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Interim Remedial Action Test Report,Former Unocal#0187,Stockton,CA Asst 15,2001 scrubbing action of the microbubbles Overall this process will be beneficial in the long term remediation of the site GR recommends that operation of the treatment system be continued with the following modifications Discontinue the use of the four deep on-site sparge points Install two new shallow sparge points, in locations near wells SP-1 and SP-3, at a depth of 40 to 45 feet bgs This will accelerate treatment in the shallow groundwater zone on site The treatment system should also be expanded to include downgradient off-site treatment, across California Street GR recommends that two sets of sparge point wells be installed, with each set including three points at depths of approximately 110, 65, and 40 feet bgs Because the current C-Sparge panel is limited to supplying ozone/air to the eight existing points,it will be necessary to add an additional panel to use for both the additional on-site and off-site points As dissolved hydrocarbon concentrations change, active point selection will be modified This might also include the reactivation of the deep on-site points if on-site concentrations rebound, or if this is shown to accelerate reductions in off-site concentrations Additional Subsurface Assessment The dissolved hydrocarbon plume beneath and downgradient of the Unocal site has been delineated at Zone A (20 to 35 feet bgs) and Zone B (65 to 75 feet bgs) depths, but not to the east at Zone C (110 to ` 120 feet bgs) depths The Vintage and Chet's Auto Service and Repair sites situated near the eastern perimeter of the currently defined plume limit the extent that additional subsurface assessment can be conducted without encountering dissolved hydrocarbons from those sites Given that the dissolved hydrocarbon concentrations detected in Zone C do not show significant attenuation downgradient toward well U-21, the Unocal site's furthest downgradient well, the plume probably extends beneath the Vintage site, where it is likely commingled with dissolved hydrocarbons from the Vintage site Also, there is no practical location downgradient of well U-21 to place an additional well in order to attempt to delineate the hydrocarbon plume in Zone C before the Vintage site Any assessment work by Unocal to delineate the dissolved hydrocarbon plume beyond the Vintage site would have to be conducted by both Unocal and the State of California, the responsible party for the Vintage site,due to a commingled plume Since the State of California has already been requested by the RWQCB in a letter dated June 27, 2001 (Appendix A)to conduct additional downgradient assessment for the Vintage site, any additional assessment by Unocal in that direction is not warranted at this time The extent to which gasoline releases from the former USTs at Chet's Auto Service and Repair have impacted groundwater in this area is not known Because of this uncertainty, in an attempt to delineate the east-southeast extent of the Zone C plume, GR recommends one additional well be installed to a depth of 120 feet bgs, adjacent to Vintage boring BH-10, as shown on Figure 3 If groundwater samples from this additional proposed well show increasing hydrocarbon concentrations, then it can be assumed that the increase is due to releases from Chet's Auto Service and Repair, and further assessment downgradient of the point would involve both Unocal and Chet's Auto Service and Repair due to the commingled plume 24004209-1 10