Chromium enters the water supply mainly as the a waste product of industry.
It exists as trivalent Chromium, which not only is not toxic but is considered an essential nutrient for humans, or as hexavalent Chromium, which is classed as a carcinogen by the EPA. The EPA's MCL for Cr -6 (hexavalent Chromium) is only 0.005 mg/l.
Fortunately, the occurrence of excessive Chromium is relatively infrequent. Or, at least that was believed to the the case until 2010 or so when news stories revealed that it is more prevalent than previously believed.
California has released a draft of a "public health goal" for a safe level of hexavalent chromium in drinking water: 0.06 parts per billion. If the state sets a limit, it would be the first in the nation.
Hexavalent chromium (Chromium VI) has had widespread, long-term use in industry for its ability to inhibit the formation of rust. It is also a known human carcinogen that has impacted drinking water aquifers in some states, resulting in well shutdowns. There are few federal or state regulatory standards for hexavalent chromium; however, that is changing. For now, many regulatory standards being used apply to total chromium levels -- the combined concentrations of trivalent and hexavalent chromium. In some cases, if the total chromium concentration in a sample meets or exceeds a certain level, you must test for hexavalent chromium.
EPA statement on Chromium: "Chromium is a naturally occurring element in the earth's crust. The three main forms of chromium found in the environment are chromium (0), chromium (III), and chromium (VI), also known as hexavalent chromium. Chromium is widely used in manufacturing processes, and it can be found in many consumer products such as wood treated with copper dichromate, leather tanned with chromic sulfate, and stainless steel cookware. Chromium is released to the environment from natural and anthropogenic sources, with the largest releases occurring from industrial sources. The general population may be exposed to chromium by inhaling ambient air, and ingesting food and drinking water containing chromium. Dermal exposure to chromium can occur from skin contact with certain consumer products or soils that contain chromium." (Full Document.)
Treatment: Removal from drinking water is accomplished by reverse osmosis (90% to 97%) or by distillation. For larger quantities, the best treatment for CR-6 is strong base anion exchange, which must be regenerated with caustic soda.
More extensive information on chromium appeared in the December 2010 issue of the Pure Water Occasional.