But it is plausable that WW's would be contaminated with Ca (from battery plates) and Zn (from zinc weights) just because the companies that make them don't care how the metal casts -- they use die casting machines. All they should care about is that the weight is consistent within their tolerances, the lead is hard enough that the weights don't fall off, and they don't ruin alloy wheels.
But I've never had any trouble with them, except for that one little batch of stick-on wieghts that I melted down before I realized that some of them were zinc