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• and clay Groundwater in these alluvial deposits is unconfined to semi-confined (GTI, March 1994) Groundwater in the Stockton area is primarily affected by significant groundwater extraction for irrigation and public water supply and by the canals and waterways of the Stockton Harbor Regional groundwater flow in the vicinity of the site has historically been toward the northeast This flow direction is the result of groundwater extraction which has reduced groundwater elevations in Stockton to approximately-30 to -40 feet mean sea level Subsurface investigation indicates that the site is underlain by interbedded clayey silts, silty clays with lenses of sands to the total depth explored (70 feet bgs) Historically, depth to groundwater has ranged from 35 36 feet bgs (Well MW-8, February 7, 1989) to 58 01 feet bgs (Well MW-10, October 26, 1992) Quarterly groundwater monitoring data from November 1988 to present indicate that groundwater beneath the site consistently flows in a northeasterly direction at an approximate gradient of 0 002 to 0 003 foot per foot 2.4 Interim Remediation Site remediation to date has consisted of removing product storage and distribution equip- ment, site assessment activities, groundwater monitoring, and installation and operation of an SVE system The SVE system operated from November 1990 to November 1992, with a 7-month hiatus from February to September 1992 The wells used for vapor-phase hydrocar- bon removal were SVE Wells V-I through V-5, and Monitoring Wells MW-3 through MW-6 The vacuum radius of influence was estimated to be in excess of 25 feet for SVE Wells V-1 through V-5 (EA, May 12, 1989) A subsequent vacuum radius of influence analysis was conducted using Well MW4, the estimated effective radius was 112 feet (EA, March 25, 1991) During the SVE system operation period, a total of approximately 12,000 pounds (1,967 gallons) of petroleum hydrocarbons were removed Based on data collected from borings drilled before and after the SVE system was activated, hydrocarbon concentrations in soil and groundwater were significantly reduced as a result of SVE system operations Soil chemistry data from borings indicate that TPH-g mass nn the vicinity of the former USTs was estimated to be 26,000 pounds mass prior to SVE system operation, currently TPH-g mass m the vicinity of the former USTs is estimated to be 4,000 pounds mass Based on these figures, a total reduction of TPH-g mass of approxi- mately 85 percent was achieved during the time period between SVE system activation and the present Since SVE system operational data indicate that approximately 12,000 pounds of petroleum hydrocarbons were removed by the SVE system, the remaining 10,000 pounds were removed as a result of natural attenuation processes The migration potential has there- fore been reduced by the reduction of total mass beneath the site This conclusion recognizes the TPH-g mass influence on migration transport mechanisms 3201337B/1918REV 6 August 31, 1995