(NEXSTAR) — Concerns about the water quality in the Seine River led officials to call off the swimming portion of an Olympic triathlon training session for a second straight day Monday, but organizers remain optimistic the conditions will improve in time for several planned events.

The representatives for Paris 2024 and triathlon’s international federation said tests conducted in the Seine on Sunday showed water quality levels leading into the training session that “did not provide sufficient guarantees to allow the event to be held.” The delegation blamed the recent rain.

Swimming in the Seine has been banned for over a century because of the poor water quality. Organizers have invested 1.4 billion euros ($1.5 billion) to prepare the river ahead of the Olympics.

French sports minister Amélie Oudéa-Castéra told French news channel CNEWS on Monday that officials are “absolutely serene about all of this.” The plans they put in place to control bacteria levels in the river have been effective, but the weather is beyond their control, she said.

Organizers say the backup plan is to postpone the triathlon. If the elevated bacteria levels persist, the swimming portion of the event will be abandoned, and the athletes will compete in a duathlon.

In addition to the swimming part of the men’s triathlon on Tuesday, the women’s triathlon on Wednesday and the triathlon mixed relay on Monday, the Seine is expected be used for the marathon swimming competitions on Aug. 8 and 9.

The marathon swimming competitions could be relocated to the Vaires-sur-Marne Nautical Stadium in the greater Paris region, which already hosts rowing and canoeing competitions and can accommodate up to 15,000 spectators, organizers said earlier this month.

Daily water quality tests in early June indicated unsafe levels of E. coli bacteria, followed by recent improvements. Some of the measures put in place to improve the water quality include the construction of a giant basin to capture excess rainwater and keep wastewater from flowing into the river, renovating sewer infrastructure and upgrading wastewater treatment plants.

High levels of E. coli in water can indicate contamination from sewage. Most strains are harmless and some live in the intestines of healthy people and animals. But others can be dangerous. Even a mouthful of contaminated water can lead to diarrhea, and the germ can cause illnesses such as infections in the urinary tract or in the intestines.

Paris Mayor Anne Hidalgo took a very public swim in the river about two weeks before Olympic events were set to start, hoping to ease fears about the long-polluted waterway being clean enough to host swimming competitions.

The Associated Press contributed to this report