Lead isotopes

Anniina Kittilä, ETH Zürich, Institute of Geophysics, Geothermische Energie u. Geofluide. Sonneggstrasse 5, 8092 Zürich, Switzerland e-mail: anniina.kittila(at)erdw.ethz.ch

Introduction and description of the method

Lead isotopes are studied to determine possible sources of lead in aqueous environments and mixing of different waters, and to elucidate groundwater flow paths. The method has been applied in numerous and various cases, for example in anthropogenic contamination studies, such as gasoline-contamination (Landmeyer et al. 2003) and in traffic-related pollution (Walraven et al. 2014), in mineral exploration (De Caritat et al. 2005), in studying an impact of local contamination in continental-scale and in mapping natural geochemical background (Reimann et al. 2012), in determining impact of historical mining in the environment (Camizuli et al. 2014), and in studying water-rock interaction (Négrel et al. 2010). The history of environmental lead isotope studies is described, for example, by Dickin (2005).

Lead has four stable isotopes (natural abundance in brackets); 204Pb (1.4%), 206Pb (24.1%), 207Pb (22.1%), and 208Pb (52.4%). The last three of them are radiogenic, and are produced from the radioactive decay of naturally occurring uranium and thorium in rocks (e.g. Landmeyer et al. 2003, Dickin 2005, Reimann et al. 2012): 238U decays to 206Pb, 235U decays to 207Pb, and 232Th decays to 208Pb. As 204Pb is not radiogenic, it is considered to be primordial in origin (Landmeyer et al. 2003). The exact measurement of 204Pb is difficult and time consuming, but the ratios between the radiogenic isotopes, 206Pb, 207Pb and 208Pb, is easier to determine, and it can be done by inductively coupled plasma mass spectrometry (ICP-MS) (Reimann et al. 2012). The half-life of 238U is comparable to the age of the Earth, whereas 235U decays much faster, so that very few of the primordial 235U is left, as it has decayed to 207Pb. The half-life of 232Th is comparable to the age of the universe. Because of the different half-lives, the accumulation of different lead isotopes is unique, and reflects the age of the system (Dickin 2005). In environmental studies, the 206Pb/207Pb ratio is often used to determine Pb contamination sources (Reimann et al. 2012), but in general, in earth sciences the radiogenic isotopes are compared with the non-radiogenic isotope 204Pb, whereas in environmental studies the ratios are usually normalized to 206Pb or 207Pb due to the fact that the 204Pb isotope cannot be measured well enough by the classical ICP-MS (Négrel et al. 2010).

Granites and granodiorites that contain high amount of feldspars have more lead than mafic and ultramafic rock. K-feldspar and pegmatites can also have large amount of lead, slightly less in micas, and least in amphiboles and quartz. Additionally, because of the calcophilic character of the elemental lead, it is very common also in sulphide minerals with zinc and cadmium. (Lahermo et al. 1996)

As groundwater circulates in a system, its chemical and isotopic compositions are affected by residence time as well as the chemical composition and mineralogy of the host rock, and different weathering stages of the minerals. This unique fingerprint can be studied with lead isotopes, and the results can help in evaluating water-rock interaction and flow path evolution (e.g. De Caritat et al. 2005, Négrel et al. 2010).

Appropriate applications

In order to gain the most from the lead isotope measurements, it is good to use the method coupled with other geochemical studies, for example with strontium and sulphur isotopes, as was done by De Caritat et al. (2005). Négrel et al. (2010), on the other hand, were able to determine a geogenic source for lead in groundwater by comparing the data with Cl and NO3 measurements (agriculture and other anthropogenic sources in the absence of evaporates). As was established by Harlavan & Erel (2002), dissolution of rock forming minerals takes place in stages, which is controlled by the minerals’ resistance to weathering. This should be understood when interpreting the lead isotope results, because the isotope ratios can increase or decrease as new minerals dissolve to the groundwater (Harlavan & Erel 2002, Négrel et al. 2010). It should also be noted that although there are many applications for lead isotopes in flow paths and mixing studies, the natural fingerprint can be obscured by anthropogenic lead input (Négrel et al. 2010).

Method maturity

As summarized by Négrel et al. (2010), numerous applications of lead isotopes have focused on environmental questions studying anthropogenic influences on sediments. It is also noted that lead isotopes have many applications in groundwater studies, for example, in tracing pollution source to water resources or in investigating drinking water supply systems, in geologic hazards impact investigations, and in flow path studies. Altogether, it can be concluded that using lead isotopes ii groundwater studies is a well-established method.


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