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For years, Hilo residents’ drinking water has been contaminated with the chemical atrazine — a popular weed killer used on thousands of acres of sugar cane fields that lined the landscape up until a couple of decades ago.

The chemical has tested within safe limits established by the U.S. Environmental Protection Agency. But scientists and health advocates are increasingly concerned that atrazine could be harmful to human health at lower levels than previously thought.

And the experience of Hilo residents suggests that the chemical can persist in water years after it’s been applied to fields.

The EPA is set to undertake a review of the chemical in June, according to its primary manufacturer, Syngenta. The Swiss-based agribusiness company says that the chemical is safe and has been extensively vetted by the EPA.

But critics hope that federal regulators will tighten restrictions or ban the chemical.

Atrazine has been tied in scientific studies to cancer, changes in sex hormones and birth defects.

“It can’t be used and regulated in a way that human health is protected,” said Tyrone Hayes, a biologist at the University of California, Berkeley, who for years has studied the impacts of atrazine on frogs.

In addition to leaching into sources of drinking water, he says it poses a particular threat to agricultural workers in fields where it is sprayed.

Hayes says his studies show that atrazine can change male frogs into females, create hermaphroditic frogs and deplete frogs’ testosterone at levels that are 30 times lower than the EPA’s safety guidelines for drinking water. He worries that atrazine could have similar effects on humans.

Atrazine Found in Wells Statewide

Atrazine has been detected in groundwater throughout the state, according to Hawaii Department of Health . On Oahu, it’s been found in Ewa, Barber’s Point, Waialua, Haleiwa and Kunia. On Maui, it’s been detected in Kihei, Puunene and Paia. And on Kauai, atrazine has shown up in wells around Lihue and Kapaa. Not all of the wells actively provide drinking water.

Gary Gill, deputy director for environmental health, stressed that the atrazine levels are below EPA safety limits and that out of thousands of water samples taken in recent years throughout the state, only 6 percent tested positive for the chemical.

Still, Gill says atrazine is something that the health department worries about given the history of agricultural pesticides in Hawaii.

“We need to be very careful and apply the greatest amount of caution to these chemicals because we have a legacy in Hawaii of agricultural chemicals impacting our drinking water,” he said. “So we take this very seriously. There are chemicals that we are still finding in our water that have been banned for 20 years.”

The Honolulu Board of Water Supply has spent millions during the past two decades removing long-banned agricultural chemicals from drinking water in central Oahu. Currently, it costs about $2.5 million a year for scrubbing technology, according to Tracy Burgo, a spokeswoman for the department.

It also takes money to test for the chemical.

Last year, the Kauai Department of Water joined a national class action against Syngenta, and won up to $10,000 to cover costs of past atrazine testing. In total, Syngenta was required to pay $105 million.

Kauai residents have also been concerned about the presence of atrazine and other pesticides in the air resulting from spraying.

In 2011, a couple hundred residents in Waimea against Dupont Pioneer, formerly Pioneer Hi-Bred, alleging that the company’s pesticides were driving down local property values and causing anxiety over health impacts.

“There are huge amounts of drift when these things are sprayed,” said Kyle Smith, an attorney for the group. “It’s not just the dust. When these pesticides are used upwind, especially in places with tradewinds, these chemicals drift a long, long way.”

The lawsuit doesn’t allege that that the pesticides are directly harming human health, but Smith said that is the overriding concern for Waimea residents.

Laurie Yoshida, a spokeswoman for Dupont Pioneer on Kauai, said that she couldn’t comment on an active lawsuit. But she said that the company hasn’t sprayed atrazine on its Waimea seed corn fields in several years, though it does use other chemicals.

“As a member of the community and as a business, we are always concerned about employees, our neighbors and community, in general,” she said.

The case is expected to go to trial in October.

Ag Leaves Legacy Of Pesticides

While atrazine has been detected statewide, it’s tested highest in wells throughout the Hilo district.

Local water experts can’t say definitively that the atrazine showing up in the wells is leftover from the days of commercial sugar cane. But they say that’s the most likely culprit, even though the last mills shut down in the mid-1990s.

“We think it is remnants of the plantation times when sugar was still an industry here on the island because most of the sites that we have atrazine showing up is along the Hamakua Coast primarily, which was in former sugar cultivation,” said Keith Okamoto, deputy manager for the Big Island’s Department of Water Supply.

Sugar cane was one of Hawaii’s top crops until the 1990s, when the industry underwent a sharp decline.

There were 55 sugar cane fields producing 6.5 million tons of cane throughout Hawaii in 1990, but by 2002 there were only two farms, producing 2.1 million tons of cane, according to a University of Hawaii . Today, Hawaiian Commercial & Sugar Co. on Maui, is the only major sugar cane producer in the state.

While sugar cane production has declined dramatically, atrazine is still being used regularly throughout Hawaii, a review of sales records from the Hawaii Department of Agriculture show. The most heavy users appear to be seed corn growers, such as Monsanto, Mycogen and Pioneer, which have taken up an increasing presence on the islands in recent years with the decline of sugar cane and pineapple.

Biotech crops are now Hawaii’s most valuable agricultural commodity, according to from the U.S. Department of Agriculture. Corn seed makes up 96 percent of the Hawaii seed crops and is valued at $236 million annually.

That means atrazine could continue to show up in Hawaii water for years to come.

“In aquifers and groundwater, atrazine can persist for well over 10 years,” said Hayes.