https://www.sciencedaily.com/releases/2019/11/191112160211.htm

"Though often thought of as a byproduct of industrial contamination, hexavalent chromium can form naturally in groundwaters, depending on the local aquifer geology and water chemistry," said Avner Vengosh, professor of Earth and Ocean Sciences in the Nicholas School of the Environment at Duke.

Other studies have identified naturally occurring hexavalent chromium in groundwater in Arizona, California, Argentina, Brazil, Greece, Italy, and Mexico.