The environmental geochemistry of lead (Pb) has probably stimulated more scientific interest than all other metallic elements combined. The local, regional, and global biogeochemical cycles of lead have been affected by man to a greater degree than those of any other toxic element. In fact, there is probably no place on the surface of the Earth that is devoid of anthropogenic Pb. Although Pb is one of the most useful of all the metals, it is also one of the most toxic. Because it serves no biological role and occurs naturally at the surface of the Earth only in trace concentrations (parts per million in rocks, parts per trillion in waters), human exposure in pre-industrial times was probably very low.
With respect to scientific study, one great advantage of Pb compared to other metals of environmental concern is the number of stable isotopes (204Pb, 206Pb, 207Pb, 208Pb) which can be used to help "fingerprint" the predominant sources of both natural and anthropogenic Pb. These are extremely useful to help identify predominant pathways, and to study the fate of this metal in the environment. Lead from natural sources (mainly crustal rocks) and Pb from anthropogenic sources (Pb ores) have very different isotopic signatures. In addition, 210Pb, a radioactive Pb isotope with no significant anthropogenic contributions, is a valuable tracer of atmospheric scavenging, soil migration, adsorption, biological uptake and other processes affecting the behaviour and fate of Pb in the environment. If quantitative measurements of total Pb concentrations have allowed us to study the Pb problem in black and white, precise measurements of Pb isotopes allow us to see in colour.
The historical trends in atmospheric Pb are recorded in valuable archives such as polar snow and ice, peat bogs, and mosses. Because Pb has been used as an industrial material for thousands of years, long-term records of atmospheric Pb are essential to determine the natural background rates, for comparison with modern values, but also to help understand the geological processes which controlled these fluxes. Although air Pb concentrations have declined significantly following the reduction and subsequent phase-out of Pb in gasoline, analyses of recent snow in Germany, southern Canada and the High Arctic shows that aerosols are still dominated by anthropogenic Pb. Anthropogenic Pb in modern atmospheric particles not only has a different isotopic composition compared to natural Pb, but it occurs in much smaller, more soluble particles. Whereas the Pb which occurred naturally in the air was supplied by comparatively large, insoluble, soil mineral particles, most of the anthropogenic Pb is emitted from high temperature combustion processes such as metallurgical processing, coal combustion, and refuse incineration. Lead emitted from these sources is released to the air in the form of sub-micron particles: with an average atmospheric residence time of approximately one week, they are not only amenable to long-range atmospheric transport (thousands of kilometres), but they are easily respirable and the Pb they contain is much more soluble.
Millions of tons of anthropogenic Pb are now found in soils across the northern hemisphere, but the fate of this Pb is unclear: is this Pb labile and will it eventually migrate out of the soils and into surface and groundwaters, or will it remain permanently fixed by organic matter and iron oxides? The natural concentration of Pb in uncontaminated waters such as soil solutions, streams, and lakes, is extremely low, and very few laboratories are able to perform reliable analyses in this (part per trillion) concentration range. Moreover, the concentration of total dissolved Pb provides limited insight into biological availability. In fact, the chemical speciation of Pb in soil solutions and other natural waters requires thorough investigation.
Blood lead levels (BLL) have been declining in humans, but so has the concentration which is considered characteristic of subclinical Pb toxicity. In addition, the BLLs which today are considered critical for children (10 micrograms per decilitre) are far beyond the concentrations estimated to be characteristic of pre-industrial humans (0.2 to 0.02 micrograms per decilitre), based on comparative analyses of modern and ancient human bones. Moreover, the most recently published studies show that Pb has deleterious effects on neurological development of children, even at Pb concentrations of l microgram per decilitre which is ten times lower than the concentrations presently considered critical.