DOE/ORO/2327 Oak Ridge Reservation Annual Site Environmental ...

4.6.4.1.4 Volatile Organic Compounds
Annual Site Environmental Report
Because of the many legacy source areas, volatile organic compounds are the most widespread
groundwater contaminants in the East Fork regime. Dissolved volatile organic compounds in the regime
primarily consist of chlorinated solvents and petroleum hydrocarbons. In CY 2009, the highest summed
concentration of dissolved chlorinated solvents (59,767 µg/L) was again found in groundwater at Well
55-3B in the western portion of the Y-12 Complex adjacent to manufacturing facilities. The highest
dissolved concentration of petroleum hydrocarbons (14,780 µg/L) was obtained from Well GW-658 at the
closed East End Garage.
The CY 2009 monitoring results generally confirm findings from the previous years of monitoring. A
continuous dissolved plume of volatile organic compounds in groundwater in the bedrock zone extends
eastward from the S-3 Site over the entire length of the regime (Fig. 4.40). The primary sources are the
Waste Coolant Processing Facility, fuel facilities (Rust Garage and East End), Y-12 Salvage Yard, and
other waste-disposal and production areas throughout the Y-12 Complex. Chloroethene compounds
(tetrachloroethene, trichloroethene, dichloroethene, and vinyl chloride) tend to dominate the volatile
organic plume composition in the western and central portions of the Y-12 Complex. However,
tetrachloroethene and isomers of dichloroethene are almost ubiquitous throughout the extent of the plume,
indicating many source areas. Chloromethane compounds (carbon tetrachloride, chloroform, and
methylene chloride) are the predominant volatile organic compounds in the eastern portion of the
Complex.
Variability in concentration trends of chlorinated volatile organic compounds near source areas is
seen within the Upper East Fork regime. As seen in previous years, data from most of the monitoring
wells have remained relatively constant (i.e., stable) or have decreased since 1988. Increasing trends are
observed in monitoring wells associated with the Rust Garage and S-3 site in western Y-12, some
production/process facilities in central areas, and the chloroethene component of the East End volatile
organic compound plume, indicating that some portions of the plume are still showing activity.
Within the exit pathway the general trends are also stable or decreasing. The trends west of New
Hope Pond are indicators that the contaminants from source areas are attenuating due to factors such as
(1) dilution by surrounding uncontaminated groundwater, (2) dispersion through a complex network of
fractures and conduits, (3) degradation by chemical or biological means, or (4) adsorption by surrounding
bedrock and soil media. Wells to the southwest to southeast of New Hope Pond are displaying the effects
of the pumping well (GW-845) operated to capture the plume prior to migration off of the ORR into
Union Valley. Wells east of the New Hope Pond and north of Well GW-845 exhibit an increasing trend in
volatile organic compound concentrations, indicating that little impact or attenuation from the plume
capture system is apparent across lithologic units (perpendicular to strike). However, no subsequent
downgradient detection of these compounds is apparent, so either migration is limited or some
downgradient across-strike influence by the plume capture system is occurring.
4.6.4.1.5 Radionuclides
The primary alpha-emitting radionuclides found in the East Fork regime during CY 2009 are isotopes
of uranium. Historical data show that gross alpha activity consistently exceeds the drinking water
standard (15 pCi/L) and that it is most extensive in groundwater in the unconsolidated zone in the western
portion of the Y-12 Complex near source areas such as the S-3 Site, the S-2 Site, and the Y-12 Salvage
Yard. However, the highest gross alpha activity (433 pCi/L) in groundwater continues to be observed on
the east end of the Y-12 Complex in Well GW-154, east of the Former Oil Skimmer Basin (Fig. 4.41).
The primary beta-emitting radionuclides observed in the Upper East Fork regime during CY 2009 are
technetium-99, isotopes of uranium, and associated daughter products. Elevated gross beta activity in
groundwater in the Upper East Fork regime shows a pattern similar to that observed for gross alpha
activity, where technetium-99 is the primary contaminant exceeding the screening level of 50 pCi/L in
groundwater in the western portion of the regime, with the primary source being the S-3 Site (Fig. 4.42).
The Y-12 National Security Complex 4-69