Chromium-6 is an element found in rocks, animals, soil, dust (volcanic) and gases according to National Toxicology Program:
Chromium-6 is used in electroplating, stainless steel production, leather tanning, textile manufacturing and wood preservation, according to the National Toxicology Program (PDF). Chromium-6 is also found in the ash from coal-burning power plants and used to lower the temperature of water in the cooling towers of power plants. – CNN
It can cause lung cancer if airbourne and this is why US Occupational Safety and Health have strict limits for this in workplaces. In 2008, a two-year study by NTP found drinking it caused cancer in lab rats and mice.
That is the same chemical that you may have heard of in the popular movie “Erin Brockovich” starring Julia Roberts.
75% of local water utilities samples show that chromim-6 is at or above 0.03 parts per billion between the year 2013 and 2015.