Detailed site descriptions

 

Contaminated site characteristics

The 98 hectare contaminated area (see figure) is located in Bear Creek Valley (BCV) just west of the main Y-12 industrialized plant area. The majority of the area is open although there are some wooded areas. Bear Creek flows down the middle of the area adjacent to Bear Creek Road. The flow in Bear Creek is supplemented by small tributaries originating on the southern slope of Pine Ridge, and by springs emanating mainly from the base of Chestnut Ridge. The tributaries convey shallow ground water that has discharged to the surface and stormflow. In its upper reaches, Bear Creek follows a relatively straight course along the geologic strike close to the contact between the Maynardville Limestone and Nolichucky Shale. The original channel on the west side of the S-3 Ponds was filled with rubble during pond construction and rerouted to its present location (Law Engineering 1983).

Major facilities that fall within the area include the S-3 Ponds, which are currently covered with an asphalt parking lot, the West End Treatment Facility (WETF), and the S-3 Ponds Pathway 1 and Pathway 2 reactive barrier demonstration projects (see figure). Dozens of single and paired wells have been installed at the two reactive barriers sites (see figure). These wells, and dozens of other wells installed in the area by the Y-12 Ground Water Protection Program, are available for research purposes. The ORNL soil lysimeter site is located within the area as well (see figure).

General characteristics – The S-3 Ponds consisted of four unlined ponds constructed in 1951 on the west end of the Y-12 Plant. Liquid wastes, composed primarily of nitric acid plating wastes containing nitrate and various metals and radionuclides (e.g., uranium and technetium) were disposed of in the ponds until 1983. Brooks (2001) provides a more detailed description of the wastes disposed of in the Ponds and uranium chemistry. Waste disposal activities at the site have created a mixed waste plume of contamination in the underlying unconsolidated residuum (primarily saprolite and fill) and shale bedrock. Areas 1 and 3 are located adjacent and directly south and west, respectively, of the former S-3 Ponds, but Area 2 is located several hundred feet to the southwest of the Ponds (see figure).

The Nolichucky shale bedrock that dips approximately 45 degrees to the southeast and has a strike of N55E (parallel to BCV) underlies the site. Overlying the bedrock is unconsolidated material that consists of weathered bedrock (referred to as residuum or saprolite), man-made fill, alluvium, and colluvium. Silty and clayey residuum comprises a majority of the unconsolidated material in this area. The thickness of residuum overlying the Nolichucky shale is typically between 5 and 10 m (15 and 30 ft) thick. Between the unconsolidated residuum and competent bedrock is a transition zone of weathered fractured bedrock. Remnant fracturing in the residuum and transition zone increases the permeability relative to the silt and clay matrix (core pictures) .

Total dissolved solids (TDS) content of the ground water plume is > 40,000 mg/L in some areas near the ponds. The S-3 Ponds plume also contains elevated levels of nitrate, bicarbonate, and other ions, metals, uranium, technetium-99, and tetrachloroethylene (PCE). The plume is stratified, with the distribution of contaminants dependent on geochemical characteristics of the contaminants and ground water. For example nitrate and technetium, which are not highly particle reactive, have the most extensive distribution in ground water. Uranium and metals that are more reactive are not as deep and have not migrated as extensively away from the ponds.

Flow in the shallow interval is oriented predominantly along geological strike, but is influenced by local topography with discharge occurring at Bear Creek and tributaries to Bear Creek. Geochemistry indicates that greater ground water residence times and, thus, slower flow generally occur below 30-m in the intermediate interval. However, the distribution of contaminant plumes in BCV indicates that more rapid flow than predicted by major ion geochemistry may occur in an along-strike direction in preferential pathways in the intermediate interval and some flow may occur up to 61-m in depth (nitrate from the S-3 Site has migrated approximately 1-km or more in the Nolichucky Shales since1950). One of these preferential pathways appears to be located in the southern portion of Area 3. A block diagram showing a conceptual rendering of contaminant transport near the S-3 Ponds is provided in this figure. Ground water levels in the vicinity of the S-3 Ponds on 12/28/00 are provided in this figure.

A number of tracer tests have been conducted within the background and contaminated areas to evaluate transport behavior and to identify key processes affecting transport. Two processes contribute significantly to retardation of solute transport and the storage of solute mass in the matrix: sorption and matrix diffusion. High clay content within the weathered matrix coupled with high porosity and small pore size impart a large surface area for sorption of reactive solutes within the matrix and, secondarily, on fracture surfaces. In addition, these same characteristics result in a large, relatively immobile volume of porewater that acts as a reservoir for storage of solutes that diffusive into the matrix through the fracture walls. The result is a significant slowing of the transport rates and the creation of secondary sources within the matrix that can and do release solutes over long periods of time. Because fracture flow rates are high, mass can be transported rapidly through preferred fracture flow pathways. This is particularly true of colloids and bacteria that reside only in the fractures due to size exclusion from the matrix. However, the overall mass flux may be low because of the low overall fracture porosity and, in the case of solutes, because of mass transfer into the matrix pores and onto solid surfaces.

The transport behavior described above has been demonstrated in laboratory tests in undisturbed cores (Sanford et al. 1996; Jardine et al. 1988, Jardine et al. 1993a; Jardine et al. 1993b; Reedy et al. 1996; Moline et al. 1997) as well as field-scale tracer tests conducted at the background area (Lee et al. 1992; Moline et al. 1998; Sanford and Solomon 1998) and similar sites on the Oak Ridge Reservation (Wilson et al. 1993; Jardine et al. 1999). Cook et al. (1996) discuss the implications of the matrix diffusion process on the use of environmental tracers for dating the age of ground water and interpretation of the data for inferring such parameters as recharge rates and vertical ground water velocities.

Waste disposal units– Since the late 1940s, hazardous and radioactive materials from Y-12 Plant operations have been disposed at various sites in BCV (USDOE 1997). The principal waste areas and contaminant sources in BCV are located in the upper 3.5-km of the valley on the outcrop of the Nolichucky Shale. Of these waste disposal units, the S-3 Ponds and Boneyard/Burnyard (BY/BY) are the primary contributors of contamination to the area (see figure).

The former S-3 Waste Disposal Ponds consisted of four unlined surface impoundments that were constructed in 1951. They received liquid nitric acid and uranium-bearing wastes via a pipeline at a rate of approximately 10 million liters/year until 1983. The Ponds were unlined and approximately 122-m × 122-m in dimension and 5.2-m deep. Infiltration was the primary release mechanism to soils and ground water. The S-3 Ponds were neutralized and biodenitrified in 1984 and subsequently closed and capped in 1988. The site is currently a large asphalt parking lot.

The BY/BY is not located within the FRC contaminated area but is one of the primary sources of uranium contamination in the Maynardville limestone (see figure). The BY/BY includes the 1) boneyard, which consisted of unlined shallow trenches that were used to dispose of construction debris and to burn magnesium chips and wood; 2) burnyard, which was used from 1943 to 1968, and received wastes, metal shavings, solvents, oils, and laboratory chemicals that were burned in two unlined trenches; and 3) Hazardous Chemical Disposal Area (HCDA), which was built over the burnyard and handled compressed gas cylinders and reactive chemicals. The residues from the cylinders and reactive chemicals were placed in a small, unlined pit. Although the HCDA has been capped, the rest of the BY/BY has not been capped.

Geology– The boundary of the western portion of the FRC contaminated area was selected to include the S-3 Ponds ground water plume in the Nolichucky Shale. The eastern half of the area overlies the portion of the Maynardville Limestone that contains the co-mingled S-3 Ponds Plume and BY/BY uranium ground water plume (see this figure and this figure).

The geology of BCV displays an inclined layer-cake-style stratigraphy that is observed on a regional scale where limestone- and dolomite-dominated rock groups are interbedded with predominantly clastic shale groups, and on the scale of outcrops where clastic beds are interlayered with carbonate beds. The orientation of geologic units is parallel and coincident to the valleys and ridges. Three primary fracture sets have been identified—parallel to bedding, perpendicular to bedding along strike, and vertical parallel to dip (Hatcher et al. 1992; Solomon et al. 1992). Additional fracture sets may also exist, and local deformation may alter the orientation of the fracture sets relative to the regional structural grain. Fracture density ranges from about 15 to 30 fractures per meter based on rock coring and geophysical logging (Lee et al. 1992).

Bear Creek Valley is underlain by rocks of three regionally important stratigraphic units: the Rome Formation, the Conasauga Group, and the Knox Group, that typically dip 45º to the southeast and have a geologic strike of N55E (see this figure and this figure). All of these rocks were formed over 500 million years ago in the Cambrian geologic age. General descriptions of the stratigraphy of geological units on the Oak Ridge Reservation are provided in the BCV Remedial Investigation Report (USDOE 1997) and Hatcher et al. (1992). These units can be grouped into those that are mainly clastic (i.e., shales) and generally have lower permeability, and those that are mainly carbonates and are generally more permeable (Solomon et al. 1992).

The Rome Formation and the Conasauga Group crop out in BCV on Pine Ridge and dip to the southeast beneath BCV. The primary geologic units of interest that underlie the FRC Contaminated Area are the Maynardville Limestone (67 to 136-m thick) and Nolichucky Shale (190-m thick), both of which are subunits of the Conasauga Group. The Maynardville Limestone and Nolichucky Shale underlie both the contaminated and background areas (see figure). With the exception of the Maynardville Limestone, the Conasauga Group is a sequence of shale, siltstone, and thin-bedded limestone. Some formations, however, include laterally continuous limestone beds that can be several meters thick, and high permeability zones parallel to bedding planes may exist, especially where karstification has enlarged fractures in limestone beds. The Maynardville Limestone, the uppermost member of the Conasauga Group, is a massively bedded limestone and dolomite with fracturing and karstification. The Maynardville Limestone forms the floor of BCV and contains the channel of Bear Creek along most of the valley. The Nolichucky Shale is located just up slope and stratigraphically lower than the Maynardville Limestone. The Knox Group (i.e., Copper Ridge Dolomite) underlies and forms Chestnut Ridge, the southern boundary of BCV.

The primary porosity of the rocks underlying the area is low, typically less than 2% in the Maynardville Limestone and approximately 10% in the Nolichucky Shales (Dorsch et al. 1996; Goldstrand 1995). The porosity of the residuum is typically 30 to 50 percent. Diagenesis, fracturing, and in the case of the carbonates, solution weathering (i.e., karstification) of bedrock have resulted in secondary porosity and increased permeability through which most fluid movement occurs (Solomon et al. 1992). Shevenell and Beauchamp (1994) have shown that the occurrence of cavities in the Maynardville Limestone decreases significantly below a depth of approximately 23-m. Below a depth of 23-m, water bearing zones are generally associated with fractures. Cavities are generally filled with silt and range in size from <0.3-m to >3-m, although, over 70% of cavities are <1.2-m.

Overlying the bedrock on the Oak Ridge Reservation are unconsolidated material that consists of weathered bedrock (referred to as residuum or saprolite, (see figure), man-made fill, alluvium, and colluvium. Silty and clayey residuum comprises a majority of the unconsolidated material in this area. The depths to unweathered bedrock differ throughout the Oak Ridge Reservation because of the varying thicknesses of fill and alluvium, and the particular weathering characteristics of the bedrock units. The total thickness of these materials typically ranges from 3 to 15-m (Hoos and Bailey 1986). The thickness of residuum overlying the Nolichucky shale within the contaminated area is typically between 5 and 10-m thick. This material will be the primary target for in situ sampling and research. The average thickness of residuum overlying the Maynardville Limestone is typically less than 3-m. Between the unconsolidated residuum and competent bedrock is a transition zone of weathered fractured bedrock.

Hydrology – The contaminated area receives an average of 137 cm of precipitation per year, much of it occurring in the winter months (see figure). This figure shows a conceptual rendering of ground water flow and contaminant transport in upper BCV. The hydrogeology of BCV differs significantly between the mainly clastic formations (i.e. the Nolichucky Shale) and mainly carbonate formations (i.e., the Maynardville Limestone). In BCV, the contact between the Maynardville Limestone and the Nolichucky Shale roughly corresponds to the axis of the valley and marks a major transition from predominantly lower permeability clastic formations to higher permeability carbonate dominant formations. Ground water in the clastic formations generally migrates along-strike in the unconsolidated residuum, transition zone and/or bedrock until eventually discharging to a tributary of Bear Creek. This surface water can enter the Maynardville ground water system through losing sections of Bear Creek.

The orientations of well-connected fractures or solution conduits are predominantly parallel to bedding planes (i.e., geological strike) and enhance the effect of anisotropy caused by layering. Remnants of this bedding plane fracturing are also present in the unconsolidated zone. This results in dominance of strike-parallel ground water flow paths in both the unconsolidated zone and bedrock. Fracture aperture width generally decreases with depth in all formations and thus restricts the depth of active ground water circulation. Active (or open) fractures occur at greater depths in the Knox Group and Maynardville Limestone than in the shale members of the Conasauga Group, and therefore, active ground water circulation is deeper in these carbonate formations.

Ground water flow in the unconsolidated zone and Nolichucky shales – Flow in the shallow interval is oriented predominantly along geological strike, but is influenced by local topography with discharge occurring at Bear Creek and tributaries to Bear Creek. Geochemistry indicates that greater ground water residence times and, thus, slower flow generally occur below 30-m in the intermediate interval. However, the distribution of contaminant plumes in BCV indicates that more rapid flow than predicted by major ion geochemistry may occur in an along-strike direction in preferential pathways in the intermediate interval and may occur up to 61-m in depth [nitrate from the S-3 Site has migrated approximately 1-km or more in the Nolichucky Shales since1950].

A bromide tracer study test conducted in the unconsolidated zone at the Pathway 2 reactive barriers site (located within the contaminated area, see this figure and this figure) resulted in an average peak concentration arrival velocity of approximately 2-m/day. However, a large mass of bromide remained in the vicinity of the injection well suggesting matrix diffusion has a significant impact on transport at this site. Testing conducted with a colloidal borescope at the same site provided similar results. Pathway 2 is located near the location of an old Bear Creek stream channel and some springs in Bear Creek, therefore, transport rates in the residuum at other locations is expected to be as much as an order of magnitude less than this.

A pumping test was recently conducted (ENTECH 1998) in a well located adjacent to NT-1 within the FRC Contaminated Area. The well is screened across approximately 150 ft of the Nolichucky Shale bedrock. At a pumping rate of 3 gpm and duration of seven days, a transmissivity of 4.6 × 10-6 m2/s and storage coefficient of 1.4 × 10-3 was calculated from the drawdown data in nearby wells. These values correspond favorably with data collected from other hydraulic testing conducted in the Nolichucky bedrock interval.

Ground water flow in the Maynardville limestone – The Maynardville Limestone is available, but is not the primary geologic unit targeted for in situ sampling and research. There are several locations where sufficient wells are located along contaminant transport pathways that investigators could conduct transport studies on fractured carbonate rock if desired. The Maynardville Limestone crops out along the southern side of the BCV floor and acts as a hydraulic drain for the valley. Flow in these formations is predominantly along strike and parallel to the maximum hydraulic gradient. The hydrostratigraphy in the carbonate formations is less defined than that in the Nolichucky Shale (Solomon et al. 1992). The definitions of shallow, intermediate, and deep regimes are not rigid and rely on cumulative evidence from fracture and cavity occurrence (Shevenell et al. 1992; Shevenell and Beauchamp 1994), and hydraulic and geochemical responses during storms and distribution of contamination (Shevenell 1994; USDOE 1997).

The shallow interval includes ground water to approximately 30-m depth. The channel of Bear Creek can be considered conceptually as one of the main hydraulic conduits in this system. In this interval, ground water flow is relatively rapid. ground water and contaminant transport in the residuum found over the Maynardville Limestone is less important than it is over the Nolichucky shales because the residuum over the Maynardville Limestone is thinner and transports a smaller percentage of ground water flow. The intermediate interval occurs between approximately 30-m and 100-m depth. Solution cavities and solutionally enlarged fractures exist in the Maynardville Limestone in this interval and are probably well connected by fractures. Because of its depth, this zone is isolated from dilution effects seen in shallower zones. Thus, flow rates are probably slower than those in the shallow interval, but contaminant plumes are more persistent and extend farther along the valley. This zone constitutes an important contaminant transport pathway and could be the targeted for research by investigators. In the deep interval [greater than 100-m depth], flow through fractures dominates ground water movement, and flow zones become less frequent as fracture density decreases with depth.

Only one pumping test has been conducted in the Maynardville Limestone in BCV. The wells used to conduct this test are located in the background area. A transmissivity of 1.2 x 10-3 m2/s and storage coefficient of 6.5 × 10-4 was calculated from the drawdown data in nearby wells. The hydraulic conductivity of the Maynardville was estimated to be 8.4 × 10-3 cm/sec.

Geochemistry – For data analysis purposes during the BCV remedial investigation, waste areas and plumes were divided into functional areas (FAs). The two FAs that fall within the FRC contaminated area include the S-3 Ponds FA in the Nolichucky Shale unconsolidated zone and bedrock and the Maynardville Limestone FA. A summary of the analytical results for wastes, soil, ground water, and surface water samples collected in the S-3 Ponds FA and ground water and surface water in the Maynardville Limestone are provided in this table and this table respectively (USDOE 1997). Data on the frequency of detection, maximum and average detected values, and whether an analyte is a site-related contaminant are also provided in these tables. This table provides data on ground water quality in the saprolite around the S-3 Ponds. This table and this figure summarize microbial data collected from the saprolite near the S-3 Ponds reactive barrier sites.

Soils – The residuum in the vicinity of the former S-3 Ponds is the primary source of soils contaminated with radionuclides and metals. Several potential test plots are shown on this figure. This table lists the contaminants detected in the boreholes drilled in the residuum near the S-3 ponds for the remedial investigation. The residuum is contaminated with metals (barium, copper, lead, mercury, nickel, vanadium and zinc), radionuclides (uranium, Tc-99, and Th-230), and organics (acetone, methylene chloride tetrachloroethylene, and toluene) above background concentrations. There are other contaminants detected above background levels but at a lower frequency and concentration. U-233/234 was detected at a maximum concentration of 17 pCi/g and average concentration of 2.1 pCi/g. U-238 was detected at a maximum concentration of 43 pCi/g and average concentration of 4.6 pCi/g. Concentrations of metals and radionuclides were highest near the southwest corner of the former S-3 Ponds. This is the primary area targeted for in situ sampling and research. An additional 10 borings were drilled and sampled for a technology demonstration project in the residuum southwest of the S-3 Ponds. The samples were analyzed for uranium only. The maximum concentration of U-238 detected in these samples was 162 pCi/g (490 ppm). All of the samples from the residuum are depleted relative to the amount of U-235 present (i.e., U-235/U-238 is <4.6%). There appears to be zones of elevated uranium in the unsaturated zone at a depth of 2 to 3 ft near the water table at a depth of 8 to 10 ft, and near the bottom of the residuum at a depth of 19 to 20 ft (see figure).

The concentration of uranium in the Nolichucky Shale and Maynardville Limestone bedrock has not been characterized. The concentrations of uranium in the Maynardville Limestone are likely to be low or not detectable because the concentrations detected in ground water are relatively low (< 0.30 ppm) and the Maynardville is not directly below a source area. However, the concentration in the shallow Nolichucky shale in the vicinity of the S-3 Ponds may be higher because concentrations of uranium in ground water have historically been detected at concentrations as high as 44 ppm with an average of 2.9 ppm.

Ground water – Contaminants in the commingled S-3 Ponds and BY/BY plume include radionuclides (uranium, Sr-99, and Tc-99), metals (strontium, cadmium, barium, boron, mercury, chromium), volatile organic contaminants (VOCs), nitrate, and lesser amounts of other contaminants. Although, the S-3 Ponds site is a source of all of these contaminants, the BY/BY site has contributed primarily uranium and VOCs to the Maynardville Limestone. There are also some unknown sources of VOCs (PCE, TCE and DCE) in the Maynardville Limestone.

The S-3 Ponds and BY/BY are located on top of the unconsolidated Nolichucky Shale, and historical waste discharges have contaminated shallow ground water beneath the waste sites. Due to the high dissolved solids contents of the liquid wastes disposed at the S-3 Ponds site, contamination has migrated to depths as great as 130-m in the Nolichucky Shales. The S-3 ponds site is located on a ground water divide so contamination has migrated to both the west and east. A block diagram showing a conceptual model of ground water flow around the S-3 Ponds is shown in this figure. The shallow unconsolidated zone would be the primary hydrogeologic environment for in situ sampling and research. The extent of nitrate, gross alpha (indicator of uranium) and gross beta (indicator of Tc-99) are shown on this figure. Nitrate is an excellent indicator of the extent of the S-3 Ponds plume. Most of the uranium (gross alpha) contamination detected in the Maynardville Limestone west of the BY/BY probably originated from the BY/BY. This figure shows the typical concentration of uranium detected in shallow (<10-m deep) push-probe piezometers installed in the unconsolidated zone around the S-3 Ponds. A list of contaminants above background concentrations is summarized in this table (Nolichucky Shale) and this table (Maynardville Limestone). Typical ground water quality of shallow piezometers installed in the unconsolidated zone near the S-3 Ponds as part of the reactive barriers project is summarized in this table.

Background concentrations of dissolved oxygen in the residuum at the reactive barrier sites are generally 2 ppm but vary between 1 and 4 ppm, Eh varies between 100 and 300-mV, and pH varies between 5 and 6.5. The terminal electron acceptor process in the shallow residuum is likely driven by oxygen but there is also a significant potential for nitrate to be important to this process. Deep ground water in the Nolichucky Shale and Maynardville Limestone can be anaerobic and have a negative Eh (< -250-mV). Under these ground water conditions nitrate is likely to be the most important to the electron acceptor process. The reactive barriers at the S-3 Ponds site were installed using guar gum. After the guar was broken down by injecting an enzyme, microbial activity in the gravel filled trenches associated with the barriers increased dramatically particularly with sulfur and iron reducing microbes (see figure). Pre-guar gum concentrations of nitrate (>1,000 ppm) and uranium (>2 ppm) in these trenches were reduced dramatically to the low ppb level after the guar gum was injected, suggesting the potential for removal of these contaminants through microbial activity.

Contaminants migrate away from the waste disposal units using the following pathways (see figure). Contaminated shallow ground water in the unconsolidated residuum and bedrock at sources on the Nolichucky Shale generally migrates along geological strike with local influences from topography, and discharges to tributaries or directly to Bear Creek causing Bear Creek to become contaminated. Contaminants in intermediate ground water in the Nolichucky Shale also migrate through fractures along strike and discharge to tributaries (see figure).

After entering tributaries, contaminants migrate in surface water directly to Bear Creek. Bear Creek intermittently loses and gains water from ground water in the Maynardville Limestone throughout the length of the valley. Losing reaches of Bear Creek cause ground water contamination in the Maynardville Limestone (see figure). Intermediate and deep ground water in the Maynardville Limestone [30 to 90-m depth] constitutes less than 4 percent of water flowing along the valley. Concentrations of contaminants in this and in the deep ground water pathway are not attenuated as rapidly as those in shallow ground water, therefore, this pathway is an important source of long distance ground water transport along BCV.


Area 1 characteristics

Area 1 consists of a small 7 m by 25 m field plot just south of the S-3 Ponds (see figure). Well locations in Area 1 are shown on this figure and data summaries for the shallow FRC wells and select cores within Area 1 are provided in the data summary tables. Existing field plots and other locations within Area 1 are available for NABIR research. Thirteen monitoring wells have been installed in the field plot (see figure). The wells are generally 1.25 inches in diameter, about 7 meters deep and have a 5-foot length of screened interval at the bottom of the well. The wells have been used used by Jonathan Istok and his colleagues at Oregon State University to conduct push-pull tests of various types. The impact of these push-pull tests probably does not extend beyond the 7 m by 25 m field plot.

A typical geologic profile at the Area 1 field plot would consist of about 1.5 meters of reworked fill and saprolite at the surface underlain by about 7 meters of intact saprolite with weathered shale bedrock below the saprolite. Hydraulic conductivity of the saprolite is fairly low (about 0.26 m/day) with maximum pumping rates of < 1 liter/minute. Hydraulic monitoring at the site indicates that the depth to ground water is approximately 3.5 meters from the surface and the hydraulic gradient is fairly flat. Contaminants include all the contaminants generally associated with the S-3 Ponds ground water plume (e.g., nitrate, technetium, uranium, volatile organic compounds and relatively high concentrations of other common anions and cations).

Concentrations of contaminants in ground water and soil vary from well to well, but tend to be fairly stable over time within individual wells. Nitrate concentrations at the Area 1 field plot in ground water range from 48 to 10,400 mg/l, uranium ranges from 0.01 to 7.5 mg/l, and technetium-99 ranges from 66 to 31,000 pCi/l. Wells with high uranium (e.g., >1 mg/l) tend to have high to moderate nitrate (>1,000 mg/l) and high technetium concentrations (>12,000 pCi/l). The pH of ground water at Area 1 tends to be more acidic than Area 2 but ranges between 3.25 and 6.5 with dissolved oxygen content about 1-2 mg/L. Sulfate concentrations range between 219 mg/l and 1 mg/l, and chloride concentrations range between 22 and 760 mg/l. Aluminum can be as high as 620 mg/l and nickel is found at concentrations of 8.6 mg/l. Calcium, sodium, magnesium, and manganese are other metals detected at significant concentrations (>100 mg/l) at the site. Tetrachloroethylene (0.120 mg/l), acetone (0.230 mg/l), and some other volatile organic compounds (VOCs) are also detected at the Area 1 field plot. As much as 375 mg/kg of uranium is associated with the solid phase material.


Area 2 characteristics

Data summaries for the FRC wells and select cores within Area 2 are provided in the data summary tables. Contaminants detected at Area 2 were probably transported to Area 2 through an historic stream channel of Bear Creek during operation of the Ponds. Some contaminated residuum and sediments in Area 2 were excavated and deposited in the S-3 Ponds, however, much contaminated residuum remains and contributes to the ground water contamination currently detected in Area 2.

A typical geologic profile at Area 2 would consist of about 6 meters of reworked fill and saprolite at the surface underlain by 2 meters of intact saprolite with weathered bedrock below the saprolite. As much as 300-500 mg/kg of uranium may be associated with the solid phase material. The reworked fill tends to have a higher hydraulic conductivity than the native saprolite. Based on data from a tracer study test conducted in 1998 (Watson et. al.1998) the rate of interstitial ground water movement in the unconsolidated fill was calculated to range from 0.7 m/day to 4.5 m/day, with an average rate of about 2.2 m/day. Hydraulic monitoring at the site indicates that the depth to ground water is approximately 4.5 meters from the surface and the hydraulic gradient ranges between about 0.01 and 0.025 to the southwest towards the Creek. Vertical upward gradients between the shale bedrock and unconsolidated zone are as great as 0.25.

The Area 2 site is a shallow pathway for the migration of ground water contaminated with uranium (12 mg/L) to seeps in the upper reach of Bear Creek (which is adjacent to Area 2). Nitrate concentrations are generally lower (e.g., <100 mg/L) at Area 2, than at the other contaminated sites, but have been detected above 1,000 mg/L in several wells. Technetium is generally detected below 600 pCi/L, and total dissolved solids concentrations (approximately 1,000 mg/L) are generally lower than Areas 1 and 3. The pH of ground water at Area 2 tends to be between 6 and 7 with dissolved oxygen content about 1-2 mg/L. Areas of higher and lower uranium and nitrate exist at Area 2. For example TPB-16 which is representative of an area with higher uranium and lower nitrate contains 28 mg/L nitrate, 98 mg/L sulfate, 310 mg/L chloride, 60 mg/L inorganic carbon, 2 mg/L organic carbon, and 1.3 mg/L uranium; while well FW003 which is representative of an area with higher nitrate and lower uranium contains 1059 mg/L nitrate, 16 mg/L sulfate, 183 mg/L chloride, 89 mg/L inorganic carbon, 13 mg/L organic carbon, and 0.01 mg/L uranium.

An 8-9 meter-deep trench bisects Area 2 in an east to west direction. The trench was filled with gravel except for an 18-meter-long section in the middle, which was filled with zero-valent iron. Guar gum slurry was added during excavation to prevent the trench walls from collapsing. The trench is oriented nearly parallel to the direction of ground water flow and is designed to use both the natural ground water gradient and the permeability contrast between the gravel and iron in the trench and the native silt and clay outside the trench to direct flow through the iron treatment zone. Over 50 wells have been installed at the site (see figure). Two 20m by 20m plots (one located on either side of the trench) that are high in uranium are available for use by NABIR PIs for field research (see figure). Plots with other geochemical conditions such as ones low in uranium but high in nitrate are also available.


Area 3 Characteristics

Area 3 located just west of the S-3 Ponds (see figure), contains a small (10m by 10m) field plot. Well locations in Area 3 are shown on this figure and data summaries for the FRC wells and select cores within Area 3 are provided in the data summary tables. The existing field plot and other locations within Area 3 are available for NABIR research. Several monitoring wells and multi-port wells have been installed in the field plot to a depth of 15.6 meters (see figure). Each multi-port well can monitor 7 different depth intervals. The site is the location for field-scale bioreduction of uranium experiments by researchers at Stanford University and partners at ORNL.

A typical geologic profile at the Area 3 field plot would consist of about 1 meter of reworked fill and saprolite at the surface underlain by about 2 to16 meters of intact saprolite with weathered shale bedrock below the saprolite. Based on geophysical studies conducted at Area 3 and observations made during the installation of monitoring wells the thickness of saprolite in the southern portion of Area 3 is about 15.6 meters thick but only about 2 to 3 meters thick in the northern portion of the study area. Hydraulic conductivity of the shallow saprolite is fairly low (<0.26 m/day) with maximum pumping rates of < 1 liter/minute. However, the permeability of the deeper saprolite (9 to 15.6 meters) in the southern portion of Area 3 is higher and can be pumped at rates of 5 liters/minute or more. Ground water flux within this interval is estimated to be an average of 0.5 m/d based on point dilution measurements. This deep saprolite zone is probably a preferential pathway for contaminants away from the S-3 Ponds. The Area 3 field plot was installed in this preferential pathway.

Hydraulic monitoring at the site indicates that the depth to ground water is approximately 3.5 meters from the surface and the hydraulic gradient is fairly flat (see figure). In the course of reviewing historical air photographs (LMES, 1997), it was reported that a small stream channel had been filled in, crossing roughly through the center of the study area. The precise location of this filled stream is not known (see figure).

Due to it's close proximity to the S-3 Ponds, contaminants (e.g., nitrate, technetium, uranium, volatile organic compounds and other common anions and cations) are detected at the greatest concentration of all the FRC Contaminated Areas. Data tables for Area 3 are grouped into shallow saprolite wells, intermediate saprolite wells that are in the preferred flow path (e.g. FW024 and FW026), and deep bedrock wells (GW-243). Nitrate concentrations at the Area 3 field plot in ground water are approximately 9,000 mg/l, uranium is as high as 50 mg/l, and technetium-99 is as high as 40,000 pCi/l. The pH of ground water at the Area 3 field plot tends to be more acidic than Area 2 (< 4.0), and dissolved oxygen content is about 1-2 mg/L. Sulfate concentrations can be as high as 1,000 mg/l and aluminum can be as high as 541 mg/l. Calcium, sodium, magnesium, and manganese are other metals detected at significant concentrations (>100 mg/l) at the site. Tetrachloroethylene (3.3 mg/l), acetone (0.7 mg/l), and some other volatile organic compounds (VOCs) are also detected at the Area 3 field plot.


Area 4 and Area 5 characteristics

Areas 4 and 5 are located along the primary ground water contaminant flow paths to the east and west of the former S-3 Ponds. These sites are within a known geochemical ground water and sediment gradient (e.g., pH increases and nitrate decreases) as the plume moves away from the source area-the S-3 Ponds. The first phase of site characterization (Geoprobe work conducting electrical conductivity logging and well installation, ground water sampling and analysis, surface geophysics including resistivity and seismic tomography, and water levels) has been completed. Uranium was detected in Area 4 at concentrations as high as 28 mg/l and in Area 5 up to 44 mg/L. Nitrate was detected in Area 4 at concentrations up to 25,000 mg/L and in Area 5 at concentrations up to 6,100 mg/L. Phase 2 coring, well installation, sampling and analysis, pumping tests, and tracer tests are underway to help identify and characterize new field plots within the S-3 Ponds plume. (For more information, see the FRC Site Characterization Plan Addendum.)

Area 4 and 5
Location of Area 4 and Area 5 wells


Background site characteristics

The Background Area consists of approximately 163 hectares located in West Bear Creek Valley, about 2 km from the Contaminated Area (see figure). The area lies directly along the geologic strike of the Contaminated Area and is, therefore, underlain by nearly identical geology, mineralogy, and structure. No known contaminants have been disposed at this location throughout the history of DOE operations. The majority of the area is heavily wooded, with the exception of the Bear Creek floodplain. Several perennial and ephemeral crosscutting streams run through the area and feed into Bear Creek. A gated gravel forestry road runs the length of the area parallel to Bear Creek, providing controlled access. Additional gravel roads provide access to the interior region of the area, including existing well locations.

Dozens of single and paired wells have been installed within the past 12 years and the locations are shown in this figure . In addition, two heavily instrumented experimental field sites are located within the boundaries of the area, providing an existing resource for intensive hydrologic testing and transport studies in the clastic formations (Fig. 1, Fig. 2, Fig. 3, Fig. 4). An automated continuous tracer injection technique using solar-power has been deployed and used in multi-year long ground water transport investigations (Sanford and Solomon 1998; Jardine et al. 1999). Geologic, hydrologic, and geochemical information is summarized in the following sections and data summary table.

Geology – The background area is underlain by rocks of the Cambrian Conasauga Group (Hatcher et al. 1992), including the Dismal Gap Formation (formerly Maryville Limestone) and the overlying Nolichucky Shale (see figure). Both are interbedded shales, siltstones, and limestones that have undergone post-depositional deformation and dip at 45° to the south. The Maynardville Limestone underlies the southern portion of the area near Bear Creek. Although some of the discussion that follows is relevant to the Maynardville Limestone, most of the information pertaining to the background area was obtained from the two experimental field sites that are instrumented in the clastic Nolichucky Shale and Dismal Gap formations.

The bedrock beneath the experimental field sites weathers into an unconsolidated clay-rich saprolite that retains the original fractures and bedding structures (see figure). Depth to competent bedrock is between 5 and 10 m based on continuous coring (Schreiber et al. 1999) and depth to auger refusal (Lee et al. 1992), and appears to be variable based on the weathering characteristics of the underlying rocks. In general, the shales tend to be fissile while the carbonate interlayers are more resistant, imparting a "washboard" shape to the saprolite/bedrock interface. The saprolite is overlain by a thin veneer of organic- and clay-rich soil with a thickness of 0.5 to 3 m that approximates the depth of the root zone. The soil layer thickens in undisturbed, wooded areas.

Mineralogical analyses conducted by Lee et al. (1991) and Schreiber (1995) show that the predominant minerals in the shales include illite, quartz, kaolinite, chlorite, calcite, and plagioclase feldspar. Fracture coatings of calcite, goethite, and kaolinite were identified by Schreiber (1995). The carbonates contain low-Mg calcite, dolomite, and ferroan dolomite (Hatcher et al. 1992). The high clay content of the weathered saprolite creates a high porosity (30 to 50%), low permeability matrix that has an enormous impact on flow and transport characteristics since it serves as a source/sink during solute migration.

Hydrology – Because of the large hydraulic conductivity contrast between the permeable soil and the underlying low permeability saprolite, a perched zone of saturation (stormflow zone) is created within a meter of the surface. The majority of infiltrated water moves parallel to the land surface through this zone, discharging to local cross-cutting streams (Solomon et al. 1992). The remaining recharge migrates both vertically (Solomon et al. 1992) and laterally (Moline et al. in prep) through saturated fractures to the water table zone. Within the upper 8.5 m, recharge fluxes have been estimated at between 0.2 and 0.4 m/y with a mean vertical water velocity within the fractures of approximately 0.12 m/day based on CFC, tritium, and helium dating methods (Cook et al. 1996).

The majority of ground water flux in the saturated zone is focused within an interval defined by the interface between the competent bedrock and overlying highly-weathered saprolite, commonly referred to as the "transition zone." This zone is generally defined as the interval between auger refusal and good core recovery by conventional rotary or cable tool methods. Characteristics of this zone are dense fractures and the relative absence of weathering products that contribute to poor core recovery and greater resistance to auger penetration. These characteristics also result in a significantly higher permeability as compared to both the overlying saprolite and underlying bedrock (Moline et al. 1998). The water table fluctuates from less than a meter near streams to three meters or more seasonally and during individual storm events. The transition zone tends to be saturated throughout most or all of the year and ground water moves through this zone laterally in directions that are heavily influenced by fracture orientation and characteristics (Moline et al. 1998). Tracer tests in the shallow ground water zone have demonstrated strike-preferential flow oblique to the average hydraulic gradient (Moline et al. 1998; Schreiber et al. 1999; Lee et al. 1992). Pumping tests conducted at two sites (Gierke et al. 1988; Lee et al. 1992) have demonstrated greater transmissivity along strike, and estimates of horizontal anisotropy based on the pump test range from 8:1 to 30:1, strike to dip.

Porosity within the matrix ranges from approximately 30-50% in the weathered saprolite (Jardine et al. 1988; Jardine et al. 1993a; Jardine et al. 1993b; Wilson et al. 1992) to 3-15% in the underlying shallow bedrock (Dorsch et al. 1996). The effective porosity is likely to be significantly lower, however, based on modeling of tracer tests at both laboratory (Koch et al. 1999) and field scales. The fitting of tracer breakthrough curves requires an effective porosity in the saprolite closer to 20%, meaning that one third to one half of the total pore volume in the matrix is isolated and therefore inaccessible to advective and diffusive transport (see Jardine et al. 1988). Fracture porosity is generally less than 1% (Solomon et al. 1992).

Bulk hydraulic conductivity in these geologic materials varies over seven orders of magnitude (10-2-10-9 cm/s), depending on the presence or absence of fractures within the volume of influence and their characteristics (e.g., aperture, spacing, connectivity), (Wilson and Luxmoore 1988; Wilson et al. 1989; Wilson et al. 1992). The low end most likely represents unfractured matrix. In general, hydraulic conductivity decreases with depth (Wilson and Luxmoore 1988; Solomon et al. 1992). Storativities within the Nolichucky Shale bedrock range from 1x10-3 to 5x10-4 based on analysis of pumping test results using a variety of approaches (Gierke et al. 1988).

A number of tracer tests have been conducted within the Background Area to evaluate transport behavior and to identify key processes affecting transport. Two processes contribute significantly to retardation of solute transport and the storage of solute mass in the matrix: sorption and matrix diffusion. High clay content within the weathered matrix coupled with high porosity and small pore size impart a large surface area for sorption of reactive solutes within the matrix and, secondarily, on fracture surfaces. In addition, these same characteristics result in a large, relatively immobile volume of porewater that acts as a reservoir for storage of solutes that diffusive into the matrix through the fracture walls. The result is a significant slowing of the transport rates and the creation of secondary sources within the matrix that can and do release solutes over long periods of time. Because fracture flow rates are high, mass can be transported rapidly through preferred fracture flow pathways. This is particularly true of colloids and bacteria that reside only in the fractures due to size exclusion from the matrix. However, the overall mass flux may be low because of the low overall fracture porosity and, in the case of solutes, because of mass transfer into the matrix pores and onto solid surfaces.

The transport behavior described above has been demonstrated in laboratory tests in undisturbed cores (Sanford et al. 1996; Jardine et al. 1988, Jardine et al. 1993a; Jardine et al. 1993b; Reedy et al. 1996; Moline et al. 1997) as well as field-scale tracer tests conducted at the background area (Lee et al. 1992; Moline et al. 1998; Sanford and Solomon 1998) and similar sites on the Oak Ridge Reservation (Wilson et al. 1993; Jardine et al. 1999). Cook et al. (1996) discuss the implications of the matrix diffusion process on the use of environmental tracers for dating the age of ground water and interpretation of the data for inferring such parameters as recharge rates and vertical ground water velocities.

Geochemistry – Extensive sampling of water chemistry within the stormflow zone, vadose zone, and shallow ground water zone was conducted at the experimental field sites instrumented in the shallow unconsolidated zone and bedrock within the background area from 1995 through 1998. Sampling initially consisted of two separate sampling events conducted during a wet seasonal (January 1995) and dry season (July 1995) period within the multilevel wells at one of the field research sites (Schreiber 1995; Schreiber et al. 1999). These data demonstrated that the shallow ground water at the field site is neutral to slightly alkaline (pH 7-8) and can be divided into four major water types: Ca-HCO3, Ca-Na-HCO3, Na-Ca-HCO3, and Na-Ca-HCO3-SO4 waters.

A follow-on investigation was conducted that included intense sampling of stormflow tubes, vadose zone multilevel samplers, and numerous multilevel and standard wells for ion chemistry, stable isotopes, helium, and CFCs (Van der Hoven et al. 1997). These sampling events were conducted monthly for a period of 14 months from November 1996 through January 1998 to capture seasonal changes in water chemistry. In addition, storm-event sampling was conducted using a subset of the well network to measure high frequency changes in ground water chemistry and during these sampling events precipitation and water from a nearby cross-cutting stream were also sampled.