An environmental group says laboratory tests show the presence of a toxic chemical in 31 of 35 cities, with Honolulu having the second to highest level.

The nonprofit Environmental Working Group said hexavalent chromium, the same chemical that led to a $333-million legal settlement described in the movie “Erin Brockovich,” is present in higher than recommended levels in Honolulu’s water supply.

Drinking water from one location that was tested in Honolulu had 2 parts per billion of hexavalent chromium, compared to .06 ppb proposed as a public health goal by the California Environmental Protection Agency, the group said.

Rebecca Sutton, the group’s lead investigator, said the Honolulu Board of Water Supply did not know the source of the hexavalent chromium. She said the chemical is a common industrial contaminant and can also originate from natural, geologic sources such as ground water.

“Regardless of the source of contamination, we need safe drinking water,” she said.

The Honolulu Board of Water Supply said it doesn’t test for hexavalent chromium because the federal Environmental Protection Agency does not require it.

Board spokesman Kurt Tsue said he was “surprised” at the Environmental Working Group’s evaluation since it ranked Honolulu sixth as the best drinking water in the nation earlier this year.

Tsue said the board does test for chromium, but cannot tell how much of it is hexavalent chromium. He said the board has met the federal guidelines for chromium, with a maximum contaminant level of  100 ppb.

Sutton said the single water sample tested from Honolulu was taken from a residence in the general vicinity of Wilhemina Rise in east Oahu. She said taking more samples would have been better but her organization was constrained by the cost of testing.

She said she felt confident that sampling other residences would bear similar results.

As part of the procedure, Sutton said, the water in the pipes were flushed “full blast”  for several minutes before the sample was taken.

Tsue said although Honolulu’s water system is interconnected, wells in different regions of the island contribute to the supply and the trace levels of chemicals vary.

“It does vary across the island depending on where we pump from,” he said.

Tsue and state Department of Health spokesman Janice Okubo said they would generally follow the guidelines and recommendations of the U.S. Environmental Protection Agency.

Federal officials have linked hexavalent chromium to lung cancer in the workplace,  and Sutton’s group said federal tests show clear evidence of its cancer-causing activity in drinking water.

Federal labor safety standards restrict the exposure of workers to hexavalent chromium, saying it can cause lung cancer in workers who breath airborne particles and damage to the nose, throat, eyes and skin.

But federal officials have set no maximum contaminant level for hexavalent chromium alone in the drinking water.

Drinking water from Norman, Okla., ranked first, with a concentration of 12.90 ppb for hexavalent chromium.

The Environmental Working Group criticized the EPA for failing to set a maximum contaminant limit to hexavalent chromium in drinking water and to require utilities to test for its presence.

The agency has set a maximum contaminant level of 100 ppb for chromium, including the combination of hexavalent chromium and the less toxic trivalent chromium. But it has set no maximum contaminant level for hexavalent chromium alone.

The agency yesterday said it is  reviewing new science and has launched “rigorous and comprehensive” review of the effects of hexavalent chromium on human health.

“When this scientific assessment is finalized in 2011, EPA will carefully review the conclusions and consider all relevant information, including the Environmental Working Group’s study, to determine if a new standard needs to be set,” it said.