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'r Ms.Lori Duncan Page 5 of 9 San Joaquin County Environmental Health Department August 25,2004 i northward extension of the plumes depicted in Figures 6 through 8 was caused by the release of product into soil during the drilling of B-18. This release resulted in groundwater contamination around VW4, which has now become commingled with the larger plume around the former UST complex. Figures 18 through 20 show contaminant concentrations versus time in MW-5. MW-5 is,most— t^o A�- � representative of site conditions because it is downgradient from the contaminant plume and has +AnA been sampled continuously since 1995. These figures show that contaminant concentrations have been reduced at the site and that the plume size has diminished. Figures 21 through 23 show decreasing contaminant concentrations with increasing distance downgradient from the former UST complex. These figures show that the lateral extent of contamination in the downgradient direction is defined. Based on the minimal amount of plume migration since groundwater monitoring and SVE/AS mitigation of the plume began, which is shown by the decreasing concentrations over time, groundwater impacts at the site do not present a threat to the municipal supply wells identified in the sensitive receptor survey. HYDROCARBON MASS ESTIMATES In 1994,EMCON calculated a petroleum hydrocarbon mass of 383 lbs at the site. During the operation time of the SVE/AS system, approximately 2,000 lbs of petroleum hydrocarbons were removed from the subsurface at the site. The following paragraphs discuss the assumptions used to update petroleum hydrocarbon mass estimates and the calculations performed. They also present the results of the new mass estimates. Where appropriate,conservative assumptions are made to insure the protection of human health and the environment. Assumptions Soil porosity is assumed to be 35%. Although the site is predominantly silts, which have lower porosity, a value of 35% was chosen for a conservative estimate of impacted groundwater volume. Hydrocarbon mass adsorbed in soil is assumed to be negligible at the site. This assumption is based on historical soil analytical data, SVE/AS system performance, and the rise in groundwater elevations. EMCON's report from 1994 shows that significant impacted soil does not occur above 30 feet bgs. The only analytical data showing impacted soil above 30 feet bgs was collected during the tank excavation in 1998 (Appendix D). These data indicate that minor soil impacts existed about 15.5 feet bgs in the old tank cavity,prior to remediation. SVE wells VW-5S and VW-5D were screened in and below the old tank cavity to remediate soil impacts in the area. The vapor extraction wells were calculated to have a radius of influence of 30 feet. That radius of influence is large enough to allow VW-5S and VW- 5D to remediate soil impacts in and around the tank cavity. Furthermore, since site investigations began in 1991, groundwater has risen from 50 feet 4' bgs to approximately 28 feet bgs, effectively submerging impacted soil at the site. The submersion KAWprocess`l5 BP ARCON0054$12004 SCM\Site Conceptual Model V2.doc