Using Tracers and Opportunistic Geochemical Signatures to Inform Modeling of Cr(VI) Migration at LANL - 21081

Chromium contamination from legacy operations at Los Alamos National Laboratory was discovered in the regional aquifer underlying the Laboratory at a depth of nearly 300 m below ground in 2005. Since 2014, several different types tracers tests and opportunistic observations of geochemical signatures have been used to provide insights into the transient behavior of the Cr(VI) contamination in the aquifer and to help develop a remediation strategy, including interim measure pump-and-treat operations. Borehole dilution tracer tests have been conducted in ten different wells across the Cr(VI) plume to obtain estimates of local groundwater flow velocities and Cr(VI) transport fluxes. Two push-drift-pull tracer tests involving multiple nonreactive tracers with different diffusion coefficients were conducted to interrogate dual-porosity behavior in the aquifer and also to provide confirmatory groundwater flow velocity estimates in the most contaminated portions of the aquifer. A cross-hole tracer test involving three different injection locations and a single extraction well revealed complex flow behavior in the aquifer, including local sub-horizontal aquifer stratification and the lack of a flow connection between the closest injection well and the extraction well. The cross-hole tracer test also provided information on natural flow velocities and dual-porosity behavior in the aquifer. Tracers have also been deployed into interim measure injection wells and observed at monitoring or extraction wells to help understand details of plume-scale groundwater flow dynamics under the influence of interim measure extraction and injection. In addition to the tracer results, monitoring-well observations of various geochemical signatures in injection water are providing insights into plume-scale groundwater flow dynamics, including estimates of flowing porosity and how volumetric flow is distributed within the aquifer porosity over large interrogation volumes. These observations are collectively being used to refine numerical models and to support development of remediation strategies. (authors)