I've not done one of these geographical associations for some time. I find them useful not so much as proof, which of course they aren't - there is no rigorous statistical analysis, nor does it take into account any of a myriad of confounding issues - but to kick-out obviously flawed hypotheses.

Compare:
A. The geographical prevalence of PD in the US [1].
B. Zinc in the soil in the US [2].

Eye-balling the maps I suspect that there is a small negative association between the two. Thus, we can't throw out the hypothesis that low levels of zinc are linked to PD.

I would suspect that the causal mechanism by which zinc levels in the body associate with those in the soil is through the food chain and the water supply.

References:

[1] "Geographic and Ethnic Variation in Parkinson Disease: A Population-Based Study of US Medicare Beneficiaries"
Allison Wright Willis,a,* Bradley A. Evanoff,b Min Lian,b Susan R. Criswell,a and Brad A. Racette
Geographic and Ethnic Variation in Parkinson Disease: A Population-Based Study of US Medicare Beneficiaries
Figure 1.

[2] "Element Concentrations in Soils and Other Surficial Materials of the Conterminous United States"
US Geological Survey
https://pubs.usgs.gov/pp/1270/pdf/PP1270_508.pdf
Original page 102

John