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

Oak Ridge Reservation
are the S-2 and S-3 sites. The extent of the nitrate plume is essentially defined in the unconsolidated and
shallow bedrock zones. In CY 2009, groundwater containing nitrate concentrations as high as 8,960 mg/L
(Well GW-275) occurred in the shallow bedrock just east of the S-3 Site (Fig. 4.38). These results are
consistent with results from previous years.
4.6.4.1.3 Trace Metals
Concentrations of arsenic, barium, beryllium, cadmium, chromium, lead, nickel, and uranium
exceeded drinking water standards during CY 2009 in samples collected from various monitoring wells
and surface water locations downgradient of the S-2 Site, the S-3 Site, the Salvage Yard, and throughout
the complex. Elevated concentrations of those metals in groundwater were most commonly observed
from monitoring wells in the unconsolidated zone. Trace metal concentrations above standards tend to
occur only adjacent to the source areas due to their low solubility in natural water systems.
Concentrations of uranium exceed the standard (0.03 mg/L) in a number of source areas (e.g., S-3
Site, production areas and the Former Oil Skimmer Basin) and contribute to the uranium concentration in
Upper East Fork Poplar Creek.
One trace metal absent from the list of those that exceed drinking water standards in CY 2009 is
mercury. Mercury has a very high affinity for clay-rich soils such as those on the ORR, and hence
exhibits little tendency for extensive transport in diffuse groundwater plumes. Additionally, the
hydrogeologic complexities of the fracture/conduit flow system underlying Y-12 make it challenging to
delineate the vertical and horizontal extents of any groundwater contamination. Elevated mercury
concentrations (above analytical detection limits) in groundwater have been consistently observed only
near known source areas (Fig. 4.39). In the past, mercury concentrations above the drinking water
standard (0.002 mg/L) have been observed in groundwater monitoring wells at the identified source areas
presented in Fig. 4.39.
Due to past processes and disposal practices, mercury is a legacy contaminant at Y-12. It is
commonly found in the soils near specific areas where it was used in processes in the 1950s and 1960s.
This metal is a contaminant of concern in surface waters discharging from these areas. It appears that high
mercury concentrations in water within the storm drain network in those areas arise from the oxidation
and dissolution of mercury from metallic mercury deposits in close proximity to flowing water that
produces high localized concentrations of dissolved mercury that infiltrate the storm drain system. A
similar process occurs in reaches of the open stream where mercury metal is buried under gravel
sediments. When dissolved mercury is discharged from the storm drain system into the open creek
channel, it is rapidly sequestered by particulate materials, and fluvial sediment/particle transport becomes
the primary mechanism of mobility.
In tightly fractured shale and other noncarbonate bedrock, the natural flow paths are such that
significant transport of mercury is not likely. In industrialized areas of Y-12 where the shallow subsurface
has been reworked extensively, some preferential transport along building foundations and underground
utilities is occurring. This is evident from elevated surface water concentrations of mercury.
Interconnection between the surface water and groundwater systems have been demonstrated by
tracer investigations (DOE 2001) and the discharge of elevated concentrations of mercury from a buried
spring (i.e., OF-51) adjacent to East Fork Poplar Creek. This discharge is presently captured and treated to
remove the mercury at the Big Springs Water Treatment System. It has been proposed that dissolved
mercury in the spring discharge arises from metallic mercury that has infiltrated into the solution cavities
and conduits of the karstic Maynardville Limestone Formation below the water table. Although
the subsurface inventory of metallic mercury in the limestone bedrock has not been determined, it could
account for a part of the quantities lost during operational use. Subsurface storage may also minimize
mobility of mercury due to decreased surface area (i.e., immobilized in filled seams, fractured, and
conduits, in contrast to the surface exposure of mercury as particles) (Rothschild et al. 1984).
4-66 The Y-12 National Security Complex