Hawaii is the only U.S. state that didn’t exceed federal testing limits for lead in drinking water during the past three years, and health department officials say it’s because the state’s fortunate to have lots of volcanic soil and few lead pipes.

“We are lucky,” said Joanna Seto, chief of the Hawaii Department of Health’s Safe Water Drinking Branch. “The biggest thing is that we have no lead pipes in Hawaii in our drinking systems and we have mostly ground water.”

An analysis of lead contamination data from the U.S. Environmental Protection Agency by The Associated Press found that Hawaii and Washington, D.C., were the only areas whose water systems didn’t test over the federal limit for lead in the past three years.

Hawaii health and water officials say there’s multiple factors, including abundant groundwater sources, few industrial pollutants and pipes built without lead, that help keep Hawaii’s tap water clean.

The AP found that across the nation, nearly 1,400 water systems that serve 3.6 million Americans have violated the federal lead standard at least once between Jan. 1, 2013, and Sept. 30, 2015. Of those, over 275 exclusively serve schools and daycares, some of which reported lead at dangerous levels and among the highest in the nation.

While no amount of lead exposure is considered safe, the federal rule calls for water systems to keep levels below 15 parts per billion. Lead is a potent neurotoxin that can damage child brain development and cause behavioral problems, and also can sicken adults.

Despite Hawaii’s good track record, some of the islands’ water systems have shown up with amounts of lead under the federal limit. For instance, EPA data shows recent drinking water tests at Marine Corps Base Hawaii and Naval Magazine Lualualei on Oahu yielded over 8 parts per billion.

Seto said in the last 10 years, only six water systems have tested over the limit of 15 parts per billion.

County water officials in Maui, Kauai, Oahu and Hawaii counties say the state’s water is cleaner because many of the main water systems aren’t built with lead pipes.

“To the best of our ability, we haven’t been able to find any records that we have lead pipes in our system,” said Erwin Kawata, program administrator for the Honolulu Board of Water Supply’s Water Quality Division.

Kawata added that Hawaii’s groundwater isn’t acidic, which means it’s less likely to corrode metal pipes. Much of Hawaii’s water comes from aquifers where it’s filtered underground by volcanic rock for years, unlike surface water found in rivers and streams that’s more susceptible to pollution, he said.

For example, an ongoing crisis in Flint, Michigan, has left residents without tap water for months after the decision was made to switch from the Detroit water system to the Flint River in April 2014. Testing last fall showed that levels of lead in the blood of some Flint children rose after the water systems were switched.

But in Hawaii, most water comes from underground, Kawata said.

“We’re very blessed,” Kawata said. “It’s what nature gives us.”