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^(234)U-^(238)U-^(230)Th-^(232)Th systematics in saline groundwaters from central Missouri

Banner, Jay L. and Wasserburg, G. J. and Chen, James H. and Moore, Clyde H. (1990) ^(234)U-^(238)U-^(230)Th-^(232)Th systematics in saline groundwaters from central Missouri. Earth and Planetary Science Letters, 101 (2-4). pp. 296-312. ISSN 0012-821X. doi:10.1016/0012-821X(90)90161-P.

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Saline groundwaters with 4.7 to 26‰ total dissolved solids were sampled from springs and artesian wells in Mississippian and Ordovician carbonates and sandstones in central Missouri. U—Th isotopic variations provide a means of evaluating processes of water-rock interaction and fluid mixing and estimating the time scales of element transport. Recently developed mass spectrometric techniques are used to make isotopic measurements on small-volume groundwater samples (0.1–4 l) with high precision (e.g., < ±5% for ^(234)U/^(238)U activity ratios). The groundwaters have a wide range of ^(238)U concentrations, 50 × 10^(−12) to 200 × 10^(−12) g/g; ^(234)U/^(234)U activity ratios, 2.15–16.0; ^(232)Th concentrations, 0.10 × 10^(−12) to 33 × 10^(−12) g/g; and ^(230)Th concentrations, 0.91 × 10^(−17) to 26 × 10^(−17) g/g. Unfiltered and filtered (0.4 μm, 0.1 μm) aliquots of a saline sample have the same isotopic composition and concentration of U, indicating that ^(234)U and ^(238)U occur almost entirely as dissolved species. The concentration of ^(232)Th is up to seven times lower in filtered vs. unfiltered aliquots, indicating that ^(232)Th is predominantly associated with particulates in the groundwaters. In contrast, most of the ^(230)Th is in solution. Previous geochemical studies indicate that: (1) the saline waters originated as meteoric recharge and evolved through halite dissolution, reactions with silicates and saline-dilute mixing processes during a long-distance flow history; and (2) interaction with limestone and dolomite aquifer rocks in central Missouri has been limited. A consistent relationship between U/Ca and ^(234)U/^(238)U activity ratio is observed in the groundwaters and provides constraints on the U/Ca ratios and ^(234)U/^(238)U activity ratios of end-member reservoirs and on the processes of isotopic exchange in this water-rock system. Model calculations that simulate (1) saline-dilute groundwater mixing and (2) limited extents of dissolution of carbonate aquifer minerals by the groundwaters can account for the variations in U/Ca, ^(234)U/^(238)U and ^(18)O/^(16)O in the suite of water samples. The model calculations demonstrate that dissolved U isotopic compositions can be a sensitive indicator of water-rock interaction, which in turn limits the usefulness of ^(234)U—^(238)U disequilibria for groundwater age determinations. The concentration of dissolved ^(230)Th in the groundwaters is (1) two to three orders of magnitude below ^(230)Th—^(234)U equilibrium activity levels, and (2) significantly in excess of concentrations estimated for the supply of Th to solution via desorption and dissolution. A model involving the derivation of the excess ^(230)Th from the in situ decay of dissolved ^(234)U in the groundwaters indicates the operation of an adsorption mechanism on the time scale of 10–10^3 years. The results reported here may have broader application to the assessment and management of hazardous chemical species in natural environments.

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Wasserburg, G. J.0000-0002-7957-8029
Additional Information:© 1990 Elsevier Science Publishers B.V. Received October 2, 1989; revised version accepted July 11, 1990. Funding was provided by the donors to the Petroleum Research Fund of the American Chemical Society (No. 21213-AC2), the Department of Energy (DE-FG03-88ER13851) and Texaco, Inc. to CIT, and the Basin Research Institute and the industrial associates of the Applied Carbonate Research Program at LSU. Helpful reviews were provided by R. Zielinski and J. MacDougall. Discussions with N. Sturchio, M. Stein and A.B. Carpenter were also helpful. D. Papanastassiou enabled smooth technical operations at CIT. Contribution no. 4799 (654).
Funding AgencyGrant Number
American Chemical Society Petroleum Research Fund21213-AC2
Department of Energy (DOE)DE-FG03-88ER13851
LSU Applied Carbonate Research ProgramUNSPECIFIED
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Caltech Division of Geological and Planetary Sciences4799
Lunatic Asylum Lab654
Issue or Number:2-4
Record Number:CaltechAUTHORS:20131028-111018862
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Official Citation:Jay L. Banner, G.J. Wasserburg, James H. Chen, Clyde H. Moore, 234U—238U—230Th—232Th systematics in saline groundwaters from central Missouri, Earth and Planetary Science Letters, Volume 101, Issues 2–4, December 1990, Pages 296-312, ISSN 0012-821X, (
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:42095
Deposited By: Ruth Sustaita
Deposited On:28 Oct 2013 20:14
Last Modified:10 Nov 2021 04:38

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