The latest tests of a major source for Windward Oahu drinking water showed it to be free of E. coli, a type of bacteria.

Drinking water from the Waihee Tunnel was found to contain E. coli during routine testing Monday. Five follow-up samples taken Wednesday were found to be free of E. coli, according to Erwin Kawata, Board of Water Supply program administrator, speaking by phone Friday in an interview. The Waihee Tunnel serves Windward Oahu from Kahana to Waimanalo.

The water was safe to drink even on Monday, Kawata said, because the E. coli-laced sample was taken before the addition of chlorine, which kills bacteria.

“This actual sample was collected coming out of the tunnel, before our chlorinator device injects chlorine into the pipe, and before it goes into the water supply.”

Kawata added that BWS tests the water on both sides of the chlorinator, and “after the chlorine is added, there’s no bacteria.” Monday’s post-chlorination sample was free of E. coli.

Kawata said the reason for the presence of E. coli in the pre-chlorination sample had not yet been determined.

“E. coli is a coliform bacteria commonly found in fecal matter of animals but is also commonly found in the environment,” he said. “It’s very prevalent in subtropical climates like Hawaii’s, where it’s found within soils.”

Asked whether waste from humans or animals such as feral pigs could have infiltrated the tunnel water, “It does happen from time to time, but only very occasionally,” Kawata said.

It was highly unlikely that an animal could have gotten into the tunnel, he added. “The tunnel is bulkheaded and gated, and everything is sealed, so it’s not like lake water where it’s open to the atmosphere.”

And if an animal had gotten in, “the levels of E. coli would be extremely high, and we would continuously find it” in subsequent samples, he said, which was not the case in this instance.

Still, the impact of humans, in particular, should not be discounted, said John Reppun, a lifelong Windward Oahu resident who worked for 35 years at Key Projects in Kahaluu. “There are no restroom facilities in these mauka lands, which BWS also controls, above the Waihee Tunnels,” Reppun said, noting that no-trespassing signs are posted.

He said that if people relieve themselves along the hiking trails that have become widely popularized by social media, the waste can eventually filter down through the soil and rock into the Waihee Tunnel, he said.

“On any given three-day weekend, as many as 3,000 people are trespassing in BWS’ mauka watershed,” he added.

The Waihee Tunnel is a natural geologic formation “where rainwater percolates,” Kawata said. “Water seeps out of the rock into the pipe” inserted by BWS.