/l to below the laboratory <br /> detection limit of 0.5 ug/l. A remedial excavation in February 2005 removed 1,524.71 <br /> tons of impacted soil from the site and disposed at Forward Landfill, Stockton. Well <br /> MW-4b was installed to replace well MW-4 which was destroyed during the remedial <br /> excavation. The chart shows clearly shows a decrease in oxygen content associated with <br /> the decrease of MTBE content and a rebound in oxygen content. A chart of MTBE and <br /> dissolved oxygen concentrations and groundwater elevations is site is presented in Figure <br /> 8. <br /> CONCLUSIONS AND RECOMMENDATIONS <br /> Shallow ground water flow was to the northeast with a very flat gradient and moderate <br /> downward vertical groundwater movement. Low concentrations of gasoline petroleum <br /> hydrocarbons were detected by the analytical laboratory in water samples collected from <br /> the monitoring well MW5 located down gradient from the former underground storage <br /> tanks. Very low concentrations of fuel oxygenates were detected in the water sample <br /> collected from the monitoring well MW4B located at and immediate down gradient from <br /> the former underground storage tanks. The contamination is defined laterally and <br /> • vertically. The dissolved oxygen low down gradient of the release indicates that the <br /> biological breakdown of the groundwater contamination is occurring. <br /> The San Joaquin County Environmental Heath Department in a March 28, 2006 letter <br /> directed that a site closure report be prepared for the Pombo Ranch site following the <br /> Regional Water Quality Board Tri — regional Guideline Appendix A. The site closure <br /> report is in preparation. Quarterly monitoring will continue as directed San Joaquin <br /> County Environmental Heath Department. <br />