Site setting


Figure 1. S–3 ponds after paving.          Figure 2. S–3 ponds before paving.

Overview [top]

The ORFRC is located in Bear Creek Valley, on the U.S. Department of Energy's Oak Ridge Reservation in eastern Tennessee. Operation of the S-3 Waste Disposal Ponds resulted in extensive areas of subsurface inorganic, organic, and radioactive contamination from historical and secondary sources. The Ponds received 3.2 x 108 liters of acidic, nitrate and uranium-bearing waste for 32 years until the Ponds contents were "neutralized", "denitrified", and capped in 1988. Although the Ponds are capped, the vast majority of contaminant mass has migrated away from the Ponds into the underlying geologic media where it has precipitated or adsorbed onto the solid phase or migrated into the matrix via diffusion creating an extensive secondary source of contamination. Ground water interaction with this secondary source of contamination has resulted in a relatively stable ground water plume that extends over 4 kilometers down Bear Creek Valley. Contaminants in the ground water plume include uranium (U), technetium-99 (Tc), nitrate, thorium, and volatile organic compounds such as acetone, methylene chloride, toluene, and tetrachloroethylene (DOE 1997). The ground water pH, which can range from 3.2 close to the Ponds to over 7.0 in wells farther down-gradient, is postulated to have a tremendous impact on subsurface processes and contaminant fate and transport. Elevated levels of dissolved organic carbon (DOC >200 ppm), primarily acetate leftover from Pond treatment, and hydrogen (>40,000 ppmv) are also associated with the plume. Other fixed gases recently detected in ground water include significant quantities of CO2, CO, N2O, N2 and CH4. Dissolved oxygen is typically low (<1 ppm) in deeper more contaminated ground water zones but can be higher (2 – 4 ppm) in areas subject to shallow recharge.

Hydrogeology [top]

The site geology at the ORFRC mimics the topography of Bear Creek Valley with stratigraphic units dipping at a 45° angle to the southwest and aligned parallel to the valley. The primary direction of contaminant transport is predictable because it parallels the strike of bedding planes and the axis of valley (Fig. 3) and tends to be constrained within discrete packages of stratigraphic layers. The site is underlain by the Nolichucky Shale although the top 4 to 15 m has weathered into unconsolidated saprolite that maintains remnant bedding structure. Some areas of the site were excavated and filled during facility construction activities. The fill is 0 to 7 m thick with a large fraction consisting of poorly sorted limestone gravel mixed with lesser native saprolite. Ground water transport near the source occurs in both the shale/saprolite (shale pathway) and fill (carbonate gravel pathway) but contaminants also migrate farther down the valley through discharge of ground water contaminants to Bear Creek and its tributaries (Fig. 3). The Maynardville Limestone (carbonate pathway), which lies stratigraphically above the Nolichucky Shale has become contaminated above drinking water standards via hydraulic interactions with Bear Creek.

Figure 3. Major source zones and flow paths in the Bear Creek Valley watershed which encompasses the ORFRC.

Secondary source zones and flow pathways [top]

S-3 ponds secondary source: Considerable research has been conducted at the contaminated area of the ORFRC that includes the ground water plume that originated from the former S-3 Waste Disposal Ponds. Much of the original contamination now is present as secondary sources bound to the sediment-rock matrix material outside of the original disposal sites (DOE 2004). The secondary source areas are extensive and encompass regions on the watershed scale (tens of km). For example, although the S-3 Ponds at Y-12 have been capped, the vast majority of contaminant mass has migrated from the Ponds into the underlying geologic media where it has precipitated or adsorbed onto the solid phase or diffused into the matrix. Leaching of these primary and secondary source zones has created contaminated ground water plumes that tend to be stable and eventually discharge to and contaminate surface water which, in turn, has deleterious ecological impacts and potential adverse consequences for human health.

Figure 4. Example of unconsolidated saprolite commonly found at the ORFRC.

Figure 5. Aerial view of the S–3 Ponds prior to capping.

Low pH shale/saprolite pathway: Between the unconsolidated saprolite and competent bedrock is a transition zone of weathered fractured bedrock. Remnant fracturing in the saprolite and transition zone increases the permeability relative to the silt and clay matrix. The transition zone, which is more highly developed to the south, tends to have higher permeability and greater contaminant transport than the underlying bedrock and overlying highly weathered saprolite due to a combination of higher fracture density and lower clay content (Phillips et al. 2006; Wu et al. 2006a). Ground water in this zone is highly contaminated near the source with U (>60 ppm) and Tc (>40,000 pCi/L) and nitrate (1,000-10,000 ppm) and the solid-phase U is as high as 1000 ppm. Dissolved H2 concentrations are also extremely high (>40,000 ppmv) at some locations near the source. Contaminants have migrated at least 950 m to the west in this pathway discharging to both Bear Creek, NT-1, and NT-2 along the pathway (Fig. 3). During dry baseflow conditions NT-1 has concentrations of nitrate as high as 10,000 ppm, U of 0.2 ppm, and Tc of 5,800 pCi/L. This pathway appears to be the primary source of nitrate and Tc contamination detected in Bear Creek and the Maynardville Limestone.

The shale pathway has a pH as low as 3.5 near the source and is highly buffered by large concentrations of Al in ground water (~500 ppm). As this low-pH plume migrates away from the source it is postulated that the acidity (Al) is titrated by the solid phase and the pH generally remains low (<4.5) until most of the Al is gone (approximately 400 m downgradient). At that point the pH rises abruptly and stays relatively neutral until the end of the plume. At the transition from low to high pH, U and Tc concentrations in the ground water and solid phase are thought to depend on Al chemistry and associated pH changes. DOC concentrations within the low pH zone are as high as 250 ppm but drops off after the pH rises downgradient (Fig. 3). An existing field plot is located in this pathway just 20 m from the S-3 Ponds, and a second cluster of nearby multi-level wells could easily be made into a new field plot. The northern portion of the shale/saprolite plume has not migrated as far from the Ponds and, in contrast to the south, has a pH of 5.5. U and Al are largely absent, nitrate concentrations are as high as 50,000 ppm, anion and cation concentrations are very high, dissolved organic carbon concentrations are 100 ppm, and dissolved H2 concentrations are lower (~100 ppm).

Neutral pH carbonate gravel pathway: A relatively narrow (20 m wide by 6 m deep) contiguous zone of fill consisting mostly of crushed Maynardville Limestone starts about 40 m west of the S-3 Ponds and extends to Bear Creek approximately 260 m from the Ponds (Fig. 3). The ground water plume in the carbonate gravel has a neutral pH (~6.5) and generally lower levels of U (1 ppm) and nitrate (<100 ppm) throughout compared to the underlying ground water plume in the saprolite. Year-round flow of Bear Creek starts where contaminated ground water from the fill discharges to seeps in the creek bottom. Concentrations of U and nitrate are similar in the seeps to the gravel pathway concentrations. The gravel pathway appears to be a major source of u contamination detected in Bear Creek and the Maynardville Limestone. Near the S-3 ponds there is an abrupt transition from low ph in the saprolite to higher ph in the overlying carbonate gravel. The upgradient portions of the gravel near the source are coated with a precipitate that is extremely high in U, on the order of a percent or more (w/w). The precipitate is probably the result of acidic ground water from the shale/saprolite pathway migrating into the higher pH gravel historically. It is not known if this process is ongoing. The U on the solid phase near the source is a secondary source suspected of being sufficient to contaminate ground water in the gravel for hundreds of years. An existing field plot has been installed in the carbonate gravel downgradient and close to Bear Creek.

Neutral pH Maynardville limestone pathway: The limestone acts as a ground water drain for the entire valley because it is located at the bottom of the Bear Creek Valley and has enlarged solutional fractures (i.e., karstic features). Bear Creek loses flow to the limestone (Fig. 3) and is dry during base flow conditions at critical locations. This submergence of Bear Creek not only contaminates the limestone above drinking water standards but has been postulated to be a source of dissolved oxygen (DO) and organic carbon (DOC) needed to maintain subsurface microbial activity for denitrification. Nitrate (900 ppm), U (0.3 ppm) and Tc (900 pCi/L) are lower in this pathway and become more dilute because of surface water/ground water interactions. Ground water in the limestone discharges to several large springs that flow back into Bear Creek. The source of nitrate and Tc is the flow discharging to Bear Creek Tributaries (NT-1 and NT-2) from the shale pathway, but the primary source of U is discharge directly to the Creek from the gravel pathway (Fig. 3).

Recharge: Recharge is not only the hydraulic driver of plume migration but can also be a significant source of dilution, DO, and DOC that can interact with contaminated media. In undeveloped areas of the ground water plume, recharge is expected to be fairly uniform but locally impacted by differences in the permeability of geologic layers that intersect the surface because they dip at 45°. Primary sources of preferential recharge near the S-3 Ponds are the unlined drainage ditch that surrounds the Ponds and the more permeable fill material. A perched zone exists beneath the drainage ditch in fill 1-2 m thick that becomes saturated during most of the year. The ground water in the perched zone has high dissolved oxygen (DO), a pH of 6.5, and contaminants include U (1.0 ppm) and nitrate (<200 ppm). As this high DO and pH perched water migrates down, it mixes and reacts with the lower pH highly contaminated ground water in the shale/saprolite pathway. Preferential recharge also occurs in the carbonate gravel pathway because the fill extends to the surface and provides an avenue for percolation and recharge of high do rainwater.

Typical of most large ground water contaminant plumes, monitoring wells provide only a limited understanding of the plume configuration. For example, aside from our small, well-instrumented ORFRC field plots near the source, monitoring within the shale preferred pathway is sparse, especially downgradient. The two wells monitoring the leading edge of the shale plume are separated by 150 m (Fig. 3) and the closest well beyond the last contaminated well is over 210 m away. Data from wells can be noisy (Fig. 6) because of differential recharge, making it difficult to determine concentration trends and to know even where the well resides within the plume and what the data from the well represent. Using only sparse monitoring well data creates some fundamental problems with assessing natural attenuation.

Figure 6. Noisy concentration trend data showing the difficulty in assessing natural attenuation with monitoring well data. The plot on the left shows seasonal trends observed in the carbonate pathway, the inset shows nitrate near the source drops during a storm as water levels rise, and the right most plot shows how nitrate concentrations at the leading edge of the saprolite plume (GW085) responds dramatically to long-term trends extending over several years (the last measurement in June 2006 was 2, 700 mg/L, the highest observed in 16 years.)

Conceptual model report [top]

The ORFRC conceptual model aggregates and synthesizes the hydrological, geochemical, and microbial processes that control the fate and transport of subsurface contaminants at the site. (More)

Figure 3. Major source zones and flow paths in the Bear Creek Valley watershed which encompasses the ORFRC.

Detailed descriptions of contaminated and uncontaminated sites [top]

Enormous quantities of data have been collected about the ORFRC and portions of the ORFRC at which field research has been conducted and samples collected for laboratory analysis. The descriptions here are organized in accordance with historical NABIR FRC research. Thus, the ORFRC is subdivided into the uncontaminated background site and contaminated sites that are designated Areas 1 through 5. (More)