WORK PLANS - Laserfiche WebLink

Home Browse Search

Final Release LLNL Site 300 (USDOE) contamination is present, and if so, to identify the magnitude and source of this contamination. Remediation strategies for each OU have been developed, evaluated, and approved by regulatory agencies and the community for implementation (LLNL 2002). Contaminants of Concern Two VOC plumes exist in the GSA: one in the eastern GSA and one in the central GSA. The plume in the eastern GSA originated in the vicinity of the debris burial trench area. This plume extended eastward and migrated northward into the Corral Hollow alluvium. TCE has infiltrated into the Tnbs2 regional aquifer at a point west of the sewage treatment pond where the Tnscl confining layer is absent. However, TCE concentrations in the regional aquifer have been decreasing since treatment commenced in 1991 (LLNL 1997). At the time remediation began at the Eastern GSA,the VOC plume extended over a mile to the northeast along the Corral Hollow stream channel. After 8 years of treatment,by the fall of 1999, the TCE plume (as defined by a 5 µg/L contour line)had been restricted to Site 300 property (LLNL 2002). Figure 5 shows that this contaminated groundwater plume has, indeed, been drawn back on site. There are three source areas for the central GSA contamination: Building 875 dry well,Building 872 dry well, and Building 873 dry well. But the contamination in groundwater has commingled and is considered one plume with contributions from three source areas(LLNL 1997). The highest groundwater concentrations of TCE (240,000 µg/L), PCE (25,000 �tg/L), and 1,2-DCE (1,000 µg/L) occurred south of Building 875 near the former dry wells in a bailed sample taken in March of 1993. If a groundwater VOC concentration is 1 to 10% of that VOCs solubility in water,the VOC may be present in the form of a dense non-aqueous phase liquid(DNAPL). This high concentration (240,000 µg/L) of TCE indicates that it is most likely present as a DNAPL. DNAPLs are denser than water and tend to sink to the lowest point in the aquifer. Boreholes and soil sample data indicate that these DNAPLs are located at a depth of 20-35 feet in the Tnbs2 regional aquifer at the contact point of the Tnscl confining layer. Because no other wells in the GSA have shown VOCs in concentrations high enough to indicate the presence of DNAPLs, LLNL concluded that DNAPLs were confined on-site to the Building 875 dry well pad area (LLNL 1997). Public Comment#'s 7(iv), 8, 9 and 10 of the Final Record of Decision for the General Services Operable Unit (LLNL 1997) concerned these DNAPLs, and the potential for them to serve as a continuing source of TCE contamination in the groundwater. LLNL is addressing this concern by employing remediation technologies, namely soil vapor extraction (SVE), to remove DNAPLs from the Building 875 dry wells area. Continued monitoring will also evaluate whether DNAPLs are acting as a constant source of groundwater contamination. The TCE plume emanating from the central GSA extends approximately 200 feet off site at concentrations approaching 80 [tg/L in off-site monitoring wells. Remediation of the plume source is on-going, and the off-site drinking water wells are currently unaffected by the Site 300 contamination. Groundwater Exposure Pathways There are currently no residential developments adjacent to Site 300. There are,however, 14 homes (4 are vacant) located on CSVRA property. Although the State of California provides bottled drinking water to all CSVRA residents near Site 300, groundwater is still used in all of 14