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Floodplain Statement of Findings for the Proposed Installation of a Multiple Permeable Reactive Barrier Within Mortandad Canyon at Los Alamos National Laboratory, Los Alamos, NM

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Information about this document as published in the Federal Register.

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National Nuclear Security Administration, Los Alamos Site Office, Department of Energy.

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Floodplain statement of findings.


This floodplain statement of findings is for the construction of a multiple permeable reactive barrier (PRB) at Los Alamos National Laboratory (LANL). The PRB will be placed across a floodplain area within Mortandad Canyon, located within the central eastern portion of LANL. In accordance with 10 CFR part 1022, the Department of Energy (DOE), National Nuclear Security Administration (NNSA) Los Alamos Site Office has prepared a floodplain/wetland assessment and will perform this proposed action in a manner so as to minimize potential harm to or within the affected floodplain.

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Elizabeth Withers, U.S. Department of Energy, National Nuclear Security Administration, Los Alamos Site Office, 528 35th Street, Los Alamos, NM 87544. Telephone (505) 667-8690, facsimile (505) 667-9998; or electronic address: For further information on general DOE floodplain environmental review requirements, contact: Carol M. Borgstrom, Director, Office of NEPA Policy and Compliance, EH-42, Department of Energy, 1000 Independence Avenue, SW., Washington DC 20585-0119. Telephone (202) 586-4600 or (800) 472-2756, facsimile (202) 586-7031.

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In accordance with DOE regulations for compliance with floodplain and wetlands environmental review requirements (10 CFR part 1022), NNSA prepared a floodplain/wetland assessment for this action. The NNSA published a notice of floodplain involvement (Volume 67, Number 236). This notice announced that the floodplain/wetland assessment document was available for a 15-day review period and that copies of the document could be obtained by contacting Ms. Withers at the above address, and that copies of the document were available for review at two public DOE reading rooms in Los Alamos and Albuquerque, New Mexico. No comments were received from the Federal Register notice on the proposed floodplain action.

Project Description: In November 2002, NNSA considered a proposal for constructing a PRB system at a narrow constriction in Mortandad Canyon within LANL where contaminated groundwater is confined to a small cross-section of alluvial materials (see figure 1). The entire PRB structure would extend about 120 feet from side-wall to side-wall within the canyon bottom. The PRB would consist of a “funnel and gate” system to direct contaminated groundwater into a centrally-located gate area of reactive materials. The impermeable funnel would be constructed of sheet piling driven to a depth of approximately 27 feet on either side of the canyon. The permeable gate would contain multiple buried cells of selected media designed to react with and reduce the concentration of contaminants in groundwater passing through the gate. The PRB would be left in place for about 5 years and its function would be monitored through a system of shallow monitoring wells that would be installed at the same time the PRB was constructed. Construction of the PRB and associated monitoring wells will commence in 2003 and be completed in less than 6 months.

Alternatives: Alternative locations for the PRB were considered but eliminated from future consideration. A combination of site factors was considered that lead to the identification of the proposed site as being the least disruptive to existing environmental resources in the area.

Floodplain Impacts: The proposed action would have the potential for minimal impacts to the floodplain. Should a rain event occur during this activity, there may be some sediment movement down canyon because of the loosened condition of the soil from the clearing and construction activities.

Floodplain Mitigation: Impacts to the floodplain would be minimized by following Best Management Practices at the construction area (such as the placement of silt fences, straw bales or wattles, or wooden or rock structures to slow down water runoff and run-on at cleared sites). Post-construction reseeding and re-vegetation along the sides of the stream channel will minimize soil disturbance and reduce or prevent the potential for soil erosion. No debris will be left at the work site. No vehicle maintenance or fueling would occur within 100 feet of the stream channel. Any sediment movement from the site would be short term and temporary.

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Issued in Los Alamos on December 20, 2002.

Ralph E. Erickson,

Manager, U.S. Department of Energy, National Nuclear Security Administration, Los Alamos Site Office.

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[FR Doc. 02-32996 Filed 12-30-02; 8:45 am]