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Hydrogeologic Evaluations at the Lexington-Bluegrass Army Depot
by Thomas Dugan
Hydrogeologist, Federal Facilities Oversight Unit
The depot is situated approximately 10 miles east of Lexington, Ky., in a rural section of Fayette County. Since this facility first began operations in 1941, some of the principal industrial activities have included metal plating and recovery of precious metals from electronic equipment. A number of wastes were created during plating operations, some of which include solvents used for metals preparations and concentrated amounts of metal contaminants such as cyanide and chromium.
These wastes were most commonly placed in three unlined landfills positioned across the depot. In addition, other sites have also been used for waste processing at the depot, some of which include the Industrial Waste Treatment Plant (IWTP) and the two lagoons used for liquid wastes dilution. Another waste burial site, Area A in the RFI, has been located near the eastern edge of the facility. This site stored plating wastes in two septic tanks after they were abandoned for a base-wide sewer treatment system. The landfills, IWTP, lagoons and other waste burial sites are currently being evaluated as potential sources of groundwater contamination due to leaching of the plating wastes through the unsaturated zone of soil and bedrock into the underlying aquifers.
The RFI generated a set of groundwater quality data during two phases of sampling in 1991 and 1993. However, a more extensive sampling program was initiated last year to gather more recent data for the interpretation of groundwater flow characteristics and contaminant concentrations in the aquifers beneath the depot. This paper details the results from the most recent sampling event in February, 1995.
RFI monitoring well installations showed how two aquifers predominate across the depot subsurface. These aquifers are located within the Tanglewood Limestone and Grier Limestone members of the Lexington Limestone. These formations were shown from RFI drilling to be composed of limestone and shale in near equal proportions. Core-drilling data indicated there was very little primary porosity in these formations, with the majority of groundwater movement taking place in fractures and possible bedding-plane dissolution features of very immature development. These mini-conduits and fracture-type porosity represent a very minor percent of the total bedrock volume, yet appear to constitute the primary zones of groundwater transmissivity.
Potentiometric surface mapping was completed for the February 1995 data generated from measurements in all monitoring wells at the depot. The majority of these wells were completed in the two principal aquifers. Figures 3 and 4 show contouring of the groundwater surface elevations in the Tanglewood, or main aquifer, and in the deeper Grier Limestone. The potentiometric surface of the shallower main aquifer generally conforms to geologic structure, as mapped on the Clintonville U.S. Geologic Quadrangle. There is a prominent groundwater divide, which trends from the far northeastern corner of the depot property down structure to the southwest (see Figure 3).
Groundwater within the main aquifer appears to move off the potentiometric high where it is impacted by contaminant leachate from the New Landfill. In the area of the Industrial Sanitary Waste (ISW) Landfill and waste burial Area A, groundwater flow in the main aquifer is shown in Figure 3 to move in two distinct directions. A northern-flow component appears to progress from Area A towards monitoring well MW35. Figure 3 shows how a contaminant plume of volatile organic compounds (VOC) moves along the northerly flow component. A second possible source for this contamination is Area B, another waste-burial site also situated along this plume.
Where groundwater movement in the main aquifer moves more southerly, it is apparent from mapping of the recent data that another contaminant plume is being initiated, this time apparently from leachate generated at the ISW Landfill and possible from remnant wastes at the IWTP. Finally, a fourth plume seems to be generated from leachate at the Old Landfill. Plume migration here appears to move in a southwesterly direction.
Mapping of the Grier Limestone, or deep aquifer potentiometric surface, has shown an overall southwesterly flow from the north-central portion of the base down to its southwestern corner. A prominent trough in this surface appears to develop in the area of the ISW Landfill (see Figure 4). Here tightly spaced contours concentrate flow along elevations as low as 890 feet above mean sea level. VOC contaminants are also shown in this mapping to be migrating within the deeper aquifer, with contaminant concentrations here in the Grier at their highest levels anywhere on the facility. Contaminant plumes in the deep aquifer appear to originate from leachate at all three of the landfills, the IWTP, lagoons and possibly from underground storage tanks whose locations are currently unknown.
Additional well installations and pump testing is planned on site to assist in design of groundwater treatment systems. It is anticipated that remediation of the numerous contaminant plumes at the depot will begin as early as the summer of 1995.
Last modified: November 28, 1995
Copyright © 1995 University of Kentucky - Kentucky Water Resources Research Institute