Groundwater contamination is pervasive at the Paducah Gaseous Diffusion Plant (PGDP). Much of this contamination is in the form of dissolved trichloroethene (TCE), a chemical once used to facilitate degreasing of parts and equipment. The other primary groundwater contaminant found at the PGDP is a radioisotope called technetium-99, or 99Tc for short
TCE and 99Tc have their similarities and their differences. TCE, a known carcinogen, entered the subsurface primarily through surface spills and subsurface pipe leaks. Being denser than water, TCE sinks. Not being very soluble in water, TCE tends to travel through the subsurface environment leaving traces behind in the soil until it reaches a barrier such as a clay horizon where it begins to pool. Once it finds a resting place, it can remain there for a long time, slowly dissolving and contaminating the surrounding groundwater. The radioactive isotope 99Tc -- also a carcinogen -- was introduced at Paducah through the reprocessing of uranium. Unlike TCE, 99Tc in the form found at the PGDP is very soluble in water. For this reason it is uncommon to find high concentrations of 99Tc in near surface soils at the site since much of it has likely already dissolved and entered the groundwater.
At the PGDP, contaminated groundwater is primarily confined to three groundwater contaminant plumes. The Northwest Plume is the largest of these, stretching about two and a half miles from the PGDP security fence to the Ohio River. This plume contains both TCE and 99Tc, with much of the 99Tc being concentrated along the plume's southeastern boundary. The Northeast Plume is approximately two miles long and extends from the plant's northeastern corner to Metropolis Lake Road. 99Tc has been detected in the plume but only near the plant's security fence. Therefore, the Northeast Plume is primarily a TCE plume. The Southwest Plume is the third and smallest of the groundwater plumes. Located just outside the western security fence, this plume is likely the youngest of the three and contains both TCE and 99Tc. Relatively low levels of groundwater contamination have also been detected in the vicinity of the C-746-S&T and C-746-U solid waste landfills.
Several actions are planned or are already underway to address groundwater contamination at the PGDP. The Northeast and Northwest Plume pump-and-treat systems represent two interim actions meant to directly address the further spread of dissolved contamination. These systems work by extracting contaminated groundwater from the centers of the two plumes. This water is subsequently treated to remove the contaminants before being discharged into a nearby stream. A final remedial solution for the plumes has yet to be developed. Other actions taken to date focus on the removal of residual and pooled TCE from the ground. The Lasagnaä treatment system was recently used to remediate a small TCE source zone located near the west central portion of the facility. This system utilized direct electrical current to force TCE-contaminated groundwater to pass through several treatment zones that served to destroy much of the TCE present in this area. Similarly, Six-Phase Heating is using electricity to extract TCE from the subsurface near the C-400 Building, the site’s primary parts cleaning facility located near the center of the PGDP. Much of the site’s subsurface TCE contamination at the site exists in this area. Six-Phase Heating utilizes several subsurface electrodes to pass an alternating electrical current through the ground. This current serves to heat and remove TCE from contaminated soils. It is hoped that this technology can be used effectively in the future on a much larger scale to remove much of the TCE that continues to contribute to groundwater pollution at the site.
Decontamination and Decommissioning of the C-410 Complex:
Across the country, aging Department of Energy facilities contain structures – typically buildings - that are contaminated and no longer serve a useful purpose. Many of these structures are in a state of disrepair and contain radioactive and other dangerous constituents. The process of addressing these structures is referred to as Decontamination and Decommissioning, or D&D. The PGDP houses numerous structures that will eventually be subject to the D&D process. Of these, only the C-410 Building Complex is currently undergoing D&D.
The C-410 Building Complex was constructed in the early to mid-50s, its primary mission being to produce UF6, the feed material used in the uranium enrichment process. The complex is composed of several adjoining structures that include the C-410/C-420 Feed Plant and the C-411 facility. The combined floor space of the complex is more than 133,100 square feet. When it was still operational, the C-420 facility would convert UO3 to UF4. The C-410 facility would then convert the UF4 to UF6. Fluorine cells housed within the complex produced the fluorine needed during the conversion process. These cells were maintained at the C-411 facility. At different periods during its operational lifetime, the complex reprocessed fuel removed from nuclear reactors. This material, while mostly uranium, also contained fission byproducts (e.g., 99Tc) produced in the reactors. The complex ceased operations in 1977.
Over the years, the physical condition of the complex has deteriorated to the point that special protective equipment is required for personnel to enter safely. Lead-based paint is peeling from the walls, and asbestos insulation hangs from overhead pipes. The complex’s roof is in a similar state of disrepair. To make matters worse, rodents and other animals have taken up residence inside the buildings.
Radiological and other forms of contamination are a problem. Uranium contamination is perhaps the greatest safety concern, although asbestos and lead-based paint also present hazards. In addition to lead, heavy metals such as cadmium are present. It is also likely that the complex contains small amounts of transuranic radioactive isotopes such as plutonium-239 and neptunium-237. These are some, but not necessarily all, of the contaminants present within the complex. There is still considerable uncertainty as to what other contaminants might be present.
Work is underway at the C-410 Complex to remove elements of infrastructure from inside the larger buildings. Trash that had been stockpiled in certain areas has been removed in order to free up space for the crews that will be dismantling the building’s interior elements. In addition, many of the old fluorine cells are being transferred offsite to a Canadian firm. This firm will likely also purchase several outdoor tanks once used to hold hydrofluoric acid. All materials removed from the complex are to be tested to determine how contaminated they might be. Materials will be managed according to the types and levels of contamination associated with them.
C-746-U Contained Solid Waste Landfill:
The C-746-U Solid Waste Landfill was constructed in the mid-1990s and began accepting waste exclusively from the PGDP in 1997. The U.S. Department of Energy (DOE) has utilized several on-site solid waste landfills in the past to dispose of everything from fly ash to cafeteria waste. The most recent of these is the C-746-U Landfill. The C-746-U Landfill currently consists of five phases, or waste storage areas, constructed on approximately 22 acres. Phases IV and V of the landfill have yet to receive waste as of late 2007. A network of 21 groundwater-monitoring wells that serve as a leak detection system surrounds the landfill.
DOE is currently disposing of certain types of contaminated soils and scrap metal at the C-746-U Landfill. Some of these wastes were generated during the course of remedial actions. While DOE is prohibited from disposing of hazardous waste in a solid waste landfill, regulations permit DOE to dispose of certain other contaminated wastes. However, DOE must provide proof to regulatory agencies that its waste disposal practices remain protective of human health and the environment. DOE submitted this information to the Division of Waste Management and is therefore being permitted to dispose of limited quantities of certain specific types of remediation wastes at this landfill.
