A state water official says a headline-grabbing water study that found a probable carcinogen in Honolulu’s tap water was “irresponsible,” since it drew its conclusion based on a single water sample.

Of the 31 cities tested, the study concluded that Honolulu has the second-highest rate of hexavalent chromium, a probable carcinogen made famous by the movie “Erin Brockovich.” The Environmental Protection Agency is considering setting a limit on the presence of the toxin in tap water.

But drawing a sweeping conclusion for an entire city’s water supply based on a single sample isn’t good science, says Watson Okubo, supervisor of monitoring and analysis for the Clean Water Branch of the Hawaii Department of Health.

“To say ‘this is the situation’ by taking one sample, is irresponsible,” he said. “That’s not a way to run a test. You don’t do something like that. It’s not scientific.”

Okubo suggested that at least 50 samples would be more appropriate.

Leeann Brown, a spokeswoman for the Environmental Working Group, which conducted the study, confirmed that the test relied on one sample from each city. “We took one sample from each city,” she said. “Each were collected from unfiltered taps in homes or in public buildings such as hospitals, libraries and malls.”

The Honolulu sample was reportedly taken from a residence in Wilhelmina Rise.

The Board of Water Supply also questioned the study’s conclusions, a spokesman told Civil Beat on Monday, because the sampling was both narrow and not necessarily representative of the water supply itself.

“If it comes from a residence, not the source, we don’t know where the hexavalent chromium comes in,” Kurt Tsue told us, pointing out antiquated pipes and other infrastructure could contain carcinogens. Chromium, according to EWG’s report, is a naturally occurring metal that is used in steel manufacturing, welding and in the plates for metal surfaces, among other things.

Despite their concerns about the small and questionable sample, officials at the Board of Water Supply are making plans to test more local water samples for the toxic compound.

For those who worry that the study’s conclusions are correct, a Civil Beat member helpfully shared that certain reverse-osmosis filters remove hexavalent chromium. He also pointed out that running to the store for bottled water is no guarantee of escaping the risk of the toxic compound. Bottled water companies, like municipalities, are not (yet) required to test for the chemical.