The U.S. Department of Energy (DOE) National Nuclear Security Administration (NNSA) published this floodplain assessment (FA) for the Pajarito Tank 4A 18-inch Transmission Water Main Repair, Two Mile Canyon, Los Alamos National Laboratory. Public comment period ends September 2, 2026.
Office of NEPA Policy and Compliance
August 18, 2026
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Floodplain Assessment for the Pajarito Tank 4A 18-inch Transmission Water Main Repair, Two Mile Canyon, Los Alamos National Laboratory; Los Alamos County, New Mexico
The Incorporated County of Los Alamos, Department of Public Utilities (LAC DPU), proposes to repair a failed 18-inch transmission water main serving Pajarito Tank 4A on U.S. Department of Energy (DOE)/National Nuclear Security Administration (NNSA) property at Los Alamos National Laboratory (LANL). The failed segment lies at the bottom of Two Mile Canyon, west of Mercury Road, within the canyon-bottom drainage corridor identified by LANL as a floodplain. The proposed action includes temporary access-road rehabilitation, staging and work areas, replacement of approximately 100 linear feet of existing 18-inch concrete cylinder pipe with 18-inch ductile iron pipe, associated fittings and a 6-inch drain assembly, new concrete encasement, grouting to backfill, testing, monitoring, and final restoration. No water will be released into the canyon from the transmission line or repairs.
The proposed project would result in negligible direct and indirect impacts to the Two Mile Canyon floodplain. Temporary disturbances with the floodplain would cease following completion of construction activities. Best management practices would be implemented. This proposed project would not modify flow paths within the floodplain from pre-project to post-project conditions. No effects on lives and property associated with floodplain modifications are anticipated.
The 15-day public comment period ends on September 2, 2026. Please provide comments on this floodplain assessment to Karen Armijo:
Email: Karen.Armijo@nnsa.doe.gov
Mail:
U.S. Department of Energy
Los Alamos Field Office
Attn: Karen Armijo
3747 West Jemez Road
Los Alamos, NM 87544
After the close of the public comment period and prior to issuing a floodplain statement of findings, DOE/NNSA will reevaluate the practicability of alternatives to the proposed floodplain action, mitigating measures, and take into account all substantive comments received during the public comment period. DOE/NNSA will endeavor to allow 15 days of public review prior to implementation of the proposed action.