Experimental Characterization of Chromium Fate and Transport in the Vadose Zone and Regional Aquifer at Sandia Canyon, Los Alamos, New Mexico

Routine groundwater monitoring conducted in 2005 identified significant chromium(Cr) contamination within the regional aquifer at Los Alamos, New Mexico. Site-specific Cr(III)/Cr(VI) transport characterization is needed (1) to refine conceptual and numerical models; (2) to evaluate long-term stability of Cr(III)/Cr(VI) in the Sandia Canyon groundwater system along the flow path from the source to the regional aquifer; and (3) to forecast natural attenuation and assess remediation possibilities. We have conducted both static (batch) and flowing (column) experiments using water chemistries and geological materials representative of potential flow pathways in Sandia Canyon. We also conducted synchrotron-based X-ray absorption spectroscopy measurements for determining Cr(III)/Cr(VI) speciation and oxidation state of Cr surface complexes in vadose zone and regional aquifer materials. In addition, characterization of materials used in experiments was performed using SEM, XRD, and BET Surface Area Measurement. Results of adsorption and flow-through experiments suggest that vadose zone and regional aquifer materials have different retention capacities for Cr(VI), reflecting large variations in measured adsorption coefficients (Kd values). Additionally, higher Kd values tend to be associated with crushed rocks and reduced particle sizes, where increased surface area and exposure of fresh mineral provides surfaces containing ferrous iron that acts as a chemical reductant. Nevertheless, X-ray Absorption Near Edge Spectroscopy (XANES) measurements suggest that reduction of Cr(VI) to Cr(III) occurred after Cr(VI) containing solution was exposed to alluvial and rock materials, both uncrushed and crushed. The results indicate that natural attenuation of anthropogenic chromium occurs within the Sandia Canyon vadose zone and in the regional aquifer. Findings from this study provide data necessary to reveal the nature and extent of site-specific long-term fate and transport of chromium in Sandia Canyon, as part of the all-encompassing legacy cleanup effort conducted at Los Alamos National Laboratory.