https://phys.org/news/2018-07-widespread-groundwater-contamination-chromium.html

The research found that naturally occurring chromium affects a larger area, more wells and a larger population throughout California than industrial sources, offering a new perspective on how California's groundwater resources should be managed.

"While the highest concentrations of chromium in any single well are clearly from industrial leaks, the greatest number of wells and the largest geographic distribution are all from this naturally occurring chromium," said co-author Scott Fendorf, the Huffington Family Professor in Earth system science at the School of Earth, Energy & Environmental Sciences (Stanford Earth). "As we continue to push the need to use and manage groundwater, understanding how these naturally occurring contaminants can jeopardize water becomes really, really important."