ARCHIVED REPORTS_XR0004222 - Laserfiche WebLink

Home Browse Search

ATTACHMENT A SUMMARY OF PREVIOUS INVESTIGATIONS In February 1995, The Mark Group performed a Phase I Environmental Site Assessment (ESA) at the site and concluded that further investigation be performed In April 1995, The Mark Group performed a preliminary subsurface investigation involving the drilling of six soil borings The Mark Group reported residual concentrations of hydrocarbons in the former UST excavation backfill Groundwater was not encountered and was reported to be in excess of 50 feet below the ground surface (bgs) In June 1996, NorCal Geophysical Consultants, Inc (NorCal) performed a geophysical survey at the site NorCal identified five anomalies associated with the waste oil tank, sump drain line, two corners of the former building where utilities entered the building, one location adjacent to the former pump islands, and one location where the former product lines and utility lines crossed Subsequent excavation in these areas revealed debris within approximately 2 feet of the surface, including asphalt, both clay and metal piping, and wires In September and October 1996, Pacific Environmental Group, Inc (PACIFIC) observed excavation of the waste oil tank and former product lines, excavation of debris from the former gasoline UST pit, and limited over-excavation of the former gasoline UST pit Gasoline hydrocarbon concentrations of up to 4,000 parts per million (ppm)were reported in the southeast corner of the former UST pit After limited over-excavation of approximately 380 cubic yards of impacted soil and 200 cubic yards of concrete debris from the former gasoline UST pit, perimeter sod samples collected from the excavation were reported nondetectable to 2,300 ppm TPH-g No petroleum hydrocarbons were detected in the soil samples colleted from beneath the former waste oil tank and former product lines As approved by San Joaquin County Environmental Health Division (SJCEHD), material excavated from the waste oil tank pit was reused as backfill in the waste oil tank excavation Excavated debris and soil from the former gasoline UST pit was disposed of offsite In May 1997, one boring (B-7) was drilled within the southeast portion of the footprint of the former gasoline UST pit Groundwater was encountered in Boring B-7 at a depth of approximately 58 feet bgs A groundwater"grab"sample collected from B-7 was reported to contain 32,000 parts per billion (ppb) TPH-g and nondetectable concentrations of benzene On April 1999, three monitoring wells were installed (MW-1 through MW-3) at the site by SECOR Soil samples collected from the installation were found to contain nondetectable concentrations of petroleum hydrocarbons (Table 1) The analysis of groundwater samples collected after monitoring well development also resulted in nondetectable concentrations of gasoline hydrocarbons (Table 2) Quarterly monitoring and sampling of site monitoring wells commenced in April 1999 Groundwater samples are analyzed for total petroleum hydrocarbons as gasoline (TPHg), MtBE, and Benzene, Toluene, Ethyl-Benzene, and Xylene(BTEX) Analytical results of the quarterly monitoring events are presented in Table 2 With the exception to samples collected on August 30, 1999, all site monitoring wells did not contain detectable concentrations of petroleum hydrocarbons The August • 30"event shows relatively low concentrations of petroleum hydrocarbons in all wells This pattern of i IChevron196258\REP0RTSISAR_2003 doc