T309 In-Situ Technologies for Energy Resource Extraction: The Role of Water

Wednesday, 28 September 2016
1:30 PM - 5:30 PM
Room 501

QUANTIFYING AQUEOUS URANIUM MOBILITY FROM IN SITU RECOVERY MINING SITES USING ISOTOPE-CONTRAINED REACTIVE TRANSPORT MODELS

Paper number: T309-12
Program: Technical Sessions
Time: 4:20 PM – 4:35 PM

Authors

Abstract

Domestic source(s) of uranium (U) ore are critical for low-carbon energy and energy security. U ore production in the USA is primarily by the in situ recovery (ISR) method where local groundwater fortified with oxygen and carbon are used to extract U as soluble U(VI)-CO3 complexes. The ISR method is economically and environmentally beneficial, producing ore from low-grade deposits with no mine tailings. Despite the advantages of the ISR technique there are significant questions about the long-term ability to restore mined aquifers to their pre-mining condition. Given the concerns about potable water availability in the western USA, there is a renewed focus on understanding the fate and transport of U in and adjacent to ISR mining operations.

We present new elemental concentration and isotopic data from recent studies in the Powder River Basin roll front U deposits. Groundwater samples and 3 sediment cores from the ore zone and down-gradient regions are used to construct reactive transport models of U mobility under pre-mining conditions. Specifically, we focus on U isotopic characterization ((234U/238U) and δ238U) of a roll-front U deposit from Smith Ranch Highland, WY, USA. Our results show substantial variations in δ238U (up to ~ 2‰) and (234U/238U) (ranging from 0.77 to 2.26) in U minerals with depth in each core and amongst the different cores. Vertically, U concentration is correlated with δ238U; samples with highest U at the center of the mineralized zone exhibit high δ238U (> 0.0‰), which decreases both upward and downward. The (234U/238U) in each core is correlated with δ238U, with the lowest (234U/238U) (<< 1.0) at the center of the mineralized zone.

Preliminary 2-D reactive transport models suggest that: 1) There is significant exchange between the solid phase U and aqueous U. 2) The primary mechanism removing aqueous U from present day groundwater is reductive precipitation. 3) The kinetics of U removal from solution are relatively fast compared to groundwater velocities (~2 m/yr). Overall, our modeling results suggest that the undisturbed groundwater is inefficient at transporting U downgradient and that restoration-remediation strategies that return the ore zone waters close to the pre-mining chemistry should effectively sequester U as precipitated U(IV) minerals inside the aquifer exemption area.

Geological Society of America Abstracts with Programs. Vol. 48, No. 7

doi: 10.1130/abs/2016AM-285354

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