> While the park trees contained lead isotopes normally associated with air pollution, the street trees had isotopes found in lead water pipes, which were made with metal from geologically old deposits in nearby mines.

I don't understand this part. We didn't use different sources of lead to make leaded gas and lead pipes, no?

Tetraethyllead production was very centralized by Ethyl corp/DuPont and required a higher purity lead ore so their isotope ratios are very well known based on the deposits that they mined. More locally sourced lead used for construction will have different isotope ratios.

When you need a lot of lead (enough to build plumbing for a neighborhood), you probably want to source it locally. When "1 part TEL to 1300 parts gasoline by weight is sufficient to suppress detonation",[1] you can source the lead from just about anywhere and ship it with the fuel.

1: https://en.m.wikipedia.org/wiki/Tetraethyllead

We didn't put elemental lead in gasoline, it was a very different molecule with a single lead atom. Given how dense lead is, you want to source it from as physically close as you can. A foundry making pipes in a city with a lead mine nearby will obviously use the local lead.

For gasoline, all production had to be centralized in a few refineries. The lead would have been shipped in, and would have been largely the same quality and age, likely coming from the same mine, or geographically close mines. Plus the absolute quantity of lead added to gasoline is relatively small. In the 60 years the US used TEL, we processed about 8 million tons of lead. Averaged out, it's 133 thousand tons a year. It would only take a few mines to provide that much. Probably not more than five or ten, but I can't immediately find good data on this.

One would expect that the lead used in gasoline is pretty homogeneous across time, and that intensive lead use (as in casting into solid metal object like pipes) would use the nearest available source, and use that source for as long as possible.

The radioactive lead isotopes come from decay of uranium and thorium, so lead from different mines will have different isotope ratios depending on how much U and Th happened to be in that ore.

Not all leaded gasoline was the same either:

> 206Pb/207Pb ratios commonly found in Pb ores throughout the world range between 16.0–18.5 and 1.19–1.25, respectively (Hansmann and Köppel, 2000). Exception to this rule is the commonly used Pb ore from the Broken Hill deposit, Australia, which is characterised by extremely low 206Pb/207Pb ratios (1.03–1.10). On the other hand, Pb originating from the Mississippi Valley ore deposit, USA, exhibits significantly more radiogenic Pb isotopic composition (206Pb/204Pb N20.0; 206Pb/207Pb= 1.31–1.35) (Doe and Delevaux, 1972). American leaded gasoline reflected therefore significantly higher 206Pb/207Pb ratios compared to European gasoline (Fig. 1). The introduction of the European leaded gasoline around 1945 resulted in a steep decrease of the 206Pb/207Pb ratio of atmospheric Pb (Weiss et al., 1999; data from peat deposits). The isotopic composition of leaded gasoline was to some extent dependent on economical factors, such as the availability and price of Pb ores and has evolved due to the different Pb ores used. For example, Pb used for French leaded gasoline originated from Australian, Moroccan and Swedish ores and the contribution of the separate ores changed during time (Véron et al., 1999). It is therefore indispensable to gather data concerning the origin of gasoline used in studied regions.

[from https://sci-hub.ru/https://doi.org/10.1016/j.envint.2007.10....]

lead from pipes was mined localy, but the lead in parks soil is from airborn pollution and so the isotope signature will be quite different