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i EXECUTIVE SUMMARY In accordance with Upgradicnt Environmental Consultants proposal 98006,which u<<s preapproved by the State UST Cleanup Fund on February 25, 1998, Upgradient performed the fourth and final quarterly monitoring event at the former Conrady Service Station in Stockton, California in December, 1998 This completes the scope of wor c authorized by the State UST Fund under this proposal number Monitoring took place on November 7 and December 7 Water depths and dissolved oxygen concentrations in the groundwater were measured Groundwater samples we r-- collected on December 7 The natural gradient is to the northeast, but the persistent trough in the gradient along the west side of the site was again evident The water to)le fell slightly from its depth of between 14 and 15 feet in September, but is still a few f eet higher than it was when Upgradient began monitoring the site in April In contrast to September, water from most of the wells had detectable concentration, of dissolved oxygen in November, although concentrations were low In December, however, only one of the wells exhibited detectable dissolved oxygen The monitoring equipment was in good working order, so it is now clear that oxygen is no longer be jig released into the groundwater from the Oxygen Release Compound injection bonng., It appears that the oxygen was depleted sometime during the second quarter, within the first five months after infection All ten water samples were analyzed for TPH-gasoline, BTEX, and gasoline oxygenates, as directed by EHD and approved by the UST Fund As in the previouw two quarter;, no oxygenates were detected, it is clear that oxygenates were not in general use prior to the 1980 removal of the USTs, and oxygenates are not a concern at this site Overall, there has been a general decrease in benzene concentrations in the impacted wells, and only MW-2 exceeded 100 ppb in December Perimeter wells, particularly MW-5, have shown an improvement since ORC infection, suggesting that the plume has shrunk in size and is less extensive than previously There is no indication of movem mt or migration of the plume downgradient Unfortunately, both the September and December samples indicate that gasoline and BTEX concentrations remain high in the core of the plume between MW-2 and SW-1 In fact, the gasoline concentration(124 000 ppb) in SW-1 in December was the highest concentration measured to date As this'Arell is screened from 32 to 30 feet and the perforations are approximately 15 feet below alae present water table, samples collected from this well probably yield lower concentrations than the maximums closer to the soil-water interface. i