State officials outline monitoring and multi-year cleanup steps for Los Alamos chromium plume

Michael Peterson, a hydrogeologist with the New Mexico Environment Department's Hazardous Waste Bureau, told the Los Alamos County Board of Public Utilities on April 16 that state and federal agencies continue to characterize and treat a chromium plume linked to historic releases from the T A-3 power plant at the laboratory.

“To date, there was about an estimated 60,000 pounds of potassium dichromate that went down the canyon. To date, the interim measures have removed about 2,000 pounds of that,” Peterson said, describing the gap between total released and current removals.

The presentation summarized recent technical work and the short- and medium-term plan. NMED and Department of Energy (DOE) contractors operate an “interim measures” system — a network of extraction wells that pump contaminated groundwater to a treatment plant, then reinject treated water through injection wells placed downgradient. The state said the system was partially restarted on Sept. 30, 2024, after an independent technical review (IRT) recommended turning some extraction wells back on to limit plume migration while additional data are collected.

NMED identified key near-term priorities: drilling additional monitoring wells to fill data gaps, redrilling an important but currently unusable monitoring well (R-73), installing a deeper well in the plume centroid (R-77) to better characterize deeper formations, and advancing the monitoring well Simmer 3 on Pueblo de San Ildefonso lands to determine whether contamination has reached Pueblo groundwater.

State staff explained why more wells are necessary. The green dashed line on the maps shown to the board marks areas where concentrations exceed 50 parts per billion (ppb) in the upper 50 feet of the aquifer; a blue dashed line indicates zones exceeding 50 ppb at deeper intervals. Peterson said that the plume is now appearing deeper than originally expected and that understanding vertical as well as lateral extent is essential before a final remedy can be designed.

Caitlin Martinez of NMED's Hazardous Waste Bureau added that the interim measures are “a temporary solution” designed to slow or contain the plume while the technical record is expanded so a final remedy can be designed and implemented more quickly. “We anticipate we need at least these data gap wells,” Martinez said, and from the state’s current view “we have at least, you know, five years of work before we can consider the concept of a final remedy.”

Peterson discussed options for where to inject treated water. Options examined by the IRT include constructing a deep DOE-owned injection well that could secure returned-flow credit under local water-right arrangements, repurposing an existing well (PM-3) as a high-capacity injection location, surface-water disposal, land-application spreading basins, vadose-zone injection, and spreading basins. State staff said some surface options would likely not qualify for returned-flow credit under current water-right rules.

The NMED presentation also covered a major procedural change: on Sept. 24 the agency and DOE updated the Compliance Order on Consent that governs corrective-action work at Los Alamos National Laboratory. The revised order adds a faster dispute-resolution pathway for technical disagreements: parties must first attempt resolution at technical and managerial levels, and if unsuccessful, jointly select an independent expert or panel whose decision will be binding. Peterson said that the IRT process used earlier is the model for this new mechanism.

On modeling, the IRT recommended moving from FEHM (a DOE-developed finite-element heat-and-mass-transfer code) to MODFLOW, the more commonly used groundwater modeling code. Peterson said MODFLOW will allow independent state reviewers, public stakeholders and outside experts to run and test the model more readily, and that expanded parameterization is needed — for example, to reflect hydraulic conductivity and anisotropy across heterogenous units such as cobble/conglomerate layers near the Puye Formation.

Board members asked how progress will be judged. Martinez said the interim measures are expected to hold the plume in place while information is collected; the state’s minimum need for a final remedy design includes defined source terms and a reliable estimate of the contaminated water volume. The state expects multi-year characterization work before a final remedy decision and cautioned that additional monitoring wells may be required if the five planned wells do not close remaining data gaps.

Peterson and Martinez emphasized the proximity of the plume to Pueblo de San Ildefonso land and said state and federal agencies will follow the Pueblo’s preference for water quality: the Pueblo has said it wants water “as close to background as possible” (about 7.5 ppb), rather than the state's 50 ppb standard. Martinez said if contamination is detected on sovereign Pueblo lands above background, the Pueblo will likely become a required participant in remediation planning.

The presentation concluded with technical annexes and a list of the highest-priority monitoring wells: Simmer 3 (on Pueblo lands), R-79 and R-80 (to investigate an unexpected concentration spike near R-45), the R-73 redrill, and the R-77 deeper centroid well. NMED also emphasized that model updates, vadose-zone characterization and better parameterization of aquifer properties are prerequisites to a final remedy.

Board members thanked NMED staff and asked no action on the matter was required at the meeting; the presentation served as an update to a continuing technical and regulatory effort.