Erosion Challenges at the L-Bar Disposal Site - 19561
- US Department of Energy Office of Legacy Management, Grand Junction, Colorado 81503 (United States)
- Navarro Research and Engineering, Inc., Grand Junction, Colorado 81503 (United States)
The L-Bar, New Mexico, Disposal Site is located in the northeast corner of Cibola County, 10 miles north of Laguna, and was the former site of a uranium mill and underground mine. The mill operated from 1977 to 1981 where a total of 1.9 x 10{sup 6} tonnes (2.1 million tons) of ore was processed. Solid and liquid byproduct, or tailings, from the milling process were impounded behind a dam constructed of native weathered Mancos Shale, which is a predominant geologic formation in the area. The tailings were encapsulated in place, and the resulting 40.5-hectare (100-acre) engineered disposal cell was completed in 2000. The 299-hectare (738-acre) Uranium Mill Tailings Radiation Control Act Title II site was transferred to the US Department of Energy (DOE) Office of Legacy Management under a US Nuclear Regulatory Commission general license in 2004. During reclamation activities, channels were designed and constructed into the surrounding weathered shale to protect the disposal cell from high-precipitation events. Due to the highly erodible soils and anticipated sediment deposition into the diversion channels, a sediment trap was also excavated. By excavating the channels and sediment trap, the local base elevation was altered, which exacerbated erosional forces. Recontouring surrounding drainage basins during the reclamation activities also promoted erosion by reducing natural rock armoring and vegetation that aids in soil stability. To compound the erosion issue, the L-Bar site is susceptible to short-duration, high-intensity, localized convective storms. Because the local geomorphology includes soils consisting of weathered shale, these precipitation events are not conducive to absorption into the soils, but rather to sheet flows that follow topography and concentrate in lower elevation areas where flow volumes and velocities increase. These factors result in the formation of deeply incised gullies and promote soil piping, which is an important geomorphic process in the widening of arroyos and the advancement of head-cutting, evident at the L-Bar site and surrounding areas. At the first annual inspection by DOE in 2005, it was noted that extensive erosion was occurring upgradient of the sediment trap. DOE determined that the design of the site's drainage system was the cause of extensive erosion. It is believed that by lowering the base elevation of the local drainage system through excavation of the sediment trap, the geomorphic response common to fluvial systems in a semiarid environment has increased the incision and remobilization of sediment downstream. Over time, these sediments would be deposited in the channels, which are designed to divert runoff away from the disposal cell, effectively limiting their capacities. The development of alluvial fans on one side of the channel directs flows to the opposite side of the channel, presenting the risk of cutting into the side slopes of the disposal cell. In addition, the upstream migrating gullies could intersect access roads and create hazards for adjacent stakeholders, including their grazing livestock. Current estimates conclude that potential offsite impacts could occur within the next 20 years, with gully migrations of up to 19 meters (63 feet) per year. In 2009, several erosion-control structures were constructed to reduce runoff velocities and ease the elevation differential created by the construction of the sediment trap and diversion channels. The first measurable storm events after construction occurred in August and September 2011, where one structure washed out and another structure was bypassed when runoff breached an earthen berm. DOE has conducted site assessments, continues to monitor erosion, and has applied lessons learned in the process of evaluating options to repair or modify the structures to stabilize the site and prevent potential short-term damage to adjacent properties as well as long-term damage to the disposal cell. (authors)
- Research Organization:
- WM Symposia, Inc., PO Box 27646, 85285-7646 Tempe, AZ (United States)
- OSTI ID:
- 23005419
- Report Number(s):
- INIS-US-21-WM-19561; TRN: US21V1346045753
- Resource Relation:
- Conference: WM2019: 45. Annual Waste Management Conference, Phoenix, AZ (United States), 3-7 Mar 2019; Other Information: Country of input: France; 7 refs.; available online at: https://www.xcdsystem.com/wmsym/2019/index.html
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
DRAINAGE
EARTH BERMS
EROSION
EROSION CONTROL
FEED MATERIALS PLANTS
GEOLOGIC FORMATIONS
GEOMORPHOLOGY
LAND RECLAMATION
MILL TAILINGS
MILLING
NEW MEXICO
PRECIPITATION
RUNOFF
SEDIMENTS
SHALES
SOILS
TRAPS
UNDERGROUND
US NRC