Hydrogeology

Groundwater Background

TCE Plume Map

Trichloroethene Groundwater Plumes.

Uranium has been enriched at the Paducah Gaseous Diffusion Plant (PGDP) since 1952.  Trichloroethene (TCE), a manufactured volatile organic compound (VOC), was used extensively to degrease enrichment process equipment and and routinely more than 400 miles of process piping.  From 1953 until 1993, rail tank cars unloaded TCE at the southeast corner of the C-400 Cleaning Facility where it was stored, transferred and used in heated liquid baths, high-pressure sprayers and vapor degreasing units.

TCE Cleaning Baths

Acid Cleaning Baths in the C-400 Building.

TCE is a dense non-aqueous phase liquid (DNAPL) and is more dense than water. TCE’s density causes it to sink through porous soil, aquifer materials, and groundwater. As TCE sinks through pore spaces it leaves TCE DNAPL in the interstitial pore spaces where it remains as it is slowly dissolved. When sufficient quantities of DNAPL are released, their downward movement will continue until they encounter impermeable materials such as clay and pool. Once pooled, the DNAPL will remain a long-term source to groundwater contamination.

Losses of TCE to the shallow subsurface occurred along TCE transfer system piping.
TCE- laden water from the C-400 cleaning baths was discharged through wastewater piping for transfer to the PGDP wastewater treatment facility. The discharged TCE dissolved the oakum joints used to seal the pipes of the wastewater transfer system and allowed TCE-laden water to enter the subsurface directly.

From 1953 to 1976, the PGDP feed plant reprocessed spent nuclear fuel rods containing uranium.  Reprocessing resulted in the introduction of technetium-99 (99Tc or Tc-99),  an artificial fission radioisotope, and other radioactive materials not associated with naturally occurring uranium.  Deposition in process equipment, piping and transfer equipment as well as transfer and storage of 99Tc-bearing liquids introduced 99Tc to PGDP wastewater streams.

Technetium-99 (99Tc) is a unique radionuclide in environmental settings because it easily dissolves in water where it forms the pertechnetate ion (TcO4-). The pertechnetate ion is relatively unreactive with aquifer materials and very mobile in groundwater.

The southeast and northwest corners of the C-400 Cleaning Building have been identified as major groundwater contamination sources related to the PGDP Northwest Plume which contains TCE and 99Tc (Figure 2.C.1.1.-1).  Sources of the Southwest Plume (TCE and 99Tc) include uranium burial grounds, a RCRA-closure drummed uranium above-ground burial facility, an oil land-farm, and the C-720 maintenance facility (Figure 2.C.1.1.-1).  The Southwest Plume is limited to DOE property.  On-site sources for the Northeast Plume have not been clearly defined.

The Northeast and Northwest TCE Plumes and the Northwest 99Tc Plume are the largest in the DOE complex and amongst the largest documented groundwater plumes of their kind in the world.  The depth of the contaminated aquifer, the geochemistry of the aquifer and contaminants, and the aquifer materials, which range from fine sands to cobbles all pose challenges to geoscientists and engineers in their efforts to characterize and remediate PGDP groundwater contamination.

TCE source areas

Primary Trichloroethene (TCE) source areas at the PGDP

http://www.eoearth.org/article/TCE_contamination_of_groundwater

RGA WLE Oct 2010 ‐ 2012 Location Averages, LBC Seeps, TVA