Four people have so far complained of illness in a town near Hokkaido's Niseko mountain resort after steam erupted during drilling for a geothermal energy development project, Mitsui Oil Exploration Co. said Saturday.

Hydrogen sulfide at the site in Rankoshi and a high level of arsenic in water collected in a nearby area were detected. Mitsui Oil, which is in charge of the project, said it is looking into what caused people to become ill, although it refrained from giving any details of the symptoms for privacy reasons.

Mitsui Oil began drilling on June 25 to survey the potential resource, and the steam erupted four days later.

Photo taken July 5, 2023, shows steam erupting from a geothermal power generation research site in Rankoshi, Hokkaido. (Kyodo)

The first two reports of illness came earlier this week -- a woman who delivered lunch at the site and was temporarily admitted to a hospital for hydrogen sulfide poisoning and a resident of Rankoshi -- but the company said Saturday that two more people have said they are also not feeling well.

No health problems have been reported from the 18 workers at the site at the time of the incident. No hydrogen sulfide was detected outside of restricted areas, the company has said.

According to the company and the town, the arsenic level detected from the nearby water was 1,590 times higher than the standard for drinking water and 318 times higher than that for water for agricultural use.

Mitsui Oil has been disposing of water accumulated at the site of the steam eruption by dumping it in an area approximately 500 meters away from a pond known for being a tourist site, according to a company official.

At a press conference in Rankoshi, Mitsui Oil President Hidenori Harada stated that the Hokkaido government has instructed the company to cease dumping water outside the site. However, it is expected to take approximately two weeks to construct a pipeline that will redirect the accumulated water to a well about 1 kilometer away.