"“The widespread distribution of cadmium contamination comes from both natural and anthropogenic sources,”"

"“Geochemically, certain parent rock materials [substrate below the ground], such as black shales, contain high levels of cadmium, leading to elevated concentrations in the soil due to weathering.”"

"However, the study does not assign blame to either natural or human causes."

To put that into perspective, I recall seeing one estimate that put the typical levels of arsenic in a cubic meter of soil that if concentrated would kill the average human.

Now, a cubic meter of soil is a very large amount, so the effective concentration is what actually counts. How relevant to human health is this 'typical' amount/quantity? Well, I don't know as it's not my field. I'd suggest however it cannot have a big effect on human health as we've lived and evolved for hundreds of millennia with that level of arsenic in soils—that's long before anthropogenic sources would have had time to kick in.

That said, we know that in some locations natural arsenic levels are high enough to cause significant health problems, in fact sometimes concentrations are so high as to be life-threatening. Such high levels are especially dangerous when leached out of rocks and soils and they end up in water supplies (contaminated bore water being the main culprit, it's a well-known problem).