DISTRIBUTIONS OF TRACE AND RARE-EARTH ELEMENTS IN THE ACID-EXTRACTABLE PHASE OF SEDIMENT IN THE UNSATURATED ZONE, IDAHO NATIONAL ENGINEERING AND ENVIRONMENTAL LABORATORY, IDAHO

The Idaho National Engineering and Environmental Laboratory (INEEL) is located on the Snake River Plain in southeastern Idaho. The subsurface geology consists of alternating layers of basalt with unconsolidated sedimentary interbeds, composed of poorly sorted clay, silt, sand, and gravel with a variable carbonate fraction. Areas within the 2,300 km² INEEL were used to dispose of low-level radioactive and other chemical wastes from 1952 to 1988. Waste constituents were presumed to have been sorbed by the fine-grained minerals present, including clay minerals, carbonates and Mn- and Fe- oxides common in the unsaturated zone.

Although the presence of Fe-rich oxides has been identified in the sediment of INEEL, their role in sequestering trace metals and rare-earth elements (REE) as potential analogues for radionuclides has not been clearly identified. The objectives of this study were to evaluate the trace metals and REE distributions in the acid-extractable phases (particularly the Mn- and Fe- oxides), and to examine spatial patterns of these elements in the subsurface at INEEL.

Forty-eight samples were collected from 12 archived cores. Two independent operationally-defined extractions were done on each sample; extraction A (0.1M hydroxylamine-HCl + 0.01M HNO₃) and extraction B (0.25M hydroxylamine-HCl + 0.25M HCl). Both methods will extract the exchangeable and carbonate-bound phases; A will also extract Mn- oxides (and some Fe), while B will also extract both Mn- and Fe- oxides.

The B extraction generally removed more than 10x greater concentrations of most elements as compared to the A extraction. The mean % extraction of Ca ranged from 2.4% (A) to 8% (B), and was as high as 26%. The mean % extraction with A and B respectively were: Mn 0.024% and 0.042%; Fe 0.026% and 0.48%; Al 0.063% and 6.3%; Si 0.36% and 4.6%. Iron oxide removal by extraction B correlates well (r² = 0.7 -0.9) with Cu, Cd, Pb and Zn release in samples that have less than 1% Ca. Correlations between Fe and Mn removal by the B extraction and U, Eu, Nd, and Sm release (r² = 0.5 – 0.8) in these lower Ca samples were also high.