The U contents and 234U/238U activity ratios of dissolved uranium in groundwaters from some Triassic sandstones in England

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Authors:Andrews, J. N.; Kay, R. L. F.
Author Affiliations:Primary:
Univ. Bath, Sch. Chem., Bath BA2 7AY, United Kingdom
Other:
Inst. Geo. Sci., Oxon OX11 0RA, United Kingdom
Volume Title:Chemical Geology
Source:Chemical Geology, 41(2), p.101-117. Publisher: Elsevier, Amsterdam, Netherlands. ISSN: 0009-2541
Publication Date:1983
Note:In English. 19 refs.; illus. incl. table
Summary:The isotopic compositions of dissolved U in groundwaters of age = or <36 000 yr from the E Midlands Triassic sandstone, and in saline fluids from deep geothermal boreholes in the Wessex Basin, were investigated. The E Midlands groundwaters have U contents which lie on a normal distribution and average 0.75 mu g/kg with a standard deviation of 0.4 mu g/kg. The 234U/238U activity ratios for the dissolved U in these groundwaters have minimum values between 3 and 4. Some groundwaters have further enhancement of the 234U/238U activity ratio = or <7, but this is not related to groundwater age. U solution is primarily controlled by carbonate solution involving the cement phase of the sandstone. The 234U/238U activity ratios are the results of preferential chemical etch of 234U relative to 238U. An Eh-pH stability-field diagram, calculated for the average groundwater 042HCO3- contents in the aquifer, shows that the uranyl carbonate complexes, 079UO2(CO3)22- and 083UO2(CO3)34-, stabilize U solutions in E Midlands groundwaters. In the Wessex basin, the redox conditions indicate that U deposition should occur, and this is reflected in the very low U content of the geothermal fluids. The residence time of these thermal waters probably exceeds 15 m.y. and a steady-state 234U/238U activity ratio due to alpha -recoil-induced of 234U should therefore have been attained, but the activity ratio is very much less than the theoretical steady-state ratio for recoil processes. It is concluded that the U in the interstitial surfaces must have been sealed by a phase of low U content. If the thermal water has evolved from groundwater similar to that in the E Midlands, decay of excess 234U must have occurred. The time required for this decay is = or >0.25 m.y. if no alpha -recoil solution of 234U occurs, and is 1.25 m.y. if an alpha -recoil steady state has been attained.
Subjects:Actinides; Clastic rocks; Geochemistry; Ground water; Hydrogeology; Isotopes; Mesozoic; Metals; Radioactive isotopes; Sandstone; Sedimentary rocks; Surveys; Triassic; U-238/U-234; Uranium; East Midlands; England; Europe; Great Britain; Midlands; United Kingdom; Wessex Basin; Western Europe
Record ID:1985039735
Copyright Information:GeoRef, Copyright 2018 American Geosciences Institute. Reference includes data from Mineralogical Abstracts, United Kingdom, Twickenham, United Kingdom, Reference includes data from PASCAL, Institute de l'Information Scientifique et Technique
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