ARCHIVED REPORTS_XR0007693 - Laserfiche WebLink

Home Browse Search

The screens in wells TB-7 and TB-8 end with the bottom 5 feet of screen in sand Yet, 7 the wells do not dewater because there is a single water table Historically (Thrifty 1993), depth to groundwater was measured at 75 feet bgs orjust below the sand layer indicating the water table is under unconfined conditions, for if confined conditions ' existed, the water table would have risen above the saturated sand interval mil • The contaminant plume with no degradation arrives at the receptor (nearest groundwater }L�`toe' II�-( ' production well, 1,500 feet downgradient) at approximately year 156 Based on a ' 1 lb4fa� BIOSCREEN simulation model, the MTBE plume would impact the receptor well at years 156 through 264 with no degradation and would never impact the well with the 1st Order Decay model • The greatest mass of fuel released was in the vicinity of the former USTs and associated ' piping prior to 1994, when the USTs were removed Based on early groundwater monitoring and sampling, free product was first detected in groundwater monitoring well MW-5 in June 1993, but rapidly dissipated from hand bailing and from May 1994 on, no free product has been detected • The vertical extent of hydrocarbon-impacted soil does not appear to extend below 80 feet ' bgs with the exception of low concentrations (<10 mg/Kg) of TPHg and(<0 006 mg/Kg) benzene in borings CPT-3 and CPT-4 at 90 to 100 feet bgs No hydrocarbon constituents were detected below 100 feet bgs This suggests that the fine grained interval from approximately 75 to 90 feet bgs retarded the vertical migration of hydrocarbon-impacted soil Low concentrations of toluene, ethylbenzene and xylenes are also present below 80 feet bgs Since the CPT borings were completed in the areas of greatest hydrocarbon ' impact, it appears that the vertical extent of hydrocarbon-impacted soil is delineated ' • Hydrocarbon-impacted soil is present in three main areas beneath the site including just north of the former USTs (area 1, MW-5, CPT-4, etc ), the former USTs (area 2, MW-2, CPT-2, etc ), and just north of the middle dispenser island (area 3, TDD-14, TDD-15, ' TB-3) as shown on Figures 5A, 5B, 5C, 5D, 5E, and 5F TPHg-impacted soil (>10 mg/Kg) is present to a depth of 80 feet bgs in areas 1 and 2 and 30 feet bgs in area 3 Based on average TPHg concentrations in area 1 (971 mg/Kg), area 2 (987 mg/Kg) and area 3 (8,333 mg/Kg) it is estimated that the mass of hydrocarbon in soil was approximately 112,824 pounds This is only a rough estimate based on several reasonable assumptions, the actual hydrocarbon load in the soils could dramatically differ ' from this estimate To date, the SVE system has removed a calculated 140,299 pounds of hydrocarbons Part of the reason for the discrepancy that more hydrocarbons have been removed than were estimated to be present is the hydrocarbon-impacted area may be ' larger onsite Several borings simply were not extended and sampled below 60 feet bgs such as all of the TDD borings drilled by Pacific and several of the TB borings (Figures 5D, 5E, and 5F) ' • Based on the assessment completed by AGE (2004), the lateral extent of hydrocarbon- impacted soil appears to be defined to the west and south by the presence of bonngs 12