Last week, the Los Osos Community Services District announced that testing of one of its six wells showed a violation of California’s standard for chromium 6 levels. The contaminated Third Street well has been shut down at least temporarily.
Chromium 6, also known as hexavalent chromium, is a human carcinogen.
The district reported that recent testing showed 11 micrograms per liter of chromium 6 in the Third Street well. State standards for chromium 6 are 10 micrograms per liter, while federal standards are 100 micrograms per liter.
We always err on the side of being conservative and the state standard does that. Just a little bit above the standard is not going to cause a health risk.
Bruce Macler, U.S. Environmental Protection Agency toxicologist
EPA toxicologist Bruce Macler said that a slightly excessive level of chromium 6 isn’t likely to have any impact on the community, even if the water delivered to customers had not been blended with other well water to make it safe, as the district has said it did.
Macler said the state standard for levels of the chemical in drinking water is very conservative, and it would take much higher amounts of the chemical, consumed over many years, for the health risk to be serious.
“We always err on the side of being conservative, and the state standard does that,” Macler said. “Just a little bit above the standard is not going to cause a health risk.”
Macler, who is based in San Francisco, said he’s familiar with the Los Osos area and knows that the area doesn’t have the type of industry, such as metal plating businesses or tanneries, that would cause contamination of the groundwater.
Old chromium mines, established in the World War II era, once existed between Morro Bay and Atascadero, as well as near Camp San Luis. Chromium mines also existed in San Luis Obispo’s Irish Hills, said Eva Ulz, the director of the History Center of San Luis Obispo County.
“The Dallidet family owned a chromium mine, in fact, and I believe we have journal entries and other references to their mining activities,” Ulz wrote in an email.
Macler said the mining history tells him that chromium is a naturally occurring substance in the area, and possibly some seepage occurred into the groundwater supply.
“That’s different, however, than people taking paint and dumping it into the ground,” Macler said. “Still, if people have homes with wells close to where those mines were, it may be worth checking out if there are higher concentrations of hex chrome in that water. Congress doesn’t want us regulating individual wells, but still there’s a public health issue.”
Macler said, based on the test results coming from the district, he doesn’t believe the community at large is at risk.
10 micrograms per liter is California’s standard for Chromium 6 in potable water.
100 micrograms per liter is the federal standard for Chromium 6 in potable water.
Those with reverse-osmosis systems in their homes would be able to reduce chromium 6 levels in their individual water supplies. However, those systems waste a lot of water, Macler said.
The district’s contamination issue comes at a time when the district is limiting water use to 50 gallons per person per day.
Macler said experts could test for the source of the chromium 6 in the Los Osos well, but he isn’t sure how much that would cost or how involved it would be.
“That would be something for the district to look into,” Macler said. “It could be expensive. But it may not be. Perhaps they could reach out to (Cal Poly) on that.”