Geochemical Controls on Contaminant Uranium in Vadose Hanford Formation Sediments at the 200 Area and 300 Area, Hanford Site, Washington

Abstract

Long‐term historic spills of uranium at the 300 Area fuel fabrication site (58,000 kg of disposed uranium over 32 yr) and at the 200 East Area BX tank farm (7000 kg of spilled uranium in one event), both within the Hanford formation in the Hanford Site, Washington State, were investigated by subsurface sampling and subsequent microscale investigations of excavated samples. The 200 Area sediments contained uranyl silicate mineralization (sodium boltwoodite) in restrictive microfractures in granitic clasts, in the vadose zone over a narrow range in depth. Well logging and column experiments indicated that tank wastes migrated deeper than observed in core samples. The 300 Area sediments included metatorbernite and uranium at low concentrations associated with detrital aluminosilicates, along with other mineral phases that could accommodate uranyl, such as uranophane and calcium carbonate. The association of contaminant uranyl with Hanford formation sediments provided a persistent source of uranium to groundwater. The results of both studies suggest that the formation of secondary solid uranyl‐bearing phases influences the subsequent release of uranium to the environment and that our understanding of these processes and individual waste sites is incomplete.

DNA Profile Analysis and Interpretation

“…At shallower depths, EXAFS analyses suggest that the major uranium-controlling phase was meta-torbernite, Cu(UO 2 ) 2 (PO 4 ) 2 • xH 2 O (Catalano et al 2006b;Arai et al 2007) with uranyl-carbonate precipitates, predominantly consistent with uranium-rich calcite (Catalano, personal communication). Until recently, it had been speculated that the uranyl-silicate phase present in the 300 Area sediments was either Na-boltwoodite, Na(UO 2 )(SiO 3 OH) • xH 2 O, or uranophane, Ca(UO 2 ) 2 [SiO 3 (OH)] 2 • xH 2 O-both minerals have the uranophane group structure (Liu et al 2004;Liu et al 2006;Zachara et al 2005;McKinley et al 2006;McKinley et al 2007). The minute inclusions, which prevent bulk analyses, and similarities between the uranophane group structures had precluded conclusive identification of the uranyl-silicate phase.…”

Evaluation of Soil Flushing for Application to the Deep Vadose Zone in the Hanford Central Plateau

“…The composition from the largest tank leaks (SX-108, BX-102, and T-106) varied from alkaline to caustic with high ionic strengths and temperatures, and they contained a variety of metals and radionuclides. The discharged waste was highly reactive with the sediment as infiltration occurred, which led to a geochemically heterogeneous uranium retention and phase distribution (Zachara et al 2007a(Zachara et al , 2007bMcKinley et al 2007;Um et al 2009). Sediment samples collected near BX-102 tanks have uranium-silicate precipitates including boltwoodite and uranophane (Um et al 2009).…”

“…The discharged waste was highly reactive with the sediment as infiltration occurred, which led to a geochemically heterogeneous uranium retention and phase distribution (Zachara et al 2007a(Zachara et al , 2007bMcKinley et al 2007;Um et al 2009). Sediment samples collected near BX-102 tanks have uranium-silicate precipitates including boltwoodite and uranophane (Um et al 2009). McKinley et al (2007 showed that the boltwoodite occurred through secondary mineral formation within microfractures, which was likely attributed to the alkaline and silica-poor composition of the tank leaks.…”

Scientific Opportunity to Reduce Risk in Groundwater and Soil Remediation

“…Zachara et al (2007) and McKinley et al (2007) summarized research that has been performed on both saturated zone and unsaturated zone sediments. When the North and South Process Pond excavations were open, four test pits were dug to collect sediment samples to the water table.…”