The Estes Gulch disposal cell was constructed to stabilize waste from the two former processing sites near Rifle, Colorado. Both processing sites (named Old Rifle and New Rifle) are located on the floodplain of the Colorado River valley and are north of the Colorado River. Old Rifle is just east of the city limits of Rifle, in Garfield County, Colorado. New Rifle is west of the city of Rifle, approximately 2 miles (mi) (3 kilometers [km]) from Old Rifle. The Estes Gulch disposal cell is located approximately 6 mi (9 km) north of Rifle.

Both processing sites were once owned by Union Carbide Corporation, but now are owned by the state of Colorado. The Old Rifle plant was built by the Standard Chemical Company in 1924 and in 1928 was bought by the United States Vanadium Corporation (an eventual subsidiary of the Union Carbide Corporation). The mill operated from 1924 to 1932, and again from 1942 to 1946 for recovery of vanadium. In 1946, uranium processing was added to the vanadium recovery circuit and recovery of both vanadium and uranium continued until 1958. In 1958, operations were transferred to the New Rifle mill and the Old Rifle mill was shut down.

About 761,000 short tons (tons) of ore from the nearby Meeker and Rifle Creek mines, as well as ore from the Uravan mineral belt, were processed at the Old Rifle mill. Tailings and spent processing solutions were deposited at the site. About 411,000 tons of these tailings were later reprocessed at the New Rifle mill and deposited there. Approximately 350,000 tons of tailings (approximately 259,000 cubic yards [yd³], or 197,000 cubic meters [m³]) remained at Old Rifle. In 1967, Union Carbide moved the southern edge of this tailings pile away from the Colorado River and partially stabilized the pile with a 6-inch (15-centimeter [cm]) cover of earth seeded with grasses.

In July 1958, operations began at the New Rifle mill, which produced both uranium and vanadium until December 1972. After 1972, only vanadium was produced and milling operations ceased in 1981. In addition to tailings from the Old Rifle site, uranium ores and upgrader products were processed at the New Rifle mill. A total of 2.7 million tons of tailings, ores, and upgrader products were processed at this mill.

Upgrader products came from other Union Carbide mills at Slick Rock, Colorado, and Green River, Utah. Upgrader products from Slick Rock were dried fines, dried slime concentrates, green sludge, and uranium-bearing chemical precipitates. The Green River upgrader products consisted of dried slimes and asphaltic uranium-bearing concentrates. Uranium bearing lignite ash from Belfield, North Dakota, was supplied to the New Rifle mill. Union Carbide partially stabilized the New Rifle pile with mulch, fertilizer, and native grasses.

At the Old Rifle mill, a salt roasting process initially was used to recover vanadium. Water, sodium chloride, and sulfuric acid were used in this process. When uranium processing was added to the vanadium recovery circuit in 1946, additional reagents were used: hydrochloric acid, sodium hypochlorate, sodium carbonate, ferric iron sulfate, ammonia, and ammonium chloride.

The New Rifle mill used a solvent extraction method to recover uranium. Reagents used in the process included water, sodium chloride, sulfuric and hydrochloric acid, kerosene, di(2-ethylhexyl) phosphoric acid, sodium hypochlorate, sodium carbonate, sodium hydroxide, and ammonia.

The Uranium Mill Tailings Radiation Control Act (UMTRCA) of 1978 (42 USC §7901 et seq.) gave the DOE authority to perform remedial action at both Rifle sites. The DOE evaluated the environmental impacts associated with site remedial action in an environmental impact statement (EIS) (DOE, 1990. The NRC and the state of Colorado concurred with the DOE's remedial action plan (RAP) (DOE, 1992a) to comply with the requirements of 40 CFR Part 192, Subparts A through C.

The DOE began constructing the Estes Gulch disposal cell in 1993. During 1994 and 1995, the DOE relocated uranium mill tailings and other residual radioactive materials (such as contaminated mill buildings and associated debris, windblown materials, and about 24,000 yd³ [18,400 m³] of vicinity property materials) and placed them in the Estes Gulch disposal cell. Disposal cell construction was completed in 1996 with placement of a radon/infiltration barrier and frost- and erosion-protection layers. A completion report documents compliance with the RAP and the site as-built conditions (DOE, 1997). In addition, the DOE prepared a final audit report and certification summary and submitted it along with the completion report to the NRC for concurrence. Concurrence from the NRC on the completion report is included in the permanent site file.

The Estes Gulch disposal site is in Garfield County in west-central Colorado on the western slope of the Rocky Mountains. The site is approximately 6 mi (10 km) north of the town of Rifle, Colorado (Figure 2.1) in Township 5 South, Range 93 west, Section 14. This section briefly describes the site vicinity. Detailed descriptions can be found in the site EIS (DOE, 1990) and the RAP (DOE, 1992a).

Site-specific precipitation, temperature, and wind-related data were collected from October 1993 to September 1996. Average annual precipitation ranged from about 14.3 to 18.5 inches (36 to 47 cm). Annual snowfall ranged from 22.3 to 34 inches (57 to 86 cm). The site-specific average daily temperature ranged from 15 to 86° F ( - 9.4 to 30° C).

Site-specific wind data indicate that wind direction was predominately from the west and northwest over 50 percent of the period of record. Winds were from the south 10 percent of the time. Wind intensity data are available from Morrison Knudsen-Ferguson (MK-F) (MK-F, 1996).

The Estes Gulch site ranges from about 5960 to 6200 feet (ft) (1820 to 1890 meters [m]) above mean sea level. The Grand Hogback rises to an elevation of about 7850 ft (2400 m) north of the site. To the south, 3300 ft (1000 m) the dissected pediment surfaces drop down to the alluvial valley of Government Creek at an elevation of about 5750 ft (1750 m).

The Estes Gulch site is at the head of a small drainage basin on a dissected pediment and alluvial fan surface sloping southwest toward Government Creek from the foot of the Grand Hogback monocline.

Off-site runoff water that could have affected the integrity of the cell previously came from a 20-acre (ac) (8-hectare [ha]) watershed north of the cell. Runoff from about 6 ac (2 ha) of the uppermost part of this watershed was diverted from the disposal cell by an interceptor ditch. The other 14 ac ( 5.6 ha) of the watershed between the interceptor ditch and the cell were graded and covered with erosion-resistant material. The graded area has a slight crown to shed some runoff away from the cell and onto the adjacent ground. Precipitation falling on top of the cell will drain to a toe ditch at the south end and will discharge eastward into Estes Gulch (DOE, 1992a).

There is little potential for future natural resource development in the immediate site vicinity.

This section briefly describes the disposal site; detailed descriptions can be found in the site RAP (DOE, 1992a) and completion report (DOE, 1997).
 

The 71-ac (29-ha) disposal cell is located on a gently sloping pediment betweenthe Grand Hogback and Government Creek. The area of the disposal cell is not subject to any significant hazard from slope failure processes such as landslides, debris flows, mud flows, and rock falls. The geomorphic processes posing a potential hazard to the stabilized disposal cell are ephemeral drainage channel changes, low-gradient slope erosion, and wind erosion; however, these processes are not reasonably expected to affect the disposal cell within the next 1000 years, or in any case for at least 200 years.

The disposal cell is constructed partially below grade. The disposal cell is located on a hillside and generally follows the same slope. The highest elevation of the cell and that of the adjacent ground surface is about 6240 ft (1900 m) above National Geodetic Vertical Datum (NGVD). From this location the cell slopes at 11.9 percent to an elevation of 6160 ft (1880 m) NGVD, where the grade changes to 5.5 percent. At this point, the surface of the cell is about 3 ft (0.9 m) above the adjacent ground surface. After the slope reaches an elevation of 6107 ft (1860 m) NGVD, the slope changes to a grade of 6.5 percent. At this point the cell surface is about 17 ft (6 m) above the adjacent ground surface. There is a major grade break at elevation 6087.2 ft (1855.4 m) NGVD, where the cell surface is about 30 ft (9 m) above the adjacent ground surface. At this point the slope continues at 20 percent to the toe of the cell.

The disposal cell contains approximately 3.7 million yds³ (2.8 million m³)of relocated tailings and other residual radioactive materials, primarily contaminated soils and demolition debris. The disposal cell is capped with a multiple-component cover (Figure 2.3).

A 1.5-ft (0.45-m)-thick radon/infiltration barrier is placed over the contaminated materials. The radon barrier is constructed of two layers: a 0.5-ft (0.15-m) compacted clay layer and a 1-ft (0.3-m) layer of bentonite-amended clay. The barrier is designed to reduce the radon-222 flux from the disposal cell to less than 20 picocuries per square meter per second and minimize water infiltration into the tailings. Over much of the radon/infiltration barrier an additional 0.5- to 0.7-ft (0.1- to 0.2-m) layer was placed to prevent drying. A 0.5-ft (0.15-m) -thick coarse-grained filter layer is placed on top of the radon/infiltration barrier to provide a capillary break and promote drainage of infiltrating water away from the radon barrier. A layer of compacted soil lies on top of the filter layer to insulate the radon/infiltration barrier and keep it from being adversely affected by freeze-thaw cycles. The typical thickness of this layer is 7.5 ft (2.3 m), and it has a maximum thickness of 18 ft (5.5 m) where the cell joins the natural slope. The topslopes and sideslopes of the disposal cell are capped with rock (riprap) to protect against wind and water erosion and prevent damage to the underlying frost protection and radon/infiltration layers.

The erosion-protection layer is 1-ft (0.3-m) thick. A 0.5-ft (0.15-m)-thick bedding layer is beneath the erosion-protection layer to prevent damage to the underlying frost protection layer from rocks and soil loss from runoff water. These grades, in conjunction with the bedding layer, will allow excess surface water to run off the disposal cell and be conveyed to adjacent site grades, thereby minimizing the risk of significant erosion. The components of both the

topslope and sideslope covers are intended to minimize the potential for deep percolation of precipitation into the residual radioactive material. A riprap apron and toe ditch at the toe of the disposal cell carry water away from the cell and provide erosion protection from gullying. An unlined interceptor ditch abuts the upslope portion of the disposal cell to divert surface flow away from the cell.

During design and analysis of the Rifle disposal cell in 1990 and 1991, the UMTRA Project team and the NRC were concerned that transient drainage and surface infiltration might collect near the toe of the cell and build up excessively. If tailings drainage water built up above the rim of the excavation, a surface expression (i.e., seep) along the south slope of the disposal cell could inadvertently allow radionuclides to escape. It was concluded in the RAP that to prevent a surface expression (i.e., seep) a temporary high density polyethylene (HDPE) liner would be constructed inside the toe of the disposal cell, and that a temporary leachate collection system would be constructed. The HDPE liner and a leachate collection system (standpipes), consisting of three 18-inch (46 cm)-diameter monitor wells and a granular under-drain layer beneath the tailings (see MK-F Rifle site construction drawings RFL-DS-10-0724, RFL-DS-10-0731, and RFL-DS-10-0732).

Monitoring and analysis of the standpipes will be required under the Long-Term Surveillance Program until it is determined that after review by the State and approval by NRC the standpipes can be decommissioned. The operation and contingency plan for monitoring well closure (see appendix) contains detailed requirements for monitoring the water level in the standpipes and analyzing the data to determine when the standpipes can be decommissioned.

Several phases of permeability testing of the Wasatch Formation bedrock at the Estes Gulch disposal site have been conducted. These include field studies by the Technical Assistance Contractor (TAC), Morrison Knudsen-Ferguson Environmental Services with support by Morrison Knudson-Ferguson, and Daniel B. Stephens & Associates. Laboratory permeability testing was conducted by the University of Arizona, Lambert and Associates, and Herzog Associates. A summary of testing results is presented below.

The TAC estimated a mean hydraulic conductivity of 7 x 10^-10 cm per second for the saturated deep Wasatch Formation using water level recovery data (Calculation RFL09-89-14-02-9). However, hydrostatic equilibrium in these wells, which varied in depth from 300 to 440 ft (91 to 134 m), was never achieved.

To improve sandstone and siltstone permeability estimates in the extreme upper Wasatch Formation at the bottom of the disposal cell excavation, a number of short-, intermediate-, and long-term infiltrometer tests were conducted in 1992 and 1993. This testing defined the saturated vertical permeability of sandstones at the low portion of the disposal cell and the saturated vertical permeability of siltstone in the cell foundation. The sandstone’s geometric mean permeability was found to be 4 x 10^-7 cm per second. The geometric mean permeability of the siltstone was determined to be 7 x 10^-8 cm per second.

This section briefly describes the hydrogeologic units and background ground water quality at the Estes Gulch disposal site and identifies the constituents of concern at the site. More detail on ground water characterization of the site is found in the Rifle RAP (DOE, 1992a). The justification for no ground water monitoring is provided in Section 2.6.

Ground water monitoring of the uppermost aquifer at the Estes Gulch disposal cell is not required. Postclosure ground water monitoring will not be conducted in the Wasatch siltstone and sandstone aquifer beneath the site due to the limited use designation of ground water in the Wasatch Formation and due to the Wasatch Formation’s considerable thickness (projected to be 3800 ft (1160 m) (40 CFR §192.11(e)(1)). Limited use ground water is ground water that is neither a current nor a potential source of drinking water because 1) the TDS concentration exceeds 10,000 mg per liter; 2) the existing widespread ambient contamination is unrelated to processing activities, and the contamination cannot be cleaned up using treatment methods reasonably employed in public water supply systems; or 3) the quantity of water available is less than 150 gal (570 L) per day.

3.2 INSPECTION TEAM

If problems are observed that require more investigation, follow-up inspections will be performed and teams will include one or more technical specialists in appropriate disciplines to assess the problems under investigation. For example, a follow-up inspection by a plant specialist may be required to evaluate reports of significant plant growth on the rock cover, or a soils scientist or geomorphologist may be needed to evaluate erosion processes.

The inspection team will consist of a chief inspector and one or more assistants. The chief inspector will be a geotechnical engineer, a civil engineer, or an engineering geologist knowledgeable in the process that could adversely affect the site (e.g., geomorphic agents of change).

3.3 ANNUAL INSPECTION

Site inspections will cover the disposal cell, the surrounding disposal site area, and the immediate off-site areas. Site inspections must be thorough enough to identify significant changes or active modifying processes that potentially could adversely affect the disposal cell: gully formation, slope erosion, changes to the rock cover, ephemeral drainage channel changes, and significant modifications by humans, animals, or plants.

Inspectors will measure and evaluate the leachate level in the monitor well leachate collection system (stand pipes) located on the 5 to 1 slope of the disposal cell.
 

Figure 3.1 Steps for follow-up inspections, custodial maintenance, and corrective action, at the Estes Gulch disposal site near Rifle, Colorado
 
 

Monitoring, corrective action, and closure of these wells will be performed in compliance with the plan as provided in the operation and contingency plan (see appendix).

The disposal cell has a rock cover and there is no planned vegetation on the disposal cell. However, remedial action of the areas surrounding the disposal cell included revegetation with grasses and sage brush. The area surrounding the disposal cell will be monitored to determine the success of the revegetation efforts. Inspectors also will inspect this area for evidence of erosion caused by wind, sheet wash, or changes in drainage patterns.

Site inspectors also will monitor damage to or disturbance of permanent site-surveillance features, ground water monitor wells (until they are decommissioned), fencing, locks, and the gate.

From inside the disposal site, inspectors will visually survey the area approximately 0.25 mi (0.40 km) outside the disposal site boundary for evidence of land-use changes that indicate increased human activity, such as land development or new roads and paths. Inspectors will note the condition of and changes to site access roads, surrounding vegetation, and relevant geomorphic features like gullies or ephemeral drainage channels; potential impacts to the site will be noted. Off-site DOE monitor wells will be inspected until they are properly decommissioned.

3.4 FOLLOW-UP INSPECTIONS

Notification agreements with the Garfield County Sheriff’s Office and the U.S. Geological Survey’s National Earthquake Information Center are included in Attachment 2. The DOE will also monitor the weather for the occurrence of severe storms in the disposal cell vicinity. In addition, the DOE 24-hour telephone number is posted on the site entrance sign so the public can notify the DOE if problems are discovered. If an extreme natural event or vandalism has occurred, the DOE will inspect the cell to assess the damage. The notification, response, and follow-up activities will be documented. This documentation will be included in the annual site report to the NRC and become part of the permanent site file.

The nature of the occurrence and the amount of firsthand knowledge available will determine the DOE’s response. If a situation is a threat to the public, the DOE will notify individuals who may be affected and appropriate federal, state, and local agencies, including the NRC. If necessary, the DOE will schedule a follow-up inspection to assess potential effects from the unusual occurrence, and will take necessary response action. Follow-up inspections will be conducted to determine whether processes currently active at or near the site threaten site security or stability and to evaluate the need for custodial maintenance, repair, or other corrective action. The scope of these follow-up inspections may be broad and similar in nature to routine site inspections or focused on specific areas of concern.

During the follow-up inspection, inspectors and technical specialists will investigate reported problems to determine whether the disposal cell has been damaged or threatened. The DOE will conduct additional site visits, if necessary to acquire data or plan maintenance and repairs.

3.5 QUALITY ASSURANCE

The DOE will maintain the surveillance and maintenance documentation identified in other sections of this LTSP; it will become part of the permanent site file. The DOE will update the site file as necessary after disposal site inspections, maintenance activities, or corrective actions are complete. These records will be handled in accordance with DOE directives to ensure their proper handling, maintenance, and disposition. The archival procedures set forth in 41 CFR Part 101 and 36 CFR Parts 1220-1238, Subchapter B, will be followed. The permanent site file information will be available for NRC and public review.
 

Site inspection reports will be submitted to the NRC within 90 days of the annual site inspection. Inspection reports will summarize the results of follow-up inspections and maintenance completed since the previous annual site inspection.

If unusual damage or disruption is discovered at the disposal site during an inspection, a preliminary report assessing the impact must be submitted to the NRC within 60 days. If maintenance, repair, or corrective action is warranted, the DOE will notify the NRC. The NRC will receive a copy of corrective action plans and corrective action progress reports, or the reports will be attached to the annual report.

The DOE also will provide copies of inspection reports and other generated under the long-term surveillance program to the state of Colorado as required in their cooperative agreement.

36 CFR Parts 1220-1238, National Archives and Records, Subchapter B - Records Management, National Archives and Records Administration.

40 CFR Part 192, Health and Environmental Protection Standards for Uranium and Thorium Mill Tailings, U.S. Environmental Protection Agency.

41 CFR Part 101, Federal Property Management Regulations, General Services Administration.
 
 

Order 5700.6C, Quality Assurance, 21 August 1991, U.S. Department of Energy, Washington, D.C.
 
 

42 USC §7901 et seq., Uranium Mill Tailings Radiation Control Act, 8 November 1978.