Hawaii health authorities have posted warnings of a sewage spill at a popular beach, the second time this week that a well-known stretch of Oahu's shoreline has been hit by a sewage spill.

About 1 million gallons of wastewater that ran through a treatment plant without being disinfected flowed into the ocean on Tuesday off Sandy Beach, a favorite spot for body surfers on the east end of the island, roughly 15 miles from Honolulu.

The discharge prompted the state Health Department to post advisories along more than a mile of coastline, including Sandy Beach, Allison Nunnally, an environmental health specialist for the agency, said on Thursday.

The beach itself remained opened, but the public was urged to stay out of the ocean while water samples collected in the area were analyzed to determine the level of risk, she said.

The discharge of less-than-fully treated sewage was caused by an electrical failure that cut off the flow of chlorine disinfectant into the wastewater before it gets pumped into the ocean, said Evan Jacobs, a spokesman for Hawaii American Water, which operates the treatment plant.

Wastewater discharged from the plant enters the Pacific about a quarter-mile offshore of Sandy Beach from an outfall pipe 40 feet deep, Nunnally said.

Sandy Beach, known by locals for its long shore break, made headlines last year after two Honolulu City Council members proposed renaming the spot for President Barack Obama, the first Hawaii-born U.S. president. But the resolution was withdrawn after the plan drew mixed reactions from the public.

On Monday about 400,000 gallons of wastewater tainted Waikiki and Ala Moana beaches after heavy rains associated with Tropical Storm Kilo inundated the system. Officials opened the waters Wednesday afternoon after testing showed the water was safe.

Even after authorities give beachgoers “the green light to return to the water after a sewage spill, all is not necessarily well,” local news station KITV4 reported in a web post on a study by Tao Yan, a professor at the University of Hawaii at Manoa. The study revealed that wastewater lasts “much longer in sand.”

Lori Kahikina, Honolulu's director of environmental services, said less sewage spilled than the half-million gallons the city initially reported.

She said Wednesday 129,000 gallons of wastewater flowed into the ocean. Another 264,000 spilled on land but never reached the ocean.

Recent heavy downpours have prompted Hawaii health authorities to issue a "brown-water" advisory for storm water runoff across the islands and warnings of several smaller wastewater spills on Oahu. A Category 1 hurricane, dubbed Ignacio, is expected to pass by Hawaii next week, local newspaper the Honolulu Star-Advertiser reported.

Al Jazeera and wire services