The organisers of a triathlon on a lake fed by the River Eden in Kent have launched an investigation after a number of competitors fell ill with diarrhoea and vomiting. The Castle Race Series, organisers of the triathlon and other competitions at Hever Castle last weekend, said rigorous tests in the days and weeks leading up to the event had indicated the water was safe to swim in. There have been several reports on social media of people falling unwell. One competitor wrote: “I have never had diarrhoea or stomach cramps like this in my life. I am unable to leave my house and have been off work all week.” Another wrote: “Three of us did the sprint plus on Saturday and all fell ill Sunday evening.” A Castle Race Series spokesperson said the welfare and safety of competitors and staff was a priority. They added: “We are conducting a thorough investigation in conjunction with Hever Castle and continuing our dialogue with Southern Water and the Environment Agency to understand how we can mitigate this in future.” Water quality on the River Eden can be adversely affected by Southern Water overflows and discharges from private land. The water company said its local treatment works was compliant with its environmental permits and that it was not aware of any “non-compliant discharges”. In July dozens of competitors reported sickness after sea swimming events at the World Triathlon Championship Series in Sunderland. An investigation by the UK Health Security Agency (UKHSA) found 88 participants had reported sickness and there was evidence of norovirus infection, but reported it might not be possible to establish the exact cause. The UKHSA said this weekend it was aware of reports of sickness after the Hever Castle event and was liaising with the local authorities and the organisers. The authority was unable to confirm the number of competitors who had reported being sick, or whether any had been hospitalised. Dr Rachael Hornigold, consultant in public health for the UKHSA, said: “We are working closely with partners following a number of those attending the Hever Castle triathlon event reporting symptoms. “Diarrhoea and vomiting is usually a self-limiting illness that can be recovered from without treatment.” The agency says such cases at large events are not uncommon and it is not always possible to identify a single source from testing samples. Southern Water said: “We are sorry to hear that some participants were unwell and we wish them a speedy recovery. We can confirm our local treatment works were compliant with their environmental permits and we are not aware of any non-compliant discharges taking place. “Southern Water takes bathing water quality extremely seriously but we are just one of the many custodians of these public areas. “We work closely with a range of partners including local councils, highways departments and the agricultural sector to enhance and protect watercourses.” The lake at Hever Castle, once the childhood home of Anne Boleyn, was constructed in the early 20th century. The River Eden flows into the lake before joining the River Medway. A spokesperson for Hever Castle said: “We are not aware of any upstream sewage overflows on the Eden or at the Hever Castle lake which may have operated in the days before the triathlon or on the days of the triathlon and affected the event. “The organisers (Castle Race Series) employ an external company to test the water is suitable for swimming and, to our knowledge, all tests prior to the race and on the Sunday received a pass with the water being considered ‘excellent’.” Data compiled by the Rivers Trust charity showed a sewer storm overflow on the River Eden spilled 39 times in 2022 for a total of 474 hours. The Environment Agency said on Friday it was unable to provide information on the locations of overflows on the Eden which might affect the lake, or any discharges in advance of the Hever Castle events, which it said was a matter for Southern Water.