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

Annual Site Environmental Report
western lateral extent of the plume of volatile organic compounds at the site has not changed significantly
from previous years. The continued observation of volatile organic compound contaminants over the past
several years at a well approximately 458 m (1,500 ft) southeast of the Chestnut Ridge Security Pits
shows that some migration of the eastern plume is apparent. Additionally, dye tracer test results and the
intermittent detection of volatile organic compounds (similar to those found in wells adjacent to the
Chestnut Ridge Security Pits) at a natural spring approximately 2745 m (9,000 ft) to the east and along
geologic strike may indicate that Chestnut Ridge Security Pits groundwater contaminants have migrated
much further than the monitoring well network indicates.
4.6.4.3.2 Nitrate
Nitrate concentrations were below the drinking water standard at all monitoring stations in the
Chestnut Ridge Hydrogeologic Regime.
4.6.4.3.3 Trace Metals
A chromium result exceeding the drinking water standard (0.1 mg/L) was observed in a groundwater
sample from one well (GW-305) at the Industrial Landfill IV (Fig. 4.33) with a maximum concentration
of 0.13 mg/L. The presence of this trace metal in groundwater at the Y-12 Complex, with the exception of
the S-3 Site, is not due to historical waste disposal but to corrosion of well casings. Nickel is a primary
component of stainless steel, and its presence indicates the occurrence of corrosion and subsequent
dissolution of stainless steel well casing and screen materials due to chemical or biochemical processes
(LMES 1999).
4.6.4.3.4 Volatile Organic Compounds
Monitoring of volatile organic compounds in groundwater attributable to the Chestnut Ridge Security
Pits has been in progress since 1987. A review of historical data indicates that concentrations of volatile
organic compounds in groundwater at the site have generally decreased since 1988. However, a shallow
increasing trend in volatile organic compounds in groundwater samples from monitoring well GW-798 to
the southeast and downgradient of the Chestnut Ridge Security Pits has been developing since CY 2000
(Fig. 4.40). Elevated concentrations observed in GW-798 appear to fluctuate with changing precipitation
conditions. The volatile organic compounds detected in CY 2009 in Well GW-798 continue to be
characteristic of the Chestnut Ridge Security Pits plume.
At Industrial Landfill IV, a number of volatile organic compounds have been observed since 1992.
Monitoring well GW-305, located immediately to the southeast of the facility, has historically displayed
concentrations of compounds below applicable drinking water standards, but the concentrations have
exhibited a shallow increasing trend. In CY 2009, samples slightly exceeded the standard for
1,1-dichloroethene, resulting in the issuance of a regulatory notice of violation by the Tennessee
Department of Environment and Conservation (see Section 2.5 for more details).
4.6.4.3.5 Radionuclides
In CY 2009, no gross alpha activity above the drinking water standard of 15 pCi/L was observed in
any groundwater samples collected in the Chestnut Ridge Hydrogeologic Regime. Gross beta activities
exceeded the screening level of 50 pCi/L at monitoring well GW-205 (Fig. 4.42) at the United Nuclear
Corporation site (the maximum detected activity was 64.3 pCi/L). This location has consistently exceeded
the screening level since August 1999. Isotopic analyses show a correlative increase in the beta-emitting
radionuclide potassium-40, which is not a known contaminant of concern at the United Nuclear
Corporation Site. The source of the radioisotope is not known.
4.6.4.3.6 Exit Pathway and Perimeter Monitoring
Contaminant and groundwater flow paths in the karst bedrock underlying the Chestnut Ridge regime
have not been well characterized by conventional monitoring techniques. A number of tracer studies have
The Y-12 National Security Complex 4-81