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Isotopic Disequilibrium of Uranium in Kansas Groundwaters

1990; Kansas Academy of Science; Volume: 93; Issue: 1/2 Linguagem: Inglês

10.2307/3628127

1938-5420

Dominic To,

Radioactive element chemistry and processing

Uranium isotopes were analyzed in 60 Kansas groundwater samples. Measurements of alpha activity (AR) indicated excess 234U present in almost all of these samples. The mean uranium concentration and the mean AR in these samples were 31.67 ? 53.84 DPM/liter (DPM/liter is defined as the total number of nuclear disintegrations per minute per liter of the sample) and 1.32 ? 0.28, respectively. Although the AR of these samples appeared to be independent of the uranium present over a wide range, a strong dependence of AR on iron and manganese concentration was observed. 234U (t/2 = 2.48 x 105 yr; % natural abundance = 0.0058) is generated in nature by decay from its parent 238U (t,2 = 4.51 x 109 yr; % natural abundance = 99.28) by way of two intermediate, short-lived nuclides 234Th (t,2 = 24.1 days) and 234Pa (t,/2 = 1.18 min). Until the early 1950's, it was assumed that radioactive equilibrium existed between 234U and 238U in nature since the intermediate daughters and 234U should be in secular equilibrium with their parent 238U. Secular equilibrium is a limiting case of radioactive equilibrium (the state in which the of numbers of atoms of parent and daughter become constant) in which the parent nuclide has a much longer half-life than the daughters and the radioactivity of the parent nuclide does not decrease measurably during many daughter half-lives. In secular equilibrium, the radioactivity of each member of the decay series will be the same although the actual number of atoms present may vary greatly. In 1955 Cherdyntsev et al. discovered the existence of disequilibrium in nature. Since then there has been a large amount of research works that demonstrated the disequilibrium of 234U and 238U in groundwaters, rivers, lakes, and oceans. disequilibrium has also been applied to specific problems. Rosholt et al. (1966) studied the general chemThis content downloaded from 157.55.39.58 on Tue, 15 Nov 2016 04:01:28 UTC All use subject to http://about.jstor.org/terms VOLUME 93, NUMBERS 1-2 39 ical evaluation of soils by investigating the and 234U/230Th ratios in a series of soil profiles. The ages of Pleistocene events was determined by utilizing the natural variations of 234U relative to 238U (Cherdyntsev et al., 1965). Kaufman and Broecker (1965) determined the uranium isotopic disequilibrium for 50 carbonate and marl samples from the Pleistocene basin of Lakes Lahontan and Bonneville in western USA. They concluded that the initial uranium AR's were too varied or that the system analyzed was too open to be useful in geochronological interpretation. Syromyatnikov (1965) suggested that disequilibrium determination of waters and of soils and extracts from soils and zones of cementation should enable one to establish the nature and the time of uranium accumulation. Rosholt and co-workers (1965) measured the variation of 234U relative to 238U in sandstone ore bodies. Their results suggested that in general, the unoxidized uranium ones were deficient in 234U whereas the altered ones had an excess of 234U. Other applications of disequilibrium to practical problems include geothermal circulation, earthquake precursor phenomena, and aquifer waste injection. The relative abundance of these two uranium isotopes is usually expressed as 234U/238U alpha activity ratio (AR) in the study of uranium isotopic disequilibrium in nature. A deficiency of234U is signified by an AR less than one and an excess is signified by an AR greater than one. An AR = 1 indicates 234U is in secular equilibrium with 238U and this also implies both uranium isotopes are decaying at the same rate. AR values as high as 28 and as low as 0.5 have been reported for groundwaters (Osmond and Cowart, 1976; Gilkeson and Cowart, 1982). In oceans, approximately 15% of excess 234U was found. It was inferred that the excess 234U found in oceans is balanced by a 234U-deficient reservoir elsewhere. In groundwaters, AR's exhibit much greater variations than do surface waters. These variations were attributed to the solid-liquid interfaced phenomenon which is dependent in some way on the radioactive generation of 234U and the apparent capability of the surrounding water to promote isotope separation. Two mechanisms have been put forward to explain the disequilibrium observed in water (Barnov et al., 1958; Rosholt et al., 1963; Dooley et al., 1966). The first mechanism involves the direct transfer of the alpha-recoil 234Th nuclide across the solid-liquid phase boundary followed by the decay to 234U. The second is an oxidation/selective leaching mechanism. The daughters of 238U (234Th, 234Pa and 234U) acquire recoil energies as a result of radioactive decays. These recoil energies are dissipated in the crystal lattice causing bond breakage. Eventually, the decay-generated 234U is oxidized to the more soluble hexavalent state as a result of the decay process and/or the differences in the oxidation potential between the original crystal site of 238U and the host environment of the 234U after displacement. This content downloaded from 157.55.39.58 on Tue, 15 Nov 2016 04:01:28 UTC All use subject to http://about.jstor.org/terms 40 TRANSACTIONS OF THE KANSAS ACADEMY OF SCIENCE It essentially involves the conversion of the combined 238U atom into a relatively uncombined 234U that is more vulnerable to subsequent leaching processes. Each of these mechanisms probably plays a dominant role in certain environments. The purpose of this work is to study the disequilibrium in Kansas groundwaters and to correlate the AR's to the manganese and iron contents. Since the concentrations of these two elements reflect the oxidation potentials in the source environment, the correlation between AR and manganese/iron concentration may be used as indirect evidence to support or oppose the oxidation/selective leaching mechanism. Also, this investigation of disequilibrium will aid the KDHE Radiation Laboratory to decide whether the U-nat conversion factor of 0.7 pCi/Ag (I pCi = 2.22 disintegrations per minute) could be used in future measurements of uranium content in Kansas groundwaters. The U-nat conversion factor is used to convert the measured uranium mass concentration into uranium alpha particle activity. However, the validity of applying this U-nat conversion factor depends on the establishment of secular equilibrium between 234U and 238U (i.e., AR = 1) in the sample.