Dr Robert Keirle of the Marine Conservation Society says the wet summer of 2012 is partly to blame for a decline in some Welsh bathing beaches.
Heavy rain and flooding in Wales last summer is being blamed for the drop in the number of beaches which have excellent water quality, according to the latest Good Beach Guide.
The annual results released by the Marine Conservation Society (MCS) today show that the number of beaches found as having excellent water quality has fallen from 153 in 2012 to 98 this year.
Beaches which failed to meet bathing water standards included Rhyl in Denbighshire, Criccieth in Gwynedd and Ogmore Slip in the Vale of Glamorgan.
The MCS said, "Relentless rain and flooding in many parts of the country led to an increase in the amount of bacteria and viruses ending up in our bathing waters.
"This type of pollution can originate from a variety of sources such as agricultural and urban run-off, storm waters, misconnected plumbing, septic tanks and dog faeces.
"Sewage and animal waste is full of viruses and bacteria and most water users won’t be aware that this type of pollution can increase the chance of them going home with an ear, nose or throat infection, or even gastroenteritis."
To see whether your local beach passed the test visit the Good Beach Guide website here.