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Project 113-26-04, 05, Page 13 November 17, 1995 V. SUMMARY AND CONCLUSIONS In July to October 1995, four groundwater monitoring wells were drilled and installed and fourteen (12 successful) hand augered soil borings were drilled at or near the Cheaper) store #37 located at 15 East Grant Line Rd , Tracy, Ca Thus work represented a third phase of subsurface investigation since a gasoline UST facility was replaced in 1988 In 1988, four groundwater wells (MW-1, 2, 3, 4) were installed and sampled once Groundwater monitoring was initiated in 1992 and performed quarterly for over two years Seven hand angered soil borings (B-1, 2,. 7) were advanced in 1993 to help delineate the boundaries of the previously identified contaminant plume It became apparent that much of the detected subsurface contamination had been up gradient from the UST facility The average groundwater flow direction over eight events between 1992 and early 1995 was N89°E (nearly due east) Twelve soil borings (B-8 through 21 except B-9, 19) were placed around the property and across the two adjoining streets in July to October 1995. Four groundwater wells (MW-5, 6, 7, 8) were placed two each in the up gradient (MW-5, 8) and down gradient (MW-6, 7) directions from the USTs Sixteen selected soil samples obtained continuously or at five foot intervals were analyzed at a State Certified environmental lab for TPH as gasoline with BTEX Up gradient and off site soil sample locations contained up to 440 ppm TPH as gasoline at about 6-7 feet BGS Down gradient boring and well locations gave soil samples with N D results The data point B-8, down-cross gradient from the dispensers showed 2,000 ppm TPH as gasoline at 6-6 T BGS Nineteen groundwater samples were procured from the borings and wells and analyzed for the same chemicals at the same lab The morutonng well samples and the down gradient boring sample (B-14) were found to be nearly free of the chemicals of interest B-8, down gradient from the dispensers, contained the highest concentration of TPH as gasoline in groundwater at 57 ppm The up gradient and off site groundwater grab samples from the soil borings contained between 2 4 ppm and 47 ppm TPH as gasoline with elevated BTEX The wells were surveyed so that groundwater elevations could be used to calculate the groundwater flow direction and gradient between the wells Current groundwater flow conditions indicate flow toward the north at a low gradient Site soils and groundwater up gradient from the tank pit have been contaminated in the past with gasoline The off site portion of the investigation points to the abandoned gas station across Holly Drive to the west from the site as a likely contributor to gasoline contaminants discovered on site It is conceivable that the vast majority of gasoline contamination present at the site originated from that site with . the address of 2421 Holly Drive Parker Environmental Services, 190 East 7th Street, Pittsburg, CA 94565-2317 Phone (510) 439-1024 Fax (510) 439-2566