|Date||22 March 2015|
|Location||Llyn Cwellyn||SH 55842 55411; 53.07640°N, 4.15340°W|
Llyn Cwellyn is a natural glacial lake on the Afon Gwyrfai river in the Nant y Betws valley between Mynydd Mawr and the Snowdon massif. The lake, which supports a natural population of Arctic char, has an area of 87 hectares and has a maximum depth of 37 m.
Formerly owned by the Marquis of Anglesey, Llyn Cwellyn is dammed at its nothern end and serves as a reservoir supplying water to around 76,000 people in parts of Gwynedd and south Anglesey.
In 2005 there was a local outbreak of cryptosporidiosis – a stomach bug caused by a parasitic intestinal infection – in which there were 231 confirmed cases. The source of the outbreak was traced to contaminated water from Llyn Cwellyn, with the nearby Rhyd-Ddu sewage treatment works being identified as a possible original source of the cryptosporidium, which is resistant to chlorine dosing. The water company Dwr Cymru had to issue a ‘boil water’ notice to approximately 37,000 households and introduced ultraviolet disinfection as a short-term measure to deal with the problem. The company was, however, fined £60,000 in 2007 for failing to provide drinking water fit for consumption during the incident. Dwr Cymru subsequently invested £13 million in upgrading its Cwellyn Water Treatment Works, located in Betws Garmon, 2.3 km from the northern end of the reservoir. Construction started in 2009 and the new treatment plant was commissioned in 2010.