https://www.sciencedaily.com/releases/2018/07/180723142806.htm

In a study that appeared online June 27 in Environmental Science & Technology, Stanford scientists used a statewide groundwater database and a new means of tracing sources to identify wells containing hexavalent chromium from industry versus those that became contaminated from naturally occurring sources - some of which may also have resulted from human activity.

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.