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Mr. Harlin Knoll Environmental Health San Joaquin Local Health Department 10 September 1990 Page 2 no permit to abandon in place was ever issued to Pac Bell by the SJLHD. The results of this investigation are summarized below. In September 1988 two borings (Boreholes No. 1 & No. 2) were drilled to lengths of 28 feet at angles of 20 degrees off vertical, beginning at distances of 8 feet from the east and north sides of the 550 -gallon UST. Due to their angles, these borings attained actual depths of about 26 feet bgs and crossed directly beneath the base of the UST, which was reportedly located at a depth of 20 feet bgs. One vertical boring (Borehole No. 3) was drilled to a depth of 25 feet bgs, beginning at a distance 2 feet south of the 550 -gallon UST. Soil samples were collected in the angle borings at lengths of 20 feet (19 feet bgs) and 28 feet (26 feet bgs), and in the vertical boring at depths of 20 feet and 25 feet. Laboratory analytical results indicated that no diesel fuel or gasoline was present in any of the soil samples, but that limited concentrations of total petroleum hydrocarbons (TPH) were present in the 20 -foot sample from angle Borehole No. 1 and the 20 foot sample from vertical Borehole No. 3. TPH was found in these samples in concentrations of 836 parts per million (ppm) and 25 ppm, respectively. No visual contamination or hydrocarbon odors were noted in these samples in the field. DISCUSSION Diesel fuel has a carbon molecular weight generally ranging from C10 to C14. The diesel fuel test, as conducted by Eureka Laboratory, is reportedly capable of detecting the presence of hydrocarbons in the average diesel range in addition to detecting the presence of other heavier hydrocarbons as low as C30. The TPH test identifies the presence of all ranges of petroleum hydrocarbon compounds in the soil sample. Since the diesel and gasoline tests detected no hydrocarbons, and the TPH test did yield positive results, these substances must exist in the range of C30 or below. According to organic chemists at American Environmental Laboratory Corporation (AELC), this high molecular weight, low end hydrocarbon range would indicate the presence of such substances as asphalt, grease and/or possibly form oil. The two samples testing positive for TPH were recovered at a depth of 19 feet bgs in Borehole No. 1 and at 20 bgs in Borehole No. 3. At these depths, simple mathematical calculations show that the sample locations were only 1.5 feet and 2 feet, respectively, from the edge of the concrete vault surrounding the base of the UST. During the course of construction of the concrete vault, it is possible that detectable concentrations of asphalt, grease and/or form oil may have accumulated at this depth in the excavation. The fact that no petroleum hydrocarbons were discovered beneath 20 feet bgs in any of the borings confirms the premise that no diesel fuel migrated from the base of the UST over the 40 years it has been in place. Diesel fuel is very mobile and is generally capable of migrating through the soil. Asphalt, grease and other thick oils tend to remain immobile due to their high viscosity. Removal of the 550 -gallon UST would be exceedingly difficult due to the size and construction of the vault surrounding it. The vault is composed of steel reinforced concrete walls ranging in thickness from 2.7 feet to 0.5 feet, and extends vertically to a depth of 20 feet bgs. Removal of the UST would likely require removal of the entire vault. Opening an excavation of this size could undermine the foundation of the adjacent Pac Bell building to the west and could also undermine the currently used 2,000 -gallon UST to the immediate northwest.