GREENSBORO — A yearlong series of tests during 2014 consistently showed levels of chromium-6 in the city's drinking water above what initially triggered statewide "do not drink" warnings to well owners near some of Duke Energy's coal ash ponds.

But new results from the Greensboro system last week found the carcinogen at "non-detect" levels, meaning its presence had fallen significantly beneath the level of .07 parts per billion (ppb) that reenergized the coal ash debate in recent weeks.

"We just received our latest results for chromium-6 and all samples are non-detect," Greensboro water supply manager Barry Parsons said. "This includes our raw water, both plants and our interconnects (in distribution lines)."

During 2014, Greensboro researchers found the contaminant above .07 ppb in treated drinking water leaving both the city's Mitchell and Townsend plants, and at two other points in the distribution system.

Concentrations in treated water averaged .09 ppb at the Mitchell Water Plant in 2014, .11 ppb at the Townsend Water Plant and .13 ppb in the distribution system, according to the city's annual water quality report.

The .07 ppb level for chromium is an estimate of what concentration would cause one additional case of cancer in a million people who drank it every day during a lifespan of 70 years. State health officials initially applied that as the safety standard for drinking water from wells near Duke Energy's coal-ash storage ponds and sent "do not drink" warnings to many residents as a result.

They rescinded those warnings in consultation with the state Department of Environmental Quality after it was pointed out that federal officials do not specifically regulate chromium-6 in drinking water and that many public water systems in North Carolina distribute water with significantly higher levels of chromium-6.

The revocation caused an uproar recently in the coal ash controversy when testimony in a lawsuit involving Duke Energy showed that utility officials argued for the reversal in talks with state officials, sparking criticism that the administration of Gov. Pat McCrory — a former Duke executive — had put his former employer's interests above the public's welfare.

State officials strongly deny that contention.

Chromium-6, also known as hexavalent chromium, became famous as the element that helped environmental activist Erin Brokovich make a name for herself with a successful lawsuit on behalf of residents in a California community whose water was polluted by a utility that used the substance to prevent rust. But chromium-6 concentrations in that town's groundwater measured more than 500 ppb — many times greater than what's being debated in North Carolina. 

Greensboro's drinking water was never unsafe to consume, but city water officials still are concerned and determined to keep chromium-6 in check.

“We don’t want this in our water," Parsons said. "It’s something we’re looking at very closely."

In fact, the city just added it to the list of unregulated, potential contaminants that officials will test for regularly, Parsons explained. The water manager said he was not sure what caused chromium-6 to drop in the latest tests, the first such sampling since December 2014.

"Over 90 percent of U.S. municipalities have it in their drinking water," Parsons said of the chemical.

City water officials are trying to track what caused the 2014 spike, but they remain puzzled because no chromium-6 has been found in samples of raw, untreated water from the the city's chain of three, water-supply lakes north of town.

Analysts did detect chromium-6 in some of the additives being used to purify water at the Mitchell and Townsend plants, but not enough to fully account for the higher 2014 readings, Parsons said.

City officials performed the sampling two years ago at EPA's behest. The agency required it of all major, public water systems as part of a continuing study of whether EPA should change course and set a specific, maximum level for chromium-6 concentrations in drinking water. The current, federal standard is 100 ppb of total chromium, a composite of chromium-6 and more common chromium-3.

The science surrounding chromium-6 is tricky and it is not clear how much of a health threat it actually poses from small concentrations in drinking water. The most common form of chromium is a naturally occurring substance that has long been thought necessary in small quantities for human health.

Although airborne chromium-6 is a proven carcinogen when inhaled in dust, the science remains less settled about drinking water. 

EPA began its current study after the National Toxicology Program determined rodents develop stomach cancer by drinking water heavily laden with chromium-6. 

So city officials are looking ahead to the likelihood EPA will recalculate chromium-6 as a hazardous contaminant at some point.

"It’s easy to say, 'Well, we are not regulated, therefore we will wait for more information,'" said Steve Drew, director of Greensboro's department of water resources. "But since a contaminant has been designated on the radar of concern and as one detrimental to public health — eventually to be regulated — we begin our due diligence."

Contact Taft Wireback at (336) 373-7100, and follow @TaftWireback on Twitter.

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