Hexavalent chromium is present in perched-intermediate groundwater (approximately 500 feet below ground surface) and in the regional aquifer (approximately 1000 feet below ground surface) beneath Sandia and Mortandad canyons. Investigations identified the chromium source as cooling tower effluent released near the head of Sandia Canyon between 1956 and 1972. Chromium was transported down Sandia Canyon in surface water before eventually infiltrating along a complex pathway into the regional aquifer. A portion of the chromium that was released was attenuated as trivalent chromium within an existing wetland through rapid chemical reduction caused by persistent reducing conditions associated with the abundance of decaying organic matter.
An interim measure (IM) was proposed and approved by the New Mexico Environment Department and is underway to control the migration of the chromium plume. The IM utilizes extraction wells, an aboveground ion-exchange treatment system, and strategically placed wells for injection of treated water to control migration and effectively reduce the footprint of the plume.
The end state for the IM is twofold; (1) effectively establish a 50 parts per billion plume edge within the Laboratory boundary and away from nearby water-supply wells through the operation pumping, treatment, and injection and (2) reduce the footprint of chromium contamination for the final remedy.
The IM infrastructure is nearing completion and will consist of five extraction wells, five injection wells, and associated buried piping connecting the wells to the treatment facility. Operation of the IM began in 2018 using one extraction well and two injection wells located along the Laboratory’s boundary with the Pueblo de San Ildefonso. Recent samples at regional aquifer well R-50, near the Laboratory boundary with San Ildefonso, are showing consistently decreasing trends in chromium concentrations. These data helps to indicate that hydraulic plume control under the Interim Measure is taking hold.
Data from the most recently installed injection well, CrIN-6, identified elevated concentrations of chromium consistent with some of the calibrated groundwater models used to support fate and transport analysis of the plume. Based on the results of the data from CrIN-6 and the groundwater model, the plume footprint has been updated to reflect a refined estimate of the extent of chromium in that portion of the plume. As a result, this well will be converted to an extraction well (CrEX-5) in 2019 to better address plume migration. An additional monitoring well (R-70) is being installed in the eastern portion of the plume, in between CrEX-5 and the nearest county drinking water well.
Full implementation of the IM is anticipated in 2019. Concurrent with the IM activities, characterization studies are underway to evaluate potential remedial alternatives that would be part of implementation of a final remedy for chromium in groundwater. Pilot-scale studies using a bio-amendment (molasses) and a chemical amendment (sodium dithionite) have been deployed and monitoring and evaluation is underway. Initial indications are that both approaches are showing potential for effective remediation, although the results are only preliminary. The Department of Energy’s Environmental Management Los Alamos Field Office and its cleanup contractor N3B are also evaluating the feasibility of utilizing directional drilling technology to obtain access to a much larger portion of the regional aquifer for amendment deployment than can be achieved with vertical wells. Completion of the characterization studies will lead to a recommendation for a final remediation strategy in a Corrective Measures Evaluation report.
You can also learn more about the Chromium Interim Measure and Characterization Campaign by downloading related presentations EM-LA has provided to the public on our Presentations page.