Gable Mountain and B Pond Complex
"Natural attenuation" refers to the ability of groundwater to rid itself of contamination
resulting from a spill or disposal of hazardous waste. Tests have demonstrated that it is
an effective form of remediation at sites with aquifers composed of granular sediment.
The basic concept of natural attenuation is not new. Bacteria that naturally inhabit
groundwater are able to break down chemicals that were once thought to be
non-biodegradable. For example, components of gasoline such as benzene, toluene,
xylene, and ethylbenzene are now known to biodegrade in groundwater to carbon dioxide
and water. Other contaminants, including chlorinated solvents (e.g., dry-cleaning
solvents), can also biodegrade under certain conditions. In some cases, natural
biodegradation may break down contaminants in groundwater faster than they can be
removed by engineered systems. DOE is using monitored natural attenuation in
conjunction with other technologies to remediate groundwater on the Hanford Site.
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The Gable Mountain Pond and B Pond areas are a series of interconnected ponds and ditches used to receive
large volumes of slightly contaminated steam condensate and cooling water from process operations at the
PUREX and B Plants. These sites have relatively low levels of residual contamination and few significant
groundwater plumes.
The remedy for these sites is not expected to be as extensive as at the high-risk sites. Many of these
sites do not require barriers to control infiltration since little residual contamination remains in
the vadose zone beneath these ponds and ditches. These large ponds may be candidates for surface
stabilization with clean topsoil and revegetation, as the remedy, or selective removal of contaminated
soil, stabilization and revegetation could be the preferred alternative. Monitored natural attenuation
appears to be the most likely remedy for the groundwater.
Gable Mountain Pond and B Pond are currently scheduled for remediation by 2026. The proposed CERCLA
plan will be issued for public comment in late summer 2003, with a decision expected by 2004 and completion
of remedial actions in 2009.
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