Irmeli Mänttäri, Geological Survey of Finland, P.O. BOX 96, FI-02151 Espoo, Finland, e-mail: irmeli.manttari(at)gtk.fi
In geology, uranium isotopes have been conventionally used to date rocks and geological events (U−Pb dating). In hydrogeology, they are used to measure the age of groundwaters, monitor changes in groundwater flow regimes over time (Clark & Fritz 1997), to study mixing of different water sources (Osmond & Cowart 1976), and to detect natural and anthropogenic pollution. Natural or anthropogenic uranium in drinking water is a health-risk for humans and animals and therefore monitoring and tracing of the uranium contents and sources of drinking waters is essential.
Uranium (U) is a naturally occurring radioactive element that has an atomic number of 92. It has two primordial radioactive isotopes 238U and 235 U that decay to 206Pb and 207Pb with half-lives (t½) of 4.47 x 109 y and 7.04 x 108 y, respectively. 238U decays through very short lived isotopes 234Th and 234Pa to an unstable isotope 234U with t½ of 2.45 x 105 y.
Environmental studies use activity ratio of 234U/238U ([234U/238U]AR) to identify disequilibrium/secular equilibrium. At equilibrium, the isotopic abundances of 234U, 235U and 238U are 0.00548, 0.7200 and 99.2745 atom percent and thus, at secular equilibrium the 234U/238U ratio is 0.000055 (Rosman & Taylor 1998). When [234U/238U]AR=1, both species are in secular equilibrium and if it is ≠0, then the system has suffered removal/enrichment of daughter/parent isotope. The degree of disequilibrium provides a measure of time (uranium series dating).
Isotope 236U (t½=2.342 x 107 y) occurs mostly as an anthropogenic input – produced in nuclear reactors. Several orders of magnitude of difference between the 236U/238U isotopic ratios in naturally occurring (cosmogenic) uranium (10-10–10 -14) and in spent nuclear fuel (10-2–10-4) imply that also a small contamination (nuclear fuel, bomb tests, recycling of nuclear waste) in a natural sample is able to increase the 236U/238U significantly (Steier et al. 2008).
Description of the method
Uranium enters in waters through physical and chemical processes from water-rock interaction and the dissolved concentration of uranium and the relative abundance of 234U and 238U isotopes vary over a wide range in natural waters. The isotope fractionation is caused by the addition of 234U in the aqueous phase by selective leaching of the solid phase and by direct recoil of the daughter nuclide at a solid/liquid phase boundary and results increasing [234U/238U]AR –values (Rössler 1983, Petit et al. 1985). This process is sensitive for U concentration and its distribution in source matrix and fracture surfaces, the chemical state of U in the solid phase and the geochemistry of the groundwater (Clark & Fritz 1997).
The evolution of U and 234U/238U is linked to redox conditions in water. Under oxidizing conditions, the oxidized uranium(VI)-ion readily form complexes with common water soluble anions and the U concentration is usually high. Under reducing conditions the concentration is usually < 0.1 ppb as at greater depths and reductive conditions the solubility of uranium decreases and uranium precipitates in fractures. At this stage 234U leaching does not take place anymore but the activity ratio can still increase through 234Th recoil from precipitating U. However, at the same time the 234U decays and its amount is decreasing.
The age of the groundwater ([234U/238U]AR>1) can be estimated using the equation 1 (Faure 1977). However, this applies only systems that have remained closed to uranium, i.e. deep aquifers with highly reducing conditions and very low solubility of uranium (Clark & Fritz 1997).
With respect to regional run-off waters, the [234U/238U]AR values are characteristic of the weathering regime. A high [234U/238U]AR (> 1.5) is characteristic of run-off where the ratio of chemical to mechanical weathering is low. Medium or low activity ratios are characteristic of regions where chemical weathering is more important (Osmond & Cowart 1976).
Technically a modern uranium isotope analysis from water samples includes the separation of uranium from other elements using IC (ion chromatography) or specific resin (EiChrom’s UTEVA or TruSpec) and measurement of the uranium ratios by MC-ICPMS or TIMS (multicollector inductively coupled or thermal ionization mass spectrometry). The ultra-low levels of 236U isotope are measured using recent advances in accelerator mass spectrometry (AMS) techniques (Sakaguchi et al. 2010, Steier et al. 2010).
234U/238U activity ratio vs. inverse uranium concentration has been used to evaluate groundwater quality and mixing of different water sources (Osmond et al. 1983). Also, when separate water sources have identifiable different characteristics, mixing volume calculations can be made. Other potential uses include 234U/238U dating, tracing of hydrologic systems such as flow system behaviour and uranium migration (Suksi et al. 2001, 2006) and ore prospecting (Osmond & Cowart 1976).
234U /238U ratios vary due to anthropogenic discharge. This has been applied to detect contamination from the use of depleted uranium ammunition (Vidic et al. 2013) and to trace migration of fertilizer in the environment (Zielinski et al. 2000).
The predominantly anthropogenic 236U isotope has been used as a tracer of oceanic circulation (Sakaquchi et al. 2012) and oceanic global fall-out (Winkler et al. 2012), to monitor environmental contamination arising from nuclear fuel leaking, nuclear reprocessing facilities and fall-outs after nuclear weapons testing (e.g. Boulyga & Heumann 2006, Lee et al. 2008, Srncik et al. 2010).
A case study of the use of uranium isotopes in assessing impacts of mining on natural waters is presented in the following Closedure web page:
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