Tantalum

The first systematic review on tantalum in the environment was published in 2017:

M. Filella (2017) Tantalum in the environment. Earth-Science Reviews 173, 122-140.

From the abstract:

Tantalum is a naturally occurring chemical element which belongs to Group 5 of the Periodic Table. It forms compounds with quite a variety of oxidation states from +5 to -3 but the only relevant oxidation state in environmental systems is +5.  While the mineralogy of the element is well-known due to its current economic importance, little information (if any) is available on the transformation and transport of tantalum in the different environmental compartments. Even data on tantalum concentrations in some environmental media, like natural waters or the atmosphere, are scarce and often discrepant.

Read a concise resume of the conclusions in the Environmental issues section of the Wikipedia entry on tantalum in English and in Catalan.