Tokyo 2020 organisers are determined to stage Olympic swimming events in the city centre Odaiba Marine Park venue despite continuing confusion over the water quality.
Underwater screens installed in Tokyo Bay, expected to host the swimming marathon and triathlon events, helped produce a marked reduction in bacteria, organisers say, but levels elsewhere are still causing concern.
A year ago, organisers said tests showed levels of E. Coli up to 20 times above the accepted limit and faecal coliform bacteria seven times higher than agreed at the venue.
Further tests were carried out this summer after underwater screens were installed to help improve water quality to the standards demanded by the International Triathlon Union (ITU) and international swimming federation FINA.
In areas where a triple-screen system was installed, test results were within required limits, organisers said.
"These results have been shared with the IOC as well as the international federations who will have competitions in this area," Tokyo 2020 Games Delivery Officer Hidemasa Nakamura said at the start of a briefing on Friday.
"They agree that the test results were quite favourable."
However, outside the triple-screened area results exceeded the targeted levels on 12 of the 16 test days.
Although he did not have the data to hand, Tokyo Metropolitan Government Sports Director Koji Kawase believes it was down to a typhoon which struck during the test period.
"So, talking about the Tokyo Bay in general, not just the screened area... I do not have the data regarding that from last year to this year but, the weather conditions such as rainfall and typhoons have a lot of influence on water qualities," said Kawase after consulting an advisor.
"Sorry, I do not have accurate data about exactly when the typhoon came," he added when pressed on the topic.
Japan was struck by Typhoon Jongdari in late July. Testing was cancelled on July 28 because of the bad weather.
Although the difference in water quality either side of the screens is dramatic, Kawase and Nakamura were vague when asked whether screens would be installed throughout the venue or whether they would be the same as those used in tests.
"Aside from exactly how we are going to build the screens but yes, we will use some sort of underwater screens during the Olympics," Nakamura said.
Despite the mixed results, organisers are determined to host the events at the venue instead of moving outside of Tokyo, pointing to Japan's National Triathlon Championships which are set to take place in Odaiba later this month.
"I think it is very important for us to hold this event in the middle of Tokyo with the ocean water and the skyscrapers and the urban background of Tokyo," said Nakamura.
"This will prove that Tokyo is a city with modern structures as well as the nature of the ocean."
The "no swimming" signs currently in place in Odaiba, he said were not just because of bacteria levels, but because there were "no life savers stationed there."
"Although the signs say no swimming, it doesn't mean that there is absolutely no swimming," he said.