The Washington Post is reporting "Probable Carcinogen Hexavalent Chromium Found in Drinking Water of 31 U.S. Cities". It's pretty much just the EWG press release "Chromium-6 is Widespread in US Tap Water: Cancer-Causing Chemical Found in 89 Percent of Cities Sampled".
So what to make of the claim that 74 million Americans are unknowingly bathing in, cooking with and drinking unsafe levels of "the carcinogenic Erin Brockovich chemical"? Well, as luck would have it there's a brand new (free!) paper that contains a very nice summary of the state of scientific knowledge concerning the impact of exposure to hexavalent chromium (Cr (VI)) in water. See "Application of the U.S. EPA Mode of Action Framework for Purposes of Guiding Future Research: A Case Study Involving the Oral Carcinogenicity of Hexavalent Chromium". EPA didn't think it posed a risk of cancer and epidemiological studies have, with one controversial exception, found no link between hexavalent chromium in drinking water and cancer. Nevertheless, when the National Toxicology Program gave lab rodents water containing Cr (VI) at hundreds of times the concentration of the highest levels found in human drinking water it detected a small but statistically significant increase in risk of a rare (for rodents) lower g.i. cancer. Lower doses, i.e. at levels only 300 times higher than those found in the drinking water of 95% of Americans, produced no increase in rodential risk.
Thereafter the state of California proposed a "public health goal" for Cr (VI) in water of 0.06 parts per billion (ppb) or, exactly, 1/1000th the level the lowest level found to slightly increase the risk of cancer in lab mice and rats. Somehow that 0.06ppb level for Cr (VI) in water became the "safe level" according to the EWG (and thus, necessarily, The Washington Post). Assuming, without explanation, that any level above 0.06ppb is thus unsafe it's no surprise that the EWG discovered that 89% of our drinking water is probably carcinogenic.
The Washington Post quotes Max Costa who "chairs the department of environmental medicine at New York University's School of Medicine". He calls the EWG's findings "disturbing". The WaPo doesn't say why they emailed Costa for a comment but we can guess. He was a retained expert witness for the Erin Brockovich plaintiffs. He testifies that "[h]exavalent chromium is one of the most potent carcinogens known to man. It can produce any type of cancer depending upon genetic susceptibility, quantity and route of exposure."
Ok, if 89% of Americans are ingesting dangerous levels of one of the most potent carcinogens known to man, one that can cause any form of cancer, what would you think the National Cancer Institute's Annual Report to the Nation on overall cancer rates would show? Would you be surprised if it showed that the overall incidence of cancer to be declining significantly? Would you be surprised if it showed that colorectal cancer rates (the closest thing to what was slightly increased in mice) have been dropping for a quarter of a century and that their decline is accelerating? If so you need to read "Annual Report to the Nation Finds Continued Declines in Overall Cancer Rates; Special Feature Highlights Current and Projected Trends in Colorectal Cancer".