Characterizing Natural Attenuation of Cr(VI) in a Chromium Plume at Los Alamos National Laboratory - 18594
- Los Alamos National Laboratory (United States)
Characterization of natural attenuation of hexavalent (Cr(VI)) on aquifer sediments within the Cr plume at Los Alamos National Laboratory has been conducted to (1) refine conceptual and numerical models of chromium fate and transport, (2) evaluate the long-term stability of Cr(III)/Cr(VI) in the complex groundwater system along flow paths from the source to the regional aquifer, and (3) evaluate and develop options for successful remediation. The naturally high chromium abundance in aquifer sediments at Los Alamos pose an unique challenge for characterization of chromium reduction, making conventional approaches such as uptake tests and advanced spectroscopic measurements ineffective in identifying relatively small amounts of anthropogenic Cr(VI) in the sediments. To address this challenge and as complimentary to these conventional methods, sequential leaching tests were conducted on core hole sediments obtained from sonic drilled cores taken from both contaminated and non-contaminated horizons among two mineralogically distinct formations (Puye formation and Miocene Pumiceous Unit) within the Cr plume. A series of three and coherent sequential leaching steps were conducted: 1) an alkaline leach to determine the amount of Cr(VI) sorbed on the sediments. 2) a weak acid leach (using 0.01 M and 0.1 M HNO{sub 3}) to determine the amount of anthropogenic chromium (Cr(III) and Cr(VI)) on aquifer sediment surfaces, and 3) a strong acid leach (using 2 M HNO{sub 3}) to characterize the amount of releasable Cr(III) from aquifer minerals. Additionally we tested the efficiency and detection limit of sequential leaching on bentonite, which consists mainly of smectite, used in drill muds and laboratory-grade biotite spiked with known concentrations of Cr(VI). Smectite and biotite are two phases considered to be potentially important host minerals in aquifer sediments within the plume. Thus, by comparing contaminated and uncontaminated sediment leachate results of chromium and other major and trace elements, the signature and quantity of anthropogenic Cr(VI) can be derived. Our results from the weak acid leaching test indicate that the method detection limit for Cr 'uptake' was about 1 μg/g for biotite and about 0.5 μg/g for bentonite. Our results also indicate that all samples taken from known contaminated horizons in comparison to non-contaminated ones within the Puye formation showed no statistical difference in the amount of Cr leached indicative of its insignificant Cr(VI) attenuation capacity. The amount of excess Cr leached from the magnetic fraction of the contaminated Miocene Pumiceous samples was about 2 μg/g of sediment. Considering that the magnetic fraction was about 11% by mass of the bulk sample, the excess amount of Cr leached from the bulk material would be about 0.2 μg/g of sediment, suggesting the potential for some Cr(VI) attenuation capacity in the Miocene pumice unit. (authors)
- Research Organization:
- WM Symposia, Inc., PO Box 27646, 85285-7646 Tempe, AZ (United States)
- OSTI ID:
- 22977848
- Report Number(s):
- INIS-US-20-WM-18594; TRN: US21V0476017893
- Resource Relation:
- Conference: WM2018: 44. Annual Waste Management Conference, Phoenix, AZ (United States), 18-22 Mar 2018; Other Information: Country of input: France; 9 refs.; Available online at: https://www.xcdsystem.com/wmsym/2018/index.html
- Country of Publication:
- United States
- Language:
- English
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