[retrac]

Elemental lead is fairly reactive - it forms a number of compounds, some of which are soluble in water. Dissolved compounds may precipitate but they don't always "fall out" simply because the elements in them are heavier.

> you expect to hear about water tables that have problematically-high natural levels of this or that heavy metal?

That does happen. Cadmium is probably the most common culprit.

> In groundwater in Pakistan, mean Cd concentrations of 10 μg/L originated from Jurassic sulfide-bearing sedimentary rocks (Naseem et al., 2014). In Germany, background Cd concentrations in groundwater range from 0.11 μg/L in loess aquifers below arable land to 2.7 μg/L in sandy aquifers below forested lands [1]

The limit in the USA and EU is 5 ug/L. It's quite possible to drill a well with natural cadmium levels above the generally accepted safe level. Lead, arsenic, chromium, barium, and copper are other common culprits. In particular, chronic arsenic poisoning from naturally-occurring arsenic in ground water and aquifers affects many millions of people around the world.

[1] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7147761/