A first swimming training session for Olympic triathletes in the Seine was cancelled on Sunday because of the poor water quality. Fears have grown for months that Paris 2024’s plan to hold the triathlon swimming and the open water swimming in the city’s river was putting athletes at risk due to pollution in the Seine. Organizers have insisted the plan should go ahead and the Paris mayor was among those to swim in the river to try to prove its cleanliness. But near constant rain since Friday’s opening ceremony exacerbated the problem. Only running and cycling training sessions will take place o…