PGDP, located in the Jackson Purchase region of western Kentucky, lies within the northern tip of the Mississippi Embayment portion of the Gulf Coastal Plain Province. The Mississippi Embayment is a large sedimentary trough oriented nearly north-south that received sediments during the Cretaceous and Tertiary geologic time periods. Figure D-1 is a schematic cross-section that illustrates regional stratigraphic relationships in the vicinity of PGDP.
In the geologic Cretaceous period, a coastal marine environment deposited what we now call the McNairy Formation. The environment was a marshy swamp, much the same as southern Louisiana is today. For the most part, the McNairy Formation is sandy at the bottom and silty at the top. A few exceptions to this are lenses of clay and at least one fairly continuous string of gravel.
Above the McNairy is the Clayton Formation. The Clayton was deposited during the early Paleocene geologic epoch in an environment so similar to that of the McNairy that the Clayton and Upper McNairy are indistinguishable in lithologic samples.
Later in the Paleocene, the Porter's Creek Clay was deposited in marine and brackish water environments in a sea that occupied most of the Mississippi Embayment.
These formations, the McNairy/Clayton and the Porter's Creek Clay dip 30 to 35 feet per mile to the south-southwest. This slope is probably due to seismic activity along the New Madrid fault zone.
The next record of geologic history at PGDP is a Pleistocene age river valley occupying approximately the same position as present day Ohio and Tennessee Rivers. The river, the ancestral Tennessee River, eroded any deposition that might have taken place between the Paleocene Porter's Creek Clay and the Pleistocene. The ancestral Tennessee River also cut a prominent river terrace at the south end of the plant.
North of the terrace, ancestral Tennessee River valley deposition is termed continental deposits. The river valley was made up of braided stream channels and possibly several "feeder" streams. The pattern of these channels left a scoured surface at the base of the continental deposits. The lower portion of the continental deposits is approximately 30 feet of gravel. The lower continental deposits, together with scoured gravel from the McNairy Formation adjacent to the lower continental deposits and coarse-grained sediments atop the lower continental deposits, makes up the shallowest aquifer at the plant, the regional gravel aquifer (RGA).
The fast-flowing of the ancestral Tennessee subsided into a lake. Clayey silt with thin zones of sand and occasional gravel were deposited in the upper portion of the continental deposits, the upper continental recharge system (UCRS). These deposits range from approximately 5 to 55 feet thick.
Finally, loess, a wind-blown silt, overlies the continental deposits throughout the site. Thickness of loess deposits varies from approximately 5 to 25 feet, averaging approximately 15 feet.
The local groundwater flow system at PGDP contains four predominate components: the McNairy flow system, the RGA, the UCRS, and the terrace gravels.
Flow originates south of PGDP within Eocene sands and the terrace gravels. Groundwater within the terrace gravels either discharges to local streams or recharges the RGA, though the flow regime of the terrace gravels is not fully understood. Groundwater flow through the UCRS is ultimately downward, also recharging the RGA. From the plant site, groundwater flows northward in the RGA toward the Ohio River, the local base level for the system.
PGDP is a major western Kentucky industry, employing about 1,700 people from the four state area. The West Kentucky Wildlife Management Area and some lightly populated farmlands are in the immediate vicinity of PGDP. Homes are sparsely located along rural roads in the vicinity of the site. Three communities lie within two miles of the plant: Magruder Village to the southwest, and Grahamville and Heath to the east.
Both groundwater and surface water sources have been used for water supply to residents and industries in the PGDP area. Wells in the area are screened at depths ranging from 15 to 245 feet. The screened hydrologic unit for these wells is undefined, though, they are believed to be primarily screened in the RGA. PGDP has supplied water to all residents within the area of PGDP's groundwater contamination with a municipal water supply. The residents' wells have been turned over to the Environmental Restoration Division for sampling. Residents' wells that are no longer sampled were locked and capped in 1995.