ARCHIVED REPORTS_XR0012168 - Laserfiche WebLink

Home Browse Search

Former Unocal Station No 0187 February 14, 1995 In October 1993, GSI drilled eight exploratory soil borings (U-A through U-H) in the locations of the former USTs, the former product dispenser islands, and the former hydraulic lifts in preparation for site closure During this investigation, TPHg were detected in four soil samples collected from bonngs U-C and U-D, which both had been drilled to 31 5 feet below ground surface (bgs) in the Iocation of the original gasoline USTs and former dispenser island D-4 TPHg concentrations in these four soil samples ranged from 1,600 to 2,900 ppm Benzene was not detected in these soil samples Results of this subsurface investigation are presented in GSI Report No 782702-3, dated December 15, 1992 In July 1994, GSI drilled five additional onsite soil borings (U-I through U-M) and installed three onsite groundwater monitoring wells (U-1 through U-3) TPHg were detected in soil samples collected from borings U-J (1 7 ppm at 26 5 feet bgs) and U-K (130 ppm at 41 5 feet bgs) TPHg and benzene were detected in soil samples collected from borings U-3 (up to 330 and 0 48 ppm, respectively, at 46 5 feet bgs), U-L (19,000 and 140 ppm, respectively at 45 5 feet bgs), and U-M (5 10 and 0 46 ppm, respectively at 45 5 feet bgs) Results of this subsurface investigation are presented in GSI Report No 4827 703-5, dated September 21, 1994 • Quarterly monitoring and sampling of the groundwater monitoring wells began August 19, 1994. TPHg and benzene are currently detected in all three monitoring wells, with the highest concentrations occurring in well U-3 (42,000 and 9,600 parts per billion, or ppb, respectively) Quarterly sampling and monitoring of the wells is performed by MPDS Inc of Concord, California Data from the most recent sampling event (November 17, 1994) were reported in MPDS Report No MPDS-UN0187-02, dated December 12, 1994 Soil and Groundwater Conditions Based on subsurface work performed to date at the subject site, soil underlying the site is predominantly clay to approximately 40 feet bgs A clayey sand stratum, approximately two to six feet thick, was encountered in all borings at approximately 19 feet bgs, except in well boring U-2 A clayey sand unit approximately three feet thick was encountered at approximately seven feet bgs in boring U-K, and a clayey sand unit approximately five feet thick was encountered at approximately 30 feet bgs in boring U-J These clayey sand strata do not appear to be laterally continuous across the site The clay is underlain by a sandy unit to approximately 55 feet bgs, except in well boring U-2. The sands in this unit have varying amounts of clay, and range from clayey sand to well-graded sand In well borings U-1 and U-3, clay was encountered in the bottom one-foot of each boring in well boring U-2, approximately three feet of sandy clay was 492704-1 3 GeoStrategies Inc.