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

Annual Site Environmental Report
The highest gross beta activity in groundwater was observed during CY 2009 from well GW-108
(16,200 pCi/L), east of the S-3 site.
4.6.4.1.6 Exit Pathway and Perimeter Monitoring
Data collected to date indicate that volatile organic compounds are the primary class of contaminants
that are migrating through the exit pathways in the Upper East Fork regime. Historically, the compounds
have been observed at depths of almost 500 ft in the Maynardville Limestone, the primary exit pathway
on the east end of the Y-12 Complex. The deep fractures and solution channels that constitute flow paths
within the Maynardville Limestone appear to be well connected, resulting in contaminant migration for
substantial distances off the ORR into Union Valley to the east of the complex.
In addition to the intermediate to deep pathways within the Maynardville Limestone, shallow
groundwater within the water table interval of that geologic unit near New Hope Pond, Lake Reality, and
Upper East Fork Poplar Creek is also monitored. Historically, volatile organic compounds have been
observed near Lake Reality from wells, a dewatering sump, and the New Hope Pond distribution channel
underdrain. In that area, shallow groundwater flows north-northeast through the water table interval east
of New Hope Pond and Lake Reality, following the path of the distribution channel for Upper East Fork
Poplar Creek.
During CY 2009, the observed concentrations of volatile organic compounds at the New Hope Pond
distribution channel underdrain continue to remain low. This may be because the continued operation of
the groundwater plume–capture system in Well GW-845 southeast of New Hope Pond is effectively
reducing the levels of volatile organic compounds in the area. The installation of the plume capture
system was completed in June 2000. This system pumps groundwater from the intermediate bedrock
depth to mitigate off-site migration of volatile organic compounds. Groundwater is continuously pumped
from the Maynardville Limestone at about 95 L/min (25 gal/min), passes through a treatment system to
remove the volatile organic compounds, and then discharges to Upper East Fork Poplar Creek.
Monitoring wells near Well GW-845 continue to show an encouraging response to the pumping
activities. The multiport system installed in Well GW-722, approximately 153 m (500 ft) east and
downgradient of Well GW-845, permits sampling of vertically discrete zones within the Maynardville
Limestone between 27 and 130 m (87 and 425 ft) below ground surface. This well has been instrumental
in characterizing the vertical extent of the east-end plume of volatile organic compounds and is critical in
the evaluation of the effectiveness of the plume capture system. Monitoring results from the sampled
zones in Well GW-722 indicate reductions in volatile organic compounds due to groundwater pumping
upgradient at Well GW-845 (Fig. 4.43). Other wells also show decreases that may be attributable to the
plume capture system operation. These indicators show that operation of the plume capture system is
decreasing volatile organic compounds upgradient and downgradient of Well GW-845.
Fig. 4.43. Decreasing summed volatile organic compounds observed in exit pathway Well
GW-722-17 near the New Hope Pond, 2009.
The Y-12 National Security Complex 4-73