MINERALOGICAL STUDY OF SEDIMENTS IN CHROMIUM-CONTAMINATED PLUME: IMPLICATION FOR NATURAL ATTENUATION

Options to remediate a legacy waste plume of hexavalent chromium (Cr⁶⁺) in the Los Alamos National Laboratory (LANL) regional aquifer include actively engineered remediation and passive natural attenuation. Natural attenuation studies seek to understand the mechanism, capacity and rate of reduction for sediments to convert the toxic and carcinogenic Cr⁶⁺ to the immobile, less harmful Cr³⁺. Here we report a site-specific mineralogy of the two geological units in the regional aquifer. Targeted sonic drilling in the plume area was initiated in 2014 to document lateral and vertical variabilities of contaminant concentration in the aquifer. These well-preserved samples were examined with XRD to determine specific mineralogical composition. Particular attention is given to clays, iron-bearing minerals and amorphous (volcanic glass) materials because of their potential to reduce Cr⁶⁺.

LANL is located on the Pajarito Plateau at the western edge of the Española basin. The older unit comprising the aquifer is the Miocene Pumiceous. This series of reworked, buff pumice beds have sub-units which correlate to the Peralta Rhyolite (6.9 Ma). The younger unit is the Plio-Pliestocene Puye formation, a heterogeneous volcanoclastic fanglomerate sourced predominantly from dacitic flows on the western side of the Pajarito Plateau. Preliminary mineralogical assessment of the Puye formation based on outcrop and borehole samples show the following: combined SiO₂ phases 1.3-25.2 wt. %, feldspars 31.5-54.3 wt. %, magnetite 1.03-1.98 wt. %, amorphous 9.1-43.9 wt. % and smectite 0-33.7 wt. %. Ongoing mineralogy from sonic cores is consistent with these findings, with the addition of muscovite. Continued study of sediment constituents with potential to attenuate will help inform the optimal procedure for remediation of the Cr⁶⁺ plume.