ARCHIVED REPORTS XR0001431 - Laserfiche WebLink

Home Browse Search

1 ` S 1 . Request For Closure,Farmer Unocal Station#0123,Tracy,CA April 19,2001 13) Mass balance calculations of substance treated versus that remaining. r , Mass balance calculations were done for TPHg in soil at the site based on concentrations detected in soil samples collected in May 1995 (Table 3) The mass balance calculations shown on Table 3 are based on the isoconcentration maps in Appendix C Each isoconcentration map was divided into zones based on`the assumption that TPHg concentrations in each zone are equal to the average of the TPHg concentrations detected in each zone Zone volumes were created by fitting ellipsoids to the areas shown on the maps in Appendix C TPH and benzene concentrations are assumed to be constant throughout each zone volume The calculations indicate that 557 pounds of TPHg were in the soil at that time Since historical groundwater monitoring data indicate that depth to water at the site fluctuates from approximately 9 to 10 feet bgs, the TPHg concentrations shown on Figures 6 through 9 in Appendix C are from samples collected at or fast below the water table and may represent dissolved hydrocarbons in groundwater TPHg concentrations in soil samples collected above the water table are low to non-detect, suggesting that very little if any residual hydrocarbons remain in soil at the site 14) Assumptions, parameters, calculations and model used in risk assessments, and fate and transport modeling. Not applicable for this site • 15) Rationale why conditions remaining at site will not adversely impact water quality, health, or other beneficial uses. Based on the current lack of detectable hydrocarbon concentrations in monitoring wells at the site except for well U-4, in which petroleum hydrocarbon concentrations in groundwater have diminished significantly since 1997, the consistent groundwater flow direction, and distances to the nearest receptors, it appears unlikely that any residual hydrocarbons from the Unocal site represent an existing or potential threat to nearby receptors 16) WET or TCLP results. Based on concentrations of metals detected in soil samples, WET or TCLP analyses were not required SUMMARY This site meets the four criteria for low risk groundwater designation as set forth in Appendix B of the Tri-Regional Board Staff Recommendations for- Preliminary Investigation and Evaluation of Underground Tank Sites as follows 240073 06 6